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The next annual meeting of the SPS will take place from 28 - 31 August 2018 at EPFL. The program will consist of the approved mix of plenary talks, topical sessions and poster exhibition.
The meeting is organised in collaboration with the Swiss Institute of Particle Physics (CHIPP) and the NCCR MARVEL (Computational Design and Discovery of Novel Materials). They all together guarantee an exciting conference covering physics at its best.
Abstract Submission Deadline: Extended until 13 May 2018
Please note that if you do not already have a CERN Indico account, you must create a CERN lightweight account before submitting an abstract. Procedure here:
https://account.cern.ch/account/Externals/RegisterAccount.aspx
Registration Deadline: Extended until 12 August 2018.
NOTE: With submitting an abstract you are NOT automatically registered. Use the form below to register.
Climate change is one of the most pressing societal and economic issues, but also one of the most complex scientific challenges. While there is overwhelming evidence that anthropogenic climate change is already happening, uncertainties in climate change projections have remained very large. For instance, current estimates of the equilibrium global-mean warming in response to a doubling of atmospheric CO2 concentrations amount to 3 ± 1.5 K. During the last 40 years, this estimate has neither significantly shifted nor narrowed. The main cause behind the slow progress is the representation of clouds in climate models, especially of small-scale convective clouds (i.e. thunderstorms, rain showers, shallow convective cloud layers). With the advent of high-resolution climate models, there are now promising prospects, as it becomes feasible to formulate the models much closer to first principles. The presentation will discuss this potential and present recent progress in this area.
The current energy system finds itself at the beginning of a transformation process of which until now only targets and fundamental determinants are known. To meet the strategic goal of a medium global warming of under 1.5-2 K, a drastic reduction of greenhouse gases until 2050 is necessary. The energy system in the long run will have to work without fossil hydrocarbons. Some industrial processes like production of iron and steel, cement and basic chemical products which are fundamentally dependent on hydrocarbons will meet additional challenges.
Most renewable energy sources are derived from solar radiation (solar thermal, photovoltaic, wind, biomass, partly also hydropower and wave energy) and are harvested on surfaces with limited energy density. Especially sustainably produced biomass as an energy carrier is sought after – and competes directly with the food supply, materials and ecological needs – meaning that under an overall perspective they are rather scarce.
Strongly increased efficiency technologies for the reduction of overall energy consumption are required in a first step.
The regional and temporal structure of energy use and energy production, especially electricity, will change and this in turn will call for transformations in the infrastructure as well as the relationships between actors (i.e. „prosumer“) and business models.
The transformation of the energy system is a complex task that interweaves technological as well as societal and economic requirements and developments.
Prognos AG has explored the necessary transformation processes in Germany and Switzerland in detailed energy system scenarios and presents the main results as well as open questions – especially ones with a physics context.
We live in a world where the demand for energy is increasing and where the use of fossil fuels (coal, gas, oil) is threatening the environment. While nuclear power plants produce no CO2 and no atmospheric pollutants, the issue of the long-standing problem of management of the long-lived nuclear waste is not solved. I will show how Accelerator Driven Systems, using thorium instead of uranium as fuel, represent a feasible solution and are being developed in several parts of the world. The impact of the research in these innovative systems is expected to be very high: spanning from the elimination of the military-grade plutonium, to the transmutation of accumulated nuclear waste, up to the deployment of a safer energy source.
Quantum-mechanical simulations have become massively used tools for scientific discovery and technological advancement: thanks to their predictive power they can suggest, accelerate, support or even substitute actual physical experiments. This is a far-reaching paradigm shift, replacing the cost- and time-scales of brick-and-mortar facilities, equipment, and personnel with those, very different, of computing engines - aiming at understanding, predicting, or designing the properties and performance of novel or complex materials and devices.
I will briefly highlight the current accomplishments and challenges, outline the current roadmap for materials discovery driven by the convergence of high-performance and high-throughput computing, and illustrate its potential with the example of novel two-dimensional and layered materials displaying promising electronic, optical, or topological properties.
Flavour physics represents one of the most interesting and, at the same time, less understood sector of the Standard Model. After a brief introduction to this filed, I will focus the attention to a series of recent results in B physics which seem to indicate a coherent pattern of deviations from the Standard Model predictions. I review these hints, addressing theoretical uncertainties and possible New Physics interpretations. Possible connections with direct searches of physics beyond the Standard Model at the high-energy frontier will also briefly addressed.
The early development of highly segmented and very precise silicon particle detectors happened historically in field of high energy physics in the early 1980’s. At the time this was very much driven by the needs in particle physics experiments to detect and identity pico-second long lived particles, containing beauty and charm quarks. The presentation will give a brief historical overview of this development and the driving requirements that defined its developments. Over the years the new detector technology has gone through an enormous growth and improvement in capability and complexity. This was on one hand through the growing demands on the performance of the particle tracking systems at the newest accelerators like the LHC proton-proton collider at CERN and at the same time due to the technological progress in microelectronics, symbolized by Moore’s Law. The potential of using this new and precise detector technology in other domains of physics was realized early on. By now, the use of highly segmented silicon strip and pixel detectors has made a phenomenal impact in the field of photon science at synchrotron facilities and free electron X-ray laser machines. They allow now to resolve in an almost unprecedented way the structural and functional information on complex solid state systems and biological molecules. The talk recalls how in the case of the PILATUS, MYTHEN and EIGER pixel system the silicon detector revolution in photon science has happened. Furthermore it will attempt to give an outlook on how the next generation detectors might evolve, given the requirements from future photon science experiments.
The European Physical Society celebrates its 50th anniversary. With a series of events and actions, the EPS remembers this year its foundation that took place in 1968 at CERN and at the University of Geneva. Under the impulsion of the Italian physicist Gilberto Bernardini, the creation of this truly international cooperative venture in physics became reality with the aim to contribute to the strength of European cultural unity. I shall present the history of EPS in the context of a world, which has changed quite a lot since 1968.
The restaurants on the campus are available. This lunch break is not organised.
Moving from the exact result that drainage network configurations minimizing total energy dissipation are stationary solutions of the general equation describing landscape evolution, I shall review the static properties and the dynamic origins of the scale-invariant structure of optimal river patterns. Optimal channel networks (OCNs) are feasible optimal configurations of a spanning network mimicking landscape evolution and network selection through imperfect searches for dynamically accessible states. OCNs are spanning loopless configurations, however, only under precise physical requirements that arise under the constraints imposed by river dynamics—every spanning tree is exactly a local minimum of total energy dissipation. It is remarkable that dynamically accessible configurations, the local optima, stabilize into diverse metastable forms that are nevertheless characterized by universal statistical features. Such universal features explain very well the statistics of, and the linkages among, the scaling features measured for fluvial landforms across a broad range of scales regardless of geology, exposed lithology, vegetation, or climate, and differ significantly from those of the ground state, known exactly. Results are provided on the emergence of criticality through adaptative evolution and on the yet-unexplored range of applications of the OCN concept – especially for ecological applications. A range of applications for the ecology of species, populations and pathogens of waterborne disease is explored - briefly.
Interactions between biological molecules are challenging to elucidate with current techniques. An orthogonal approach is to probe for 'response signatures' that identify specific circuit motifs. For example, bistability, hysteresis, or irreversibility are used to detect positive feedback loops. For adapting systems, such signatures are not known. Two circuit motifs generate adaptation: negative feedback loops (NFLs) and incoherent feed-forward loops (IFFLs). On the basis of computational testing and mathematical proofs, we propose differential signatures: in response to oscillatory stimulation, NFLs but not IFFLs show refractory–period stabilization or period skipping. We applied this approach to the cell cycle control system in yeast and the Caenorhabditis elegans AWA neurons and identified the key network topologies in each network.
To explain spontaneous polarization of motile cells, we have recently proposed a novel model of self-organizing cell activity, where local cell-edge dynamics depends on the distance from the cell center, but not on the orientation with respect to the front–back axis as assumed in previous models (Raynaud et al., Nat. Physics 12, 367-373, 2016). Here we show that traction-forces exerted by polarizing cells on the substrate increase with edge-center distance, and correlate in space and time with edge dynamics. Traction-forces increase gradually during cell protrusion, and maximal forces are observed shortly after the onset of retraction, coinciding with maximal edge retraction velocity and actin retrograde flow. Our results suggest that traction-forces mediate distance-sensitivity and organize cell activity during polarization.
The surface of biomolecules is composed of nano-domains characterized by varying hydrophobicity. The textbook understanding is that these domains contribute additively to interfacial properties. Yet, recent observations show limitations of this model.
Here, we use a combined experimental and simulation study to propose a generalized equation which includes terms that consider domain boundaries. To validate our theory, we fabricated model compounds differing solely in patchiness. Traditional understanding would be that these materials have the same wetting properties. Instead, we found considerable differences in work of adhesion and in the hydrogen-bonding network of the interfacial water. We also performed molecular dynamics simulations of water on patchy surfaces that allowed us to interpret our observations and develop our generalized model.
We present a novel concept of magnetoplasmonic biosensor with ultranarrow resonances and high sensitivity. Our approach is based on the combination of a specially designed one-dimensional photonic crystal and a ferromagnetic layer to realize ultralong-range propagating magnetoplasmons and to detect alteration of the environment refractive index via observation of the modifications in the Transversal Magnetooptical Kerr Effect spectrum. The fabricated sensor heterostructure shows extremely sharp, with an angular width of 0.06°, surface plasmon resonance (the propagation length as large as 142 um) and even sharper magnetoplasmonic resonance with an angular width as small as 0.02°. The magnitude of the Kerr effect of 11% is achieved which allows for detection limit of 1x10-6 .
The protein self-assembly related neurodegenerative disorders such as Alzheimer’s diseases have not been fully understood.
To obtain a better understanding on the mechanism of protein fibrillization, we mapped time-lapse images of insulin aggregates as a model of multi-stranded fibril by resolution Atomic Force Microscopy(AFM). With statistical analysis of their periodically fluctuated profile, we revealed fibrillar behavior during evolution e.g. how they twisted together and contribution of elongation and secondary nucleation on fibril formation. Moreover, we compared effects of ion strength and balance of hydrophobic and electrostatic interaction on the fibril formation and, most interestingly, variation of secondary structure of these aggregates monitored by AFM-Infrared nanospectroscopy so that we underlined the contribution of different secondary structures on fibril formation.
We analyze maximum entropy approaches to infer the functional design of elastic materials exhibiting allostery, i.e. the property of highly specific responses to ligand binding at a distant active site.
We consider the functional designs of in silico evolved allosteric architectures which propagate efficiently energy (including shear, hinge, twist) or strain (resulting in a less-constrained trumpet-shaped region between the allosteric and the active site).
We benchmark existing maximum-entropy inference methods on these computationally evolved functional architectures. We show that such approaches, similarly to a sector analysis, capture key aspects of the allosteric designs and provide quantitative predictions on the pattern of mutational effects; we provide an interpretation of the inferred couplings in terms of propagation of the elastic response.
Until two years ago, the smallest permanent magnets were single molecule magnets. Surface Science has opened up an alternative approach to this field in studying the magnetic properties of single atoms adsorbed onto surfaces. Very recently a major breakthrough was achieved by identifying systems where a single surface adsorbed atom are stable magnets. Ho atoms on two monolayer thick MgO(100) films grown on Ag(100) where found to exhibit magnetic remanence up to 30 K. Very recent STM experiments show stable magnetization over two hours in external fields of 8 T that are applied opposite to the magnetization of the atoms and at temperatures of 30 K; the first spontaneous switching is observed at 45 K.
Reliable organic spintronic devices depend on spin-injecting interfaces with precisely architectured multilayers to avoid premature failure e.g. due to current filaments building up at defects. Using X-ray Magnetic Circular Dichroism (XMCD) we present unambiguous evidence that in the sequential fabrication of organic bilayer interfaces by physical vapor deposition (sublimation), molecules re-organize perpendicular to the interface plane. On the basis of systematic experiments we analyse the mechanisms in this unexpected behaviour. Our results provide an analogon to the earlier established case of atomic inter-layer mixing of interfaces built with delta-doped layers in semiconductor devices as well as layered oxide films.
The properties of antiferromagnetic materials at the nanoscale are important for future spintronic devices. We study the magnetic properties of goethite at the nanoscale by combining X-ray linear dichroism (XLD) spectroscopy and X-ray photoemission electron microscopy. Complementary scanning electron microscopy is used to correlate the magnetic properties of individual goethite nanoparticles with their actual morphology. Temperature and orientation dependent XLD spectra of individual goethite nanoparticles, suggest a magnetic ordering transition close to 400 K corresponding to the Néel temperature of bulk goethite. Predominantly isotropic XLD spectra reveal isotropic magnetic properties of individual nanoaprticles despite of their acicular shape. We attribute this finding to the presence of multiple antiferromagnetic domains within the particles due their polycrystalline microstructure.
Correlated electron systems display electronically ordered ground states often intrinsically segregated at the nano/mesoscale. Antiferromagnetic order is a common instability of these materials potentially showing spatial inhomogeneity. I will present our recent result using a resonant soft X-ray scattering nanoprobe to image the bulk magnetic landscape in NdNiO3 thin films. Our measurements provide direct evidence for a highly textured magnetic fabric, that further exhibits return memory effects across the Neel transition. The scale-free distribution of antiferromagnetic domains and its non-integral dimensionality, together reveal a magnetic fractal geometry near criticality, which reflects the interplay between electronic correlations and an intrinsic local parameter. Our observations expose new essential details of the nanoscale structure of nickelates.
The magnetic reversal in perpendicular exchange-coupled systems consisting of a rare earth-based ferrimagnet (amorphous TbFe) and a transition metal-based ferromagnet (crystalline Co/Pt), can be surprisingly complex and different from conventional systems. We address the reversal mechanism locally by high-resolution MFM at 10K and in high fields. The TbFe layer alone reveals no change in its domain structure between 0 and 6T, allowing for a large field-span study of adjacent Co/Pt. The net moments of TbFe and Co/Pt are antiparallel aligned and as the field increases, the reversal of Co/Pt can be described by a three-stage magnetization process, where isolated grains switch over a wide field range. Unexpectedly, nanoscale inhomogeneities in both TbFe and Co/Pt are responsible for the observed behaviors.
Understanding and controlling the damping in ferromagnetic thin films is very important for emerging technologies including magnonics and spintronics. One of the possible ways to manipulate magnetic damping is injection of spin current generated due to spin Hall effect [1]. To measure the modulation of damping we use a time-resolved magneto-optical Kerr effect microscope (TR-MOKE), which has the best spatial and temporal resolution to measure the damping of the ferromagnetic film. The observations will significantly contribute to the field of spintronics.
References:
[1] L. Liu et. Al., Science, 336, 555-558 (2012)
SrRuO3-SrIrO3 bilayers have recently attracted attention due to their topological Hall effect (THE) as an evidence of interfacial Dzyaloshinskii–Moriya interaction (DMI), which may lead to the formation of skyrmion phase. We measured THE in a SRO-SIO bilayer at different temperatures between 5K and 100K, and performed MFM in external fields. We observe maze pattern of up/down domains of ~70 nm in size. At various locations domains with the same magnetization touch each other and narrow 360° walls form, which indicates the presence of DMI. In external fields, domains shrink and skyrmions with a diameter of 10-20 nm appear. Our measurements prove that a strong THE can occur even though no skyrmion phase but solely individual skyrmions and bubble domains exist.
Current-induced spin orbit torques are well established as a powerful method to manipulate magnetic thin films, with latest works demonstrating magnetization reversal down to the sub-ns scale. The reversal mechanism is a two-step process determined by domain wall nucleation and propagation. Recent work indicates that the domain nucleation process is a bottleneck for energy efficient switching. In this work we present a novel technique to fabricate coupled in-plane and out-of-plane magnetized lateral interfaces, where the Dzyaloshinskii–Moriya interaction lowers the energy barrier for domain wall nucleation and provides additional means of control over current-induced switching.
Zigzag graphene nanoribbons (ZGNRs) have attracted considerable interest due to the unique edge states they host. We recently established an on-surface synthesis approach to synthesize atomically precise 6-ZGNRs using a carefully designed precursor monomer. Furthermore, our recent experiments on exfoliated graphene nano-constrictions provide the first evidence through ballistic electron transport measurements that the edge states are spin polarized. To exploit these properties in devices we need to have ZGNRs on suitable substrates, this has been not yet achieved as the ZGNRs are extremely reactive. With the goal of passivating these edges in UHV to facilitate their transfer to ambient conditions, we present first results of how morphology and electronic properties of 6-ZGNRs change when exposed to reactive gases.
Recent hints of lepton flavour universality violation motivate searches for lepton flavour violating b-hadron decays. The LHCb experiment is particularly well suited for for these searches due to its large acceptance, high trigger efficiency and excellent tracking and particle identification capabilities. Recent results from LHCb on searches for the lepton flavour violating decays $B_{(s)}\rightarrow emu$ will be presented. An overview of the ongoing search for the $\Lambda_b \rightarrow \Lambda e \mu$ decay will also be shown.
The Lepton Flavour Universality (LFU) anomalies are currently one of the hottest topics in the particle physics community. A combination of recent results from LHCb, Belle and BaBar from semileptonic B decays have shown a discrepancy from the Standard Model prediction of ≈ 4 σ. I will review the latest lepton universality tests with semileptonic B mesons and present the prospects for an independent LFU test with baryons using semileptonic Λb→Λc decays. This test will help to corroborate the present anomalies and to provide complementary constraints on the possible origins of these anomalies beyond the Standard Model.
One of the fundamental properties of the Standard Model (SM) is lepton flavour universality (LFU): particles couple equally to the three lepton generations. Rare decays governed by the $b \to s \ell^+ \ell^-$, which are loop- and CKM-suppressed, are extremely sensitive to New Physics scenarios introducing particles preferentially coupling to the second or third generations, and they have been thoroughly studied at LHCb. In particular, measurement of ratios of branching fractions of $B\to X\ell\ell$ decays, with $X$ being a system containing a strange meson and $\ell\ell$ either a muon or electron pair, indicate significant deviation from theory expectations, hinting at possible LFU violation. This talk will review past and ongoing LHCb analyses aimed at elucidating the nature of this phenomenon.
The sensitivity of the decay $B^{0} \to K^{*0} \mu^{+}\mu^{-}$ to effects of beyond the Standard Model is well know. An anomalous behaviour in angular and branching fraction analyses of this decays has been reported, notably in one of the observables with reduced theoretical uncertainties, $P^{\prime}_{5}$. However, the vector-like nature of this pattern could be also explained by non-perturbative QCD contributions from charm loops, that are able to either mimic or camouflage NP effects In this talk I will discuss the main features of this channel and present the prospects to disentangle true New Physics effects from non-local hadronic contributions.
The equivalence of the three lepton families (except for their mass), known as lepton flavour universality (LFU), is a cornerstone of the Standard Model (SM), which can be violated in New Physics (NP) models by particles that couple preferentially to certain generations of leptons. In the last years, hints of LFU violation have appeared in both tree-level $b\to cl\nu$ and rare $b\to s\ell\ell$ beauty decays. These results, combined with tensions in angular and branching fraction measurements of rare semileptonic decays, point to a coherent pattern of anomalies that could represent the first crack of the SM. This presentation will review these anomalies, and give an outlook of the future, discussing how these measurements can be used to characterise NP scenarios.
Measuring the top quark Yukawa coupling is an important test of the standard model (SM) of particle physics and the production of a Higgs boson in association with top quarks (ttH) is the only channel that allows a direct measurement of this SM parameter. This talk will focus on a search for ttH where the Higgs boson decays to bottom quarks and the top quark pair decays hadronically. The data were collected by the CMS experiment during Run 2 of the LHC at a center-of-mass energy of 13 TeV. Because of the small cross section and challenging final state, sophisticated methods for signal/background rejection as well as signal extraction, such as the matrix element method, are required.
Limits on the top squark mass from the LHC experiments have reached the TeV scale in the most favourable scenarios. More experimentally challenging models are now being investigated, such as scenarios predicting complex decay chains and compressed mass spectra.
In this context, the prospects for an ATLAS search for top squark pair production in events with either a Z or a Higgs boson, with the full run 2 dataset will be presented.
Two final states are investigated, both containing high transverse momentum and either a high multiplicity of jets originating from b-quarks, to target events with a Higgs boson in the final state, or a large lepton multiplicity, two of which have an invariant mass compatible with the Z boson.
We present a search for direct bottom squark pair production using the full 2015-2017 ATLAS dataset in final states containing a large number of b-tagged jets and missing transverse energy. Naturalness considerations suggest that the masses of the superpartners of the third-generation quarks should be around the TeV scale and thus would be produced at a considerable rate at LHC. Depending upon the SUSY mass hierarchy considered, the bottom squark may decay through intermediate steps containing Higgs bosons. The results are interpreted in scenarios where the bottom squark decays via the second lightest neutralino and a b-jet and the neutralino subsequently decays to a Higgs boson and the lightest neutralino, whereby the primary decay mode of the Higgs is exploited.
After my studies at ETH and a PhD in semiconductor spintronics at IBM Research I decided to do something that had a more immediate impact on people's lives. I first worked for Sensirion and then moved to Davos to work in snow and avalanche research. I learned about the problems and challenges in this field and founded the company Geopraevent in 2012. Geopraevent offers services improving the safety in alpine regions. We design, develop and operate electronic warning and alarm systems for natural hazards like snow avalanches, rock fall or debris flows. Currently, we operate more than 70 systems worldwide.
I will present on the opportunity to adapt the chosen career path and to benefit from the experience in various areas, namely research, business and intellectual property (IP) law. After studying physics in Bochum and Brighton, I did a joint French-German PhD in physics. Then, I have worked for five years at IBM Zurich Research Lab. My research brought me into contact with IBM’s patent attorneys. This influenced my next career step in direction of IP management. Five years ago, I have changed company to Sensirion, which offered the opportunity to build up its patent department from scratch. Sensirion was founded in 1998 as a spin-off from ETH and has become a leading manufacturer of flow and environmental sensors solutions.
The nuclear power plant Gösgen produces 8 billion kWh of electrical energy, steam and radioisotopes for cancer diagnostics and therapy. As a part of the energy strategy 2050, major investments are taken for long term operation until renewa¬ble produc¬tion capacity is build up. This also includes for example research projects to license nuclear fuel with enhanced accident tolerance. In contrast to public perception, the field of nuclear power remains highly competitive and innovative. Initially, I studied physics and did a PhD in Quantum Optics at ETH Zürich. In 2010, I started working in the field of fuel service and R&D. Today, I am head of the nuclear fuel division and deputy head of staff of the emergency organisation.
After studying physics at ETHZ I had the opportunity to do my PhD at IBM Research Lab in Rüschlikon. In addition, I achieved my diploma in teaching physics. Subsequently, I worked for 3 years in industry before starting as a physics teacher at the Gymnasium and now being able to teach as well mathematics. I will present on the opportunity to adapt the chosen career path and to benefit from the experience in various areas, namely research, business projects, school environment and last but not least the own family (I am mother of 3 children!). Moreover, I would like to show you the possibility of combine career and family.
Liability risks are difficult to predict because of their long-tail nature and their susceptibility to legal, societal, economic, and technological changes. Modeling liability catastrophes is especially challenging, as there is limited experience and new risks keep emerging. The industry therefore cannot rely on historic loss data only.
By reflecting structured cause-effect chains, forward-looking models anticipate future business outcomes in light of changing operating environments, without having to wait for claims to emerge. This allows a transfer of insight into the future and to contexts where loss data is sparse.
Since centuries, physicists have developed structural modeling methods for models with potential for re-use in other areas. This has enabled Swiss Re to drive an industry-wide paradigm change to structural modeling.
At the end of the studies each of us is confronted with the ultimate decision: academic career or industry? The path may lie somewhere in between the two. After my studies on Raman scattering in high-temperature superconductors, I decided to get closer to “real-word problems” and started my career in industry. Beginning as R&D project manager, I worked in different fields, from the design of superconducting magnets to the development of optical sensors. After ten years in industry I decided to join the Zurich University of Applied Sciences to work on applied research in collaboration with industrial partners, in a position bridging the gap between the two worlds.
Band-gap problem is the fundamental issue which underlies the predictive inaccuracy in a number of applications, e.g., the determination of defect energy levels in solid. In this talk, we will first show that defect energy levels can be obtained reliably through hybrid-functional and many-body $GW$ calculations provided that the band gaps are well accounted for. We will then outline recent developments towards accurate fundamental band gaps in the context of self-consistent $GW$ and dielectric-dependent hybrid functionals, both of which are nonempirical in nature. Specifically, the self-consistent $GW$ with an efficient vertex-corrections scheme gives highly accurate electronic structures. The dielectric-dependent hybrid functional achieves the comparable accuracy for a variety of materials and is computationally more practical for large-scale systems.
Approximate density functionals produce total energies that do not exhibit the expected piecewise-linear behavior as a function of the particle number, leading to a discrepancy between total and partial electron removal/addition energies and poor predictive capabilities of ionization potentials. Koopmans-compliant functional enforce a generalized criterion of piecewise linearity in the energy of any density functional with respect to the partial removal/addition of an electron from/to any orbital of the system. Koopmans’ corrections to approximate density functionals, lead to orbital-density dependent potentials able to deliver accurate spectroscopic properties. Ionization potentials of a large set of molecules, photoemission spectra of organic donors and acceptors and band gaps of 35 semiconductors and insulators are presented, showing excellent agreement with experiment or higher-order theories.
Copper vanadates have recently shown promise as photoanodes for water-splitting photoelectrochemical cells. However, studies revealed that their performance is severely limited. We study, both experimentally and computationally, the electronic structure, excitonic effects and optical properties of β-Cu2V2O7. To achieve an accurate description of the properties of this complex oxide, we perform calculations within the framework of the Bethe-Salpeter equation and the QSGW method, while including effects of magnetic ordering, nuclear quantum motion and thermal vibrations. We find strongly bound excitons, that can affect the photoelectrochemical efficiency, and explain therewith the lower efficiencies compared to those implied by the band gap alone.
We present a theoretical formulation for studying the pH-dependent interfacial coverage of semiconductor-water interfaces through ab initio electronic-structure calculations, molecular dynamics simulations, and the thermodynamic integration method. The proposed method is applied to study the BiVO4(010)-water interface and yields a pH at the point of zero charge in excellent agreement with the experimental characterization. Furthermore, from the calculated pKa values of the individual adsorption sites, we construct an ab initio concentration diagram of all the adsorbed species at the interface as a function of the pH of the aqueous solution. The achieved results are used in conjunction with the band alignment at the BiVO4(010)-water interface, in order to study the pH-dependent catalytic reaction pathway for water splitting.
An accurate modeling of transition-metal compounds is central to many scientific problems and technological applications including battery materials, photovoltaics, multiferroics, superconductors.
Unfortunately, approximate DFT functionals do not capture electronic localization in low-dispersion states (e.g., d or f) and misrepresent important properties of these systems.
This work shows how extended Hubbard corrections to current functionals, including on-site and inter-site interactions, improve dramatically the description of many physical properties, capturing localization even in presence of hybridization. Materials for Li-ion batteries will be used to demonstrate how interaction parameters consistent with the electronic and crystal structures greatly improve the prediction of thermodynamic quantities and average voltages. A new algorithm to evaluate them from density-functional perturbation theory is shown to guarantee efficiency and accuracy.
DFT+U+V is a simple and powerful tool to model systems containing partially-filled manifolds of localized states. However, the Hubbard parameters are often treated semi-empirically, which is a somewhat unsatisfactory approach. Conceptual and practical methods to determine e.g. the Hubbard U parameter from first principles have nevertheless been introduced long ago, based either on the constrained random-phase approximation or on linear-response theory. Nonetheless, these approaches are often overlooked due to their cost or complexity. Here, we introduce a computationally inexpensive and straightforward approach to determine on-site U and inter-site V based on density-functional perturbation theory. Such developments open the way for deployment in high-throughput studies, while providing the community with a simple tool to calculate consistent values of U and V.
Biological membranes represent the selective barrier of every cell, where they shape organelles, steer vesicles trafficking and modulate interactions with integral and peripheral proteins. Thus, capturing their complexity in terms of lipids composition, concentration and chemical features is crucial to accurately describe protein-membrane interactions. Molecular modelling and multiscale molecular simulations seamless integrated with biophysical/biochemical and structural biology experiments have the potential to characterize the protein-membrane interface at the molecular level. We used this approach to study the functional membrane-binding properties of several protein systems, as those involved in CoQ bio-synthesis at the mitochondrial inner membrane and human acyl protein thioesterases that catalyze S-depalmitoylation regulating protein trafficking across intracellular membranes.
Pattern formation and growth of developing tissues involve the graded distribution of morphogens. Scaling of the morphogen gradient ensures proportioned morphological patterning of tissues with different sizes. On the other hand, we showed that growth of the drosophila wing disc is mediated by a mechanism in which cells compute the relative time derivative of the concentration of a morphogen: Dpp. In this context, scaling drives a homogenous increase of the morphogen concentration throughout the tissue and thereby can fuel homogeneous growth. A key question is how does the morphogen gradient scale? Scaling of the gradient implicates changes in endocytic trafficking of the morphogen and the rates of its lysosomal degradation. Effective changes of endocytic rates is fine tuned by the trafficking of the morphogen through two pathways: clathrin-mediated endocytosis and clic/geec. The molecular machinery controlling these endocytic events involve two extracellular factors, Pentagone and the HSPG Dally. We propose a mechanism by which these molecules can control scaling. The key observation is that Pentagone scale itself. We show that Pentagone scaling can drive the scaling of Dpp. How is then Pentagone scaling? We consider a model in which Pentagone scales itself by Advection/Dilution without the necessity of a further scaling factor.
Exciton dynamics in DNA oligomers plays a crucial role in photoprotection. Using ultrafast broadband deep-UV transient absorption spectroscopy on Deoxyadenosine monophosphate oligomers of various lengths, we observe the effect of π-π stacking. The latter adds new relaxation channels, which are not seen in the monomer. Furthermore, these channels also differ for strand lengths between 2 to 20 bases. Excimer states extending over more than two bases, may be the gateway for energy relaxation to the ground state.
Circular dichroism (CD) spectroscopy is a well-established tool in analytical biochemistry. In the deep-UV range (< 300 nm), it is sensitive to the spatial arrangement of transition dipoles on amino acid residues, nucleotides and peptides. Time-resolved CD spectroscopy is thus a promising experimental technique that is sensitive to changes in biomolecular configuration as a function of time, combining the time-dependent electronic information provided by traditional transient absorption spectroscopy with the structural information encoded in the chirality of molecular systems. In this context, we now report the first single-shot broadband circular dichroism spectrometer in the deep-UV (250–370 nm) with femtosecond time-resolution. Artefact-free static and transient CD spectra of enantiopure [Ru(bpy)$_3$]$^{2+}$ are successfully recorded at noise levels < 10$^{-5}$ OD.
The SAFIR collaboration is developing a PET insert for a pre-clinical MRI system, aiming at excellent temporal resolution, of ~5s time frames. Image reconstruction is performed using the Software for Tomographic Image Reconstruction (STIR). An accurate model of the scanner geometry is important for the precise reconstruction of quantitative PET data. Within STIR, a simplified model of the scanner is used. In this study, we implemented a more accurate model into the library. We evaluated this new implementation with Monte Carlo simulations. Our results demonstrate a significant improvement for the new model in terms of resolution and uniformity. Details of software implementation and the analysis of simulated data will be presented.
The aim of the SAFIR collaboration is the construction of a positron emission tomography (PET) insert for a preclinical magnetic resonance imaging device. The device will be able to handle high source activities and the data acquisition time for one PET image acquisition will be 5s or less.
The presented prototype consists of the same components as the full system. It fulfils all mechanical constraints, but has only two readout boards mounted with one detector module each. The detector modules consist of a scintillator matrix, with a matching silicon photomultiplier array and PETA6 Application Specific Integrated Circuits. After linearisation and calibration, the singles energy resolution at the photo peak is 14.7 % full width at half maximum.
Broadband transient absorption is a widely used tool in the domain of ultrafast spectroscopy. However, standard chemometrie methods for decomposing spectra into components associated to the involved species like global and target analysis or singular value decomposition cannot be used when the spectral shapes of these components exhibit temporal changes, for instance due to internal conversion, vibrational cooling or solvation dyanmics. We present a method which uses anisotropy measurements to perform such decompositions without the need of an a priori knowledge of the shape nor the kinetics of the involved spectral components.
Generation of entangled photons in semiconductor microcavities through lower and upper polariton scattering via biexciton state was theoretically predicted. However, the ideal condition for its implementation has not yet been experimentally established. A groundbreaking demonstration of a polaritonic Feshbach resonance was realized when the energy of two lower polaritons with anti-parallel spins was tuned in resonance with the biexciton state. Here, we report on the existence of a “cross” Feshbach resonance, for which the lower and upper polariton scatter efficiently to the biexciton state. The spectral width of the cross-Feshbach resonance is an important quantity that determines the rate at which the biexciton decays radiatively generating a pair of polarization and momentum entangled photons in the lower polariton branch.
Electronic spins in solid-state media have recently attracted large interest in quantum information science. Their large magnetic moments offers fast operations in computation and communication applications and high sensitivity for sensors. However, this implies also high sensitivity to magnetic noise, thus reducing coherence times. In our work, we demonstrate strong suppression of decoherence in an isotopically purified 171Yb:YSO crystal by inducing clock transitions simultaneously in the microwave and optical domains. Our technique allows to reach coherence times above 0.1ms and 1ms for optical and microwave transitions respectively, and can in principle be generalized to other media with strongly anisotropic hyperfine interaction. Our results show the great potential of 171Yb:YSO for optical quantum memories, microwave-optical transducers and single spin detection.
We present an angle-resolved photoemission spectroscopy study of low-temperature adsorption of potassium on the 1T-TiSe2 charge density wave (CDW) compound. Combined with simulations of one-electron spectral functions, our measurements allow for discussing the impact of K on the CDW order parameter, chemical potential and spin-orbit coupling. Our study reveals a coverage-dependent intercalation mechanism of K in the 1T-TiSe2 van der Waals gap, a rigid-band model of electron doping, as well as similarities between the dependence of the CDW order parameter on the K concentration and doping-dependent phase diagrams of 1T-TiSe2.
The origin of the four pre-edge peaks (1s-3d) in the Ti K-edge XAS spectrum of anatase TiO$_2$ is still actively debated. By combining X-ray Absorption Linear Dichroism with ab-initio finite difference method calculations, we provide an unambiguous assignment of these peaks. These forbidden transitions arise from a strong mixing of the 3d orbitals with 4p$_z$ and 4p$_{x,y}$ orbitals, with an angular momentum dependent chemical shift because of different Ti-O bond distances along $x,y$ and $z$. Interestingly, the above edge region also exhibits a strong linear dichroism related to a significant p-orbital contribution in the multiple scattering region.
Anatase TiO2 is a superior material for converting light into other forms of energies. Despite the extensive studies of its photophysics, only recently the importance of e-h interactions in TiO2 was understood [1]. Here, we apply a novel ultrafast broadband deep-UV spectroscopy setup to access for the first time the nonequilibrium dynamics of TiO2 at the band edge. By monitoring the evolution of exciton nonlinearities, we reveal an ultrafast electron cooling in the conduction band [2], and demonstrate a novel methodology to probe selectively the interfacial charge injection from a dye to TiO2 [3].
[1] E. Baldini, L. Chiodo, A. Dominguez, M. Palummo, S. Moser, M. Yazdi-Rizi, G. Auböck, B. P. P. Mallett, H. Berger, A. Magrez, C. Bernhard, M. Grioni, A. Rubio, and M. Chergui, “Strongly Bound Excitons in Anatase TiO2 Single Crystals and Nanoparticles”, Nat. Comm. 8, 13 (2017).
[2] E. Baldini, T. Palmieri, E. Pomarico, G. Auböck, and M. Chergui, “Clocking the Ultrafast Electron Cooling in Anatase Titanium Dioxide Nanoparticles”, ACS Photonics, DOI: 10.1021/acsphotonics.7b00945 (2018).
[3] E. Baldini, T. Palmieri, T. Rossi, M. Oppermann, E. Pomarico, G. Auböck, and M. Chergui, “Ultrafast Interfacial Electron Injection Probed by a Substrate-Specific Excitonic Signature”, J. Amer. Chem. Soc. 139, 11584-11589 (2017).
By performing Resonant Inelastic X-ray Scattering (RIXS) experiments at the O K and Ir L-edge on thin films of Sr$_2$IrO$_4$ grown on different substrates we observe the evolution of the spin, orbital and charge elementary excitations upon strain. We find the local lattice distortions to control the magnetic correlations, with the spin-wave dispersion showing an anisotropic softening affecting mainly the (π,0) direction of the reciprocal lattice. By comparison with simulations based on band structure calculations, we assign a dispersive mode at 400 meV to electron-hole pair excitations. We find both energy and bandwidth of this mode to be highly affected by strain, connecting its development to the evolution of the band structure and the Mott insulating gap upon lattice distortions.
Transition Metal Oxides (TMO) represent an ideal platform to exploit exotic phenomena in solid state physics. Conductivity and superconductivity in the Two Dimensional Electron System (2DES) at the LAO3/STO3 (LAO/STO) interface is one of them.
The 2DES sits on the STO part of the interface, in a potential well created by band bending. Reducing the thickness of the hosting STO material can be a tool for tuning interesting properties, like quantum well population and polaronic coupling.
In this work we analyze the electronic structure of a LAO/STO interface, where the STO side is a thin layer of few unit cells.
In 2012, the ATLAS and CMS Collaborations announced the discovery of a new state with a mass around 125 GeV, compatible with the Standard Model Higgs boson. The measurements of this new particle’s properties are important to test the predictions of the Standard Model.
A measurement of the Higgs-beauty quark coupling through the Higgs boson production associated with a Z or W boson, where H decays to b-bbar and the Z/W to leptons, is presented. The analysis is based on 36.2/fb data from p-p collisions at 13 TeV centre-of-mass energy, collected by CMS in 2016. An excess is observed in data with a corresponding significance of 3.3𝜎, making this measurement the first evidence for this process within the CMS collaboration.
We present the results of two measurements that combine the integrated luminosity of about 1.1 inverse attobarn collected by the B factory experiments BaBar and Belle in single physics analyses. The first measurement is a time-dependent $CP$ violation measurement of $B^{0} \to D^{(*)}_{CP} h^{0}$ decays, where $h^{0}$ denotes a light neutral hadron and the $D_{CP}$ meson decays into two-body $CP$ eigenstates. A first observation of $CP$ violation governed by mixing-induced $CP$ violation according to $\sin{2\beta}$ is reported. The second presented measurement is a time-dependent Dalitz plot analysis of $B \to D^{(*)} h^{0}$ with $D \to K_{S}^{0} \pi^{+} \pi^{-}$ decays to access $\cos{2\beta}$. We report the first evidence for $\cos{2\beta}>0$ and the exclusion of multi-fold solutions of the CKM Unitarity Triangle.
A search for CP violation in the charm sector is performed through a full amplitude analysis of the decay mode $D^0 \rightarrow K^+ K^- \pi^+ \pi^-$. Its complicated resonant structure and the interferences between the various amplitudes together with the unprecedented statistic may offer the opportunity to observe CP violation for the first time in charm mesons. This symmetry breaking is expected to be small in the standard model, which makes effect of physics beyond the Standard Model comparable, if not bigger. One of the key points of the analysis is to develop an amplitude model that describes with good approximation the five dimensional phase space of the data.
Gauge theories play a fundamental role in physics, from high energy (e.g the Standard Model) to condensed matter (e.g. as effective low energy theories of many-body Hamiltonians).
Dimensional mismatch theories are a particular example of this. These models are characterized by gauge fields that live in an higher dimensions than the matter fields.
After an overview of the application of these models, we focus our attention on 1+1 dimensional fermions interacting with higher dimensional $U\left(1\right)$ gauge fields. We show that confinement, present for the case of $1+1$ gauge fields, survives when gauge fields live in higher dimensions. This contradicts the naive intuition that, when the gauge fields are in $3+1$ dimensions, the fermions should deconfine, as it happens for QED.
We first sketch the concepts of the lattice regularisation in quantum field theory. This formulation provides a link to statistical mechanics, which enables its treatment by means of Monte Carlo simulations. They lead to non-perturbative numerical measurements of observables, such as the hadron spectrum, from first principle of Quantum Chromodynamics (QCD). We summarise the status of lattice QCD with dynamical quarks, where hadronic observables can now be evaluated to percent accuracy.
In LHCb very rare decays are defined as decays that are forbidden in the Standard Model or with branching ratios smaller than 10-8. These include purely leptonic decays as Bs → µµ, Ks → µµ, Bs → ττ, which are loop decays very suppressed in the Standard Model but can be highly enhanced in new physics scenarios. Predictions for such decays are very clean and therefore they constitute excellent tests for the Standard Model. Lepton flavour violating and Lepton number violating decays are also searched at LHCb, and, as these are effectively forbidden in the Standard Model, any observed signal would be a clear sign of new physics. An overview of analyses and searches for very rare decays at LHCb is presented.
Purely baryonic decays of baryons such as $\Lambda_b \rightarrow p \bar{p} n$ and $\Lambda_b \rightarrow \Lambda p \bar{p}$, are predicted by the Standard Model but they have never been observed. Measurements of purely baryonic decays represent a valuable test of assumptions and factorization approach used in theoretical predictions. This contribution describes the first steps and the current status of the study of purely baryonic decay processes using the LHCb detector aiming at the experimental observation of these decays, measuring their branching fractions and, if signal yield allows, carry out the CP violation measurements for those decay modes.
Radiative $b$-hadron decays are a sensitive probe for new physics and a measurement of the photon polarisation in $b\rightarrow s \gamma$ transitions can help constrain several models of new physics. The methods used by the LHCb collaboration to measure the photon polarisation include the full amplitude analysis of $B \rightarrow K\pi\pi\gamma$ decays, the decay time analysis of $B_s \rightarrow \phi \gamma$ decays, and the angular analysis of $\Lambda_b \rightarrow \Lambda \gamma$ decays. The status and sensitivity of these measurements will be presented, as well as the constraints they set on the Wilson coefficients, the parameters used in the effective description of $b \rightarrow s$ transitions.
Christian Bosshard, Vicepresident Center Muttenz, is managing the development of light structures, printed electronics and integration technologies for applications in solid state lighting, sensors (for life sciences), medical devices and optoelectronics in general. He received his degree in Physics (1986) and his doctorate (1991, Silver medal award) from ETH. In 2001, he joined CSEM to build up the microsystem packaging activities before becoming Director of CSEM Muttenz. Christian Bosshard is a Fellow of the Optical Society of America (OSA), Managing Director of the Swissphotonics technology network, Coordinator of the Photonics Platform in the Heterogeneous Technology Alliance (HTA), and a member of the University Council of the University of Basel.
Cantilever-based measurement techniques have proven to be powerful tools for Nanotechnology. We will discuss two examples from our current R&D. First, we track the mass of a single cell with high precision in physiological conditions, while simultaneously conducting optical microscopy in order to link cell mass dynamics to cell morphology and state. Second, by using a different approach, we characterize the surface of 2D materials with atomic precision at ambient conditions. For 2D sheets of material, their electric-, optical-, mechanic- and structural properties greatly depend on the crystal structure, the cleanliness of the surface, influences of the substrate and the density of defects. We show an atomic-scale characterization of various materials and indications for a structural phase transition in MoS2.
After an apprenticeship as a mechanical draftsman, Eugen Voit studied Physics at ETH Zürich. After his PhD in Nonlinear Optics he joined industry in 1988. Starting on the R-side of R&D he served in many functions over his active 30 years. For more than 10 years he was the Chief Technology Officer of Leica Geosystems AG in Heerbrugg. He always kept close contact with academia and holds a honorary professorship at Ulm University.
My scientific career started with theoretical particle physics and became increasingly more applied: Following my experimental PhD focussing on quantum electronics at the ETH Zurich, I started industrial R&D in the “Applied Physics Group” at ABB Corporate Resarch. Here, I deal with a variety of topics including thermal management, electrical properties of polymers, multi-energy systems, and energy storage.
In my presentation, I will point out differences between academic and industrial research, based on my experience and using the example of two projects: one in ‘airborne wind energy’ and the other one in energy storage. I will conclude with some final remarks regarding physics – as a knowledge base and a way of thinking – and being a physicist in industry.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
We investigate the mechanism of hole diffusion across leaky amorphous TiO$_2$ (am-TiO$_2$) layers. Through $\textit{ab initio}$ molecular dynamics simulations, we construct an atomistic model of am-TiO$_2$ consistent with the experimental characterization. We first demonstrate oxygen vacancies impossibly occur in am-TiO$_2$, which can be assimilated by the amorphous structure upon structural rearrangement. Hence, their role in hole diffusion is ruled out. In contrast, O-O peroxy linkages are formed in pristine am-TiO$_2$ upon injection of excess holes, with an associated defect level at 1.25 eV above the VBM. We show that such linkages can provide a viable mechanism for hole diffusion in am-TiO$_2$, as illustrated by a diffusion path of 1.2 nm with energy barriers lower than 0.5 eV in our model.
The Density Overlap Region Indicator (DORI) is a density-based scalar field, which reveals covalent bonding patterns and non-covalent interactions simultaneously. The present work goes beyond the traditional static quantum chemistry use of scalar fields and illustrates the suitability of DORI for analyzing geometrical and electronic signatures in highly fluxional systems. We show how DORI can capture fingerprints of CH-π and π-π interactions throughout the temperature-dependent rotational processes of a molecular rotor. It also serves to examine the fluctuating π-conjugation pathway of a photochromic torsional switch. Attention will be placed on post-processing the large amount of generated data and on reducing their dimensionality combining DORI with the sketch-map dimensionality reduction algorithm.
Despite the success of linear-response time-dependent density functional theory (LR-TDDFT) in describing the photophysics of many molecular systems, its applicability has been limited by several substantial drawbacks, such as the description of charge-transfer and doubly excited states. Moreover, the approximate TDDFT functionals are unable to describe London dispersion interactions. Here, we aim at understanding the impact of van der Waals interactions on the properties of chemical systems beyond their electronic ground state. For this, we performed excited state and molecular dynamics simulations on the prototypical cis-stilbene molecule and its 3-3',5-5'-tetra-tert-butyl derivative. While the treatment of London dispersion results in negligible changes for cis-stilbene, these attractive forces have a substantial influence on the energetics and structural evolution of its substituted derivative.
Electrocatalysis is expected to play a key role in the development of a clean energy cycle. Understanding the reaction mechanism of target electrochemical processes is highly desirable in order to facilitate the design of novel catalyst materials. We present how first-principles calculations combined with an accurate implicit solvation model can contribute to shed light on the reaction mechanism of two electrocatalytical processes: the reduction of an oxidized gold surface and the CO$_2$ reduction on copper electrodes. We also present the implementation of a size-modified Poisson-Boltzmann model, which allows one to mimic the presence of ionic species in electrolyte solutions. We show how by accurately accounting for the diffuse-layer capacitance, this model enables an improved description of electrified interfaces in solutions.
The phonon Boltzmann equation developed by Peierls describes the heat conduction in solids in terms of the dynamics of interacting phonon wave-packets. This picture holds true only in a perfectly pure and infinite crystal. Several methods have been recently developed to solve this equation in a numerically exact way, allowing to determine the thermal conductivity of crystals.
Introducing disorder, it is possible to reach a point where the phonon wave-packets do not propagate far enough to sample the crystal periodicity, rendering impossible to attribute them a wave vector or a group velocity. In this regime, the theory developed by Allen and Feldman applies.
The thermal transport in Si/Ge alloys is studied using these two approaches at variance of the composition.
Despite its simplicity the interacting homogeneous electron gas (HEG) is a paradigmatic test case in the study of the electronic structure of condensed matter. Beside being a model for valence electrons in simple metals, it also provides the basic ingredient for key electronic-structure theories. Here we propose to study it with many-body perturbation theory (MBPT), including diagrams beyond GW, to improve on the description of its spectral function. A novel numerical implementation of MBPT for the 3D non-relativistic HEG has been developed, with special focus on the treatment of the full-frequency dependence of the Green's function and self-energies.
Koopmans-compliant functionals provide a novel orbital-density-dependent framework for an accurate evaluation of spectral properties by imposing a generalized piecewise-linearity condition on the total energy of the system with respect to the occupation of each orbital. Because of the orbital-density-dependent nature of the functionals, minimization of the total energy leads to a ground-state set of variational orbitals that are localized. Here we show how to transform this variational formulation to a physically meaningful Bloch-like picture. We discuss the validity of Bloch's theorem within the context of orbital-density-dependent Hamiltonians and, as a proof of principle, we present the Koompans-compliant band structure of selected semiconductors and insulators.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
We present time- and angle-resolved photoemission spectroscopy (TR-ARPES) measurements on BixSb(2−x)TeySe(3−y) topological insulators. Exploiting circularly polarized femtosecond pulses we investigated spin-related ultrafast phenomena as photo-induced spin current and spin-dependent relaxation processes.
In particular, we report the first experimental evidence of a direct coupling between light and empty topological surface states (ESS) that triggers a flow of spin-polarized electrons in k-space i.e. a photon-induced spin-current. In addition, our data suggest an accumulation of spin-polarized electrons in the conduction bands leading to a finite polarization in the surface resonance state (SRS).
Topological insulators have emerged as a novel quantum state of matter[1,2]. The Dirac-like surface state and the peculiar spin-texture play an essential role in manipulating spin-polarization and spin-current. For future topological-based devices, a fine tailoring of electronic properties and robustness against air exposure are required. An innovative approach may rely on lead-based ternary chalcogenides[3,4]. By performing detailed ARPES and STM investigations of PbBi4Te7 and PbBi6Te10, we provide evidence of coexisting topological surface states (TSSs) due to different surface terminations and Rashba-like split states close to the Fermi level. TSSs and Rashba-like states display a prevalent two-dimensional character and are barely affected by air exposure. XPS measurements and DFT simulations suggest Rashba-like states stem from van der Waals gap expansion.
Designed band engineering via perturbative superlattice potentials, inspired by graphene on hexagonal boron nitride substrate, potentially allows to induce topological bands. Making use of the general form of a substrate potential as dictated by symmetry, we derive the low-energy mini-bands of an hexagonal superstructure. Assuming a large supercell, we focus on a single Dirac cone (or valley) and find all possible arrangements of the low-energy electron- and hole bands in a complete six-dimensional parameter space. We identify the sectors hosting topological mini bands and also characteris complex band crossings that generate a Valley Chern number atypically larger than one. Our map provides a starting point for the systematic design of topological bands by substrate engineering.
Topological insulators are commonly described using dimensional reduction from higher dimensional systems, e.g., charge transport in 1-dimensional Thouless pumps is mapped by dimensional reduction to the 2-D quantum Hall effect. Recently, a class of 2-dimesnional lattices has emerged where localized modes exist not only on the edges (1D) but also on the corners (0D). In this talk, I will present how these localized states can be understood from the boundary physics of a 4-dimensional Dirac model. Using the same paradigm of dimensional reduction, I will show that the variety of recently proposed 2-D models stems from the 4-dimensional symmetry of the parent model, in addition to finding a novel 2-D model with 0-D corner modes.
Entanglement properties are routinely used to characterize phases of quantum matter in theoretical computations. For example the spectrum of the reduced density matrix, or so-called "entanglement spectrum", has become a widely used diagnostic for universal topological properties of quantum phases. However, while being convenient to calculate theoretically, it is notoriously hard to measure in experiments. Here we use the IBM quantum computer to make the first ever measurement of the entanglement spectrum of a symmetry-protected topological state. We are able to distinguish its entanglement spectrum from those we measure for trivial and long-range ordered states.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
In beam transport systems of a proton therapy machine, it is important to have an on-line measurement of the proton-beam intensity (nA). A non-interceptive beam intensity monitor has been developed for low-intensity beams for proton therapy machines without the hindrance of interceptive monitors. It works on the principle of a reentrant cavity resonator, matching its resonance frequency of 145.7 MHz to the second harmonic of the beam pulse repetition rate of 72.85 MHz. A prototype was built based on the ANSYS HFSS driven modal setup to optimize design parameters such as the position of inductive pickups. Characterization of the prototype is performed on a stand-alone test bench. Comparison of simulated and test bench scattering parameters provides a good agreement.
An emerging new area in accelerator technologies is THz-driven devices. Optical rectification of infrared pulses gives intense phase-stable sources thus provides intrinsic synchronization without timing drifts and great temporal resolution. Generated single-cycle fields in the tens of MV/m range would allow downscaling of accelerator and beam diagnostic infrastructure.
Manipulation and characterization of low-energy few picocoulombs bunches was already demonstrated with the segmented THz electron accelerator and manipulator (STEAM) device. Simulation results will be presented on the optimization of this robust device: changing dielectric slab lengths and segmentation of the layers so that it efficiently interacts with relativistic electrons produced by SwissFEL and demonstrates terahertz streaking on a few fs scale.
We present here a method that allows data transfer through fog and clouds, based on the opto-mechanical displacement of the water droplets instead of directly interacting with them. We experimentally demonstrated this method, which is based on the use of two lasers and the detection of a 1 GHz amplitude modulated sinusoidal signal. A high peak power pulsed laser generates the cloudless channel while a co- or counter-propagating low power laser carries information through the transmittive channel. Our experiment has been carried on a laboratory-scale fog length, with a typical water droplet density more than 100 times higher than that of real fog or cloud, hence an optical density comparable to a real fog.
In ice-covered enclosed water basins, the primary source of energy is the fraction of solar radiation that penetrates across the ice and reaches waters usually found below the temperature of maximum density. The radiative buoyancy flux works directly into the bulk, forcing increments of temperature/density in the upper fluid volume, which can become gravitationally unstable and drive convection. In this work, we formulate the mechanical energy budget and its energetics in an ice-covered Boussinesq flow forced only by solar radiation, and we introduce the 'cumulative mixing efficiency' to quantify the mixing over the diurnal time-scale. This work provides a framework to examine physical processes in ice-covered lakes.
This paper explores the potential of an atmospheric single-column model to analyse the nonlinear effects of a subgrid-scale mid-latitude open freshwater body. The are to propose a coupling technique between a combined land-open water surface and the atmosphere, to evaluate the nonlinear effects as the fractional areas of both surfaces increases incrementally from land to open water, and to evaluate the predictability of the atmosphere-surface system. Results showed that the mean surface fluxes and other relevant quantities may not be arithmetically averaged on the basis of the fractional area of the land and that of the open water surfaces, as simulated quantities evolve in a highly nonlinear manner as a function of the respective land and open water areal fraction.
Heat exhaust is one of the major issues in fusion research. The TCV tokamak has a large flexibility in the magnetic geometry of the plasma, allowing alternative divertor configuration studies in order to mitigate the tremendous heat and particle fluxes on the plasma-facing components. The envisioned divertor upgrade of TCV will introduce baffles to the divertor, which were predicted to increase the neutral compression by one order of magnitude compared to the unbaffled case. This is believed to be essential for high performance operation. Predictions based on the SOLPS-ITER code package will be presented that compare the properties of both divertor configurations in the light of atomic and molecular neutral distribution.
Understanding the plasma dynamics in the scrape-off layer (SOL) is of fundamental importance on the way to fusion energy. For example, the SOL sets the boundary conditions for the tokamak core and it regulates the energy flux to the tokamak wall. With the goal of improving our understanding of the SOL, the GBS code was developed during the past years. GBS simulates the SOL turbulence by solving the two-fluid drift-reduced Braginskii equations. In this work, we describe recent simulations that consider plasma turbulence in a X-point diverted geometry.
Proper understanding of turbulent transport is crucial to achieve controlled fusion. Numerically, turbulence can be studied in the gyrokinetic framework, where turbulent fluctuations are separated from the rapid gyration motion of the charged particles. Comparison with experimental observables can then be made through the application of a synthetic diagnostic. In this work we make use of a synthetic diagnostic to simulate measurements from a tangential phase contrast imaging system on TCV. The system measures line integrated electron density fluctuations along a laser beam path that is tangent to the magnetic field. Comparison with nonlinear gyrokinetic simulations using the Eulerian GENE code with realistic TCV geometry, is provided by the synthetic tool that integrates the simulated fluctuations over diagnostic volumes.
Exposure is a short-science-film hackathon for scientists and artists. Last held in Lausanne in November 2017, it united forty young science enthusiasts to learn and develop science communication skills by producing 3-minute short science films in just 3 days. We present here the “Sun in a Box” short-film which won the joint-best film voted by an expert jury. It attempts to beautifully share the challenges of nuclear fusion to the public at large. The team and organizers of Exposure will be present to share both the joys and challenges of the experience. The film would be available for viewing through a computer screen.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
In recent years, it has become possible to investigate transport phenomena using ultracold atoms in a two-terminal configuration where two reservoirs are connected through a mesoscopic channel. The measurements, however, rely on comparing different samples because of the destructive nature of probing methods, which makes the measurements sensitive to even very weak fluctuation in the atomic sample preparation. In order to achieve more precise measurements, we will implement non-destructive measurements of atomic currents featuring the cavity QED technique. We are currently developing a new apparatus where a degenerate Fermi gas of Lithium-6 is coupled to a high-finesse optical cavity. In the poster, we will discuss the non-destructive probing scheme using the cavity and present the recent progress on the apparatus.
The application of quantum-logic techniques to the spectroscopy of trapped atomic ions has enabled the determination of atomic properties at unprecedented levels of precision. Molecules have been proposed as suitable candidates for testing possible time-variation of fundamental constants, e.g. $\frac{m_p}{m_e}$ - ratio, and as long-coherence-time qubits. Our efforts focus on N$_2^+$ which has recently been identified as a promising candidate system for precision spectroscopy. We are currently establishing a complete toolbox for high-precision spectroscopy of single molecules using quantum-logic methods, their initialisation, coherent manipulation and non-destructive interrogation by coupling them to a co-trapped single atomic ion. We have laid the experimental and theoretical foundations for hyperfine-state initialisation of the molecular ions and addressing the suitable extremely narrow infrared transitions.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
Traction forces are critical for the interaction of the cell with its environment, cell polarization and motion. We have recently observed that traction stress that the cell exerts on the substrate is correlated to the cell-edge dynamics and increases with the distance between the cell edge and the center. In this work, we test if these features of traction force distribution could be reproduced by a simple model of force transmission in the actomyosin network (Ronceray et al., PNAS, 2015, doi:10.1073). We observe that in the case of a centripetal force located in the center of the cell, the force decays with distance. Force dipoles distributed throughout the cell lead to more complex force distributions which are currently under investigation.
An infection with a pathogen is especially dangerous in cases of developed resistance to currently available drugs. Depending on the infecting agent, the onset of disease may progress quickly. Culture-based conventional susceptibility assays take days and weeks to complete, in order to chose the right drug and dose. Our work considers the nanomechanical sensor that responds to miniscule fluctuations exerted by living cells. The sensor is able to discriminate metabolically active bacteria from antibiotic-inactivated or dead ones. The technique is based on optical lever detection and a simple inexpensive prototype device has been developed. The proposed technique has been tested on numerous bacterial strains. Working principle of the technique and test cases with bacterial samples will be presented.
Nowadays, sharpened glass fiber – made probes attached to a quartz tuning fork (TF) and exploiting the shear force – based feedback are by far the most popular in the field of SNOM. These probes are expensive, very fragile and their fabrication is difficult, hard to control and in many cases a hazardous process. Here we are presenting the first SNOM probes made from different plastic optical fibers. The sharp tips were prepared by chemical etching of the fibers in dichloromethane - ethyl acetate solution and other solvents, and the probes were prepared by proper gluing of sharpened fibers onto the TF. These probes demonstrate an excellent performance in both topographical and optical channels after intense use.
With the misuse of antibiotics and the increasing number of multi-resistant
living organisms, antimicrobial resistance becomes a very serious public
health issue. Conventional techniques for antibiotic sensitivity
characterization requires 24 h to one month.
Our group developed a cantilever based nanomotion detector that provides the same result in less than one hour. The organism of interest is attached onto a cantilever and its nanoscale movements induce cantilever oscillations. If the organism is exposed to an antibiotic to which it is sensitive and that compromises its viability, the
oscillations stop.
This work proposes microfabricated device with an array of such nanomotion detectors. Miniaturization of the chip and integration with a microfluidic system will permit to monitor dozens of microorganisms simulatenously.
MRI is a versatile and widely used imaging technique, which is based on the principles of NMR. However, both techniques suffer from an inherent insensitivity due to very small levels of polarization. By combining hyperpolarization techniques with detection of the NMR signal through the asymmetry of radioactive beta or gamma decay an increase in NMR sensitivity by almost 10 orders of magnitude is achievable. This increase in sensitivity will drastically decrease NMR and MRI measurement times. First studies on 26Na in liquid samples were performed, with the aim to study Na interaction with biological samples. In addition MRI experiments using radioactive Xe isotopes (131m, 133m) are planned. These can be used in lung and brain imagining.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
Black phosphorus (BP) consists of a layered puckered structure of P atoms in a hexagonal arrangement, with outstanding optical, [1] electrical [2] and thermal properties [3].
Unlike his carbon counterpart graphene, it exhibits a thickness-dependent electronic band gap, spanning from 2 eV for phosphorene (single layer) down to 0.33 eV for bulk BP.
Our trARPES experiment now shows that the electronic structure of BP undergoes dramatic transient changes, namely a complete or almost complete gap closure, when the system is excited by an ultrafast IR pulse.
[1] Jia, Y. et al., Nat. Commun., 5, 1–6 (2014)
[2] Li L. et al., Nat. Nanotec., 9, 372–377 (2014)
[3] Luo, Z. et. al., Nat. Commun., 6, 8572 (2015)
A bottom-up approach allows for the synthesis of ultra-narrow graphene nanoribbons (GNR) with a sizeable bandgap and thus excellent candidates for switching applications. In this work we focused on 5- and 9-atom wide armchair GNRs (5-AGNR, 9-AGNR) grown under UHV conditions on Au(111) and Au(788) surfaces. GNRs were transferred using two different transfer approaches and for both 5- and 9-AGNRs Raman spectra indicated no significant degradation upon transfer as well as remarkable stability overtime. Detailed characterization of both transfers’ will be addressed. In a next step, 9AGNR-FET devices were produced using graphene electrodes, with a channel length of 1-5nm. A performance of Ion > 6μA at Vd = 0.1 V and high Ion/Ioff ratios of ~10^4 was observed
InteractiveXRDFit is a Matlab program that calculates the X-ray diffracted intensity for heterostructures. Other fitting programs are already available and efficient, but may lack some flexibility with some of the parameters that this program allows modifying. Here the user can choose the substrate and the different materials composing an heterostructure among a long list of compounds (mainly perovskite oxides), choose between (001) or (111) substrate orientation, and play with the different structural parameters (unit-cell size and number of layers). It is possible to build a superlattice composed of up to three different materials, and add a top and/or bottom layer (to simulate electrodes, spacers or capping layers). Each layer can have a different c-axis (either constant or varying as a function of depth within a layer). The simulation is quick and allows the user to compare it directly to the measurements, so as to rapidly determine the crystalline parameters of the sample.
$CsPbBr_3$ is a robust fully-inorganic semiconductor of the lead-halide family of perovskites (LHPs), whose outstanding properties and low cost promise several applications in photovoltaics, photodetectors, and optoelectronics. However, their band structure, which is fundamental to describe their interaction with light, remains poorly studied. By applying angle-resolved photoemission spectroscopy utilizing an extreme-UV light source, a direct view of the electronic structure in the whole surface Brillouin zone was achieved. Valence band mapping of $CsPbBr_3$ single crystals was performed, in agreement with available theoretical studies. The effective masses of the holes were calculated from extracted band dispersions along different high symmetry directions.
One-dimensional (1D) materials are important model systems. We present a high energy resolution ARPES study of NbSe$_{\mathrm{3}}$, a quasi-1D bulk material exhibiting three-dimensional charge density wave (CDW) phases at low temperature. Synchrotron measurements reveal CDW gaps in the electronic structure at energies well below the Fermi level ($E_{\mathrm{F}}$) [1], while ultra-high energy resolution laser ARPES at 6 eV uncovers previously obscured gaps at $E_{\mathrm{F}}$. A comparison to spectral function calculations highlights the importance of inter-chain coupling in the CDW formation. Concurrently we observe a change in the dimensional behavior of NbSe$_{\mathrm{3}}$ with temperature and extract the crossover energy scale [1]. Such considerations are generally applicable to low-dimensional materials.
Here, we have studied the magnetically ordered phase of Ca$_{2}$RuO$_4$ at T = 16 K, using Oxygen K edge Resonant Inelastic X-ray Scattering (RIXS) technique. Four excitations have been identified – two low energy excitations labelled A and B at 80 meV and 400 meV respectively and two high energy excitations labelled C and D at 1.3 eV and 2.2 eV respectively. The A and B branches are interpreted to be arising from composite spin-orbital excitations due to spin-orbit coupling and the high-energy excitations, dubbed C and D, arise from singlet-triplet excitations at the Ruthenium site set by energy scale of Hund’s coupling
Charge-carrier cooling in transition metal oxides is of outmost importance in photovoltaics where it affects the cell performances. In this study, charge-carrier cooling is investigated by femtosecond broadband UV spectroscopy with different pump wavelengths where the cooling can be monitored via a red-shifting optical bleach towards the optical gap energy. Lifetime density analysis provides the cooling time with respect to the pump excess energy. In agreement with previous theoretical works by V.P. Zhukov and coworkers, we find that the cooling time is limited by the electron cooling.
Since the discovery of high temperature superconductivity in the
cuprates, this class of materials has been heavily investigated. However,
even after 30 years of research, the mechanisms that lead to their unique
behaviours are still not fully understood. Much focus has been given to their
electronic structure. Since these materials exhibit strong electron
correlations, density-functional-theory (DFT) has been considered too
simplistic. Recently, we succeeded to resolve both the $d_{x^2-y^2}$ and d$_{z^2}$ bands in La-based cuprates directly with angle-resolved photoemission spectroscopy
(ARPES)[1]. On this poster, a comprehensive ARPES study across single layer
hole-doped cuprates is given and it will be demonstrated how standard DFT
calculations describe qualitatively the electronic structure of overdoped
cuprates.
[1]C.Matt et al.,Nature Communications 9,972(2018)
Materials with antiferromagnetic interactions between spins on a triangle lattice inherently exhibit large frustration between similar energy ground states giving rise to new behavior. The kagomé lattice is an enticing example; however, various effects hinder its highly degenerate spin-liquid state and instead select a single magnetic ground state. It is therefore worthwhile to study nearly-kagomé compounds in an attempt to discern what precisely stops formation of the spin-liquid. Since it has strong antiferromagnetic interactions on a diamond-kagomé lattice we studied the magnetic excitation spectra of Cu2OSO4 in order to help elucidate the mechanisms by which spin-liquid formation fails.
Multiferroic materials are good candidates to realize ultrafast control of the magnetic and electric polarization simultaneously. This condition is met in $CoCr_2O_4$ (CCO), showing multiferroic properties below $T_s=27K$. This investigation concentrates on the effects of Ge-doping on ultrafast demagnetization dynamics below and above the multiferroic phase transition.
Femtosecond LASER pump, X-ray probe measurements at the Co L3-edge were performed at the Femtospex beamline (BessyII,HZB) for different temperatures. The experiments show that the exponential decay times are in the order of 2 ps, which is comparable or even faster than previous results on pure CCO. In comparison, the Antiferromagnetic state (AFM) in CuO decays in about 200 fs, and the long range AFM order in TbMnO3 decays in about 23 ps.
Ca$_{3}$Co$_{2}$O$_{6}$ is a frustrated Ising-like magnet with uniaxial anisotropy along its crystallographic c-axis. Magnetic field applied along this axis induces metamagnetic transition with occurrence of a magnetization plateau at ⅓ of maximum magnetization. We aim to understand the dynamics of this transition using bulk and microscopic methods and eliminate compound specific features to identify universal aspects behind them.
Magnetization was measured on several samples as function of magnetic field at different temperatures to compare metamagnetic phase transitions in positive, negative and zero field regions, and AC susceptibility to study their frequency dependence, and determine average size and distribution of magnetic clusters. Maximum d$M$/d$H$ calculated from the magnetization measurements were not reflected in $\chi_{\rm AC}$ indicating complex size-dependence of oscillating clusters.
We performed deep-UV transient absorption spectroscopy of anatase-TiO2 sensitized by gold nanoparticles (NP). The advantage of the deep-UV is that the probe is sensitive to the excitonic transitions of TiO2. We detect electron injection upon excitation of both the interband transitions and the plasmon band of the NPs on time scales of 300 fs and < 500 fs (limitation of the time resolution), respectively. This is very different to reports using THz to visible probes that are sensitive to the NPs. We also find that the electron injection yield is ca. 5 times higher under interband excitation than under plasmonic excitation.
The bottom-up fabrication approach allows the growth of atomically precise graphene nanoribbons (GNRs). The fascinating properties, which originate from the unique electronic structure of GNRs, motivate wide interest in its potential application. In this work we synthesized 9-atom wide armchair GNRs under UHV conditions on a Au(11 12 12) crystal, allowing the growth of aligned GNRs. GNRs were successfully transferred by the electrochemical delamination method with preservation of their structural quality and uniaxial alignment. We use polarized Raman for a detailed analysis of characteristic GNR modes and scanning tunneling microscopy and atomic force microscopy for the GNR film morphology before and after transfer. Furthermore, the influence of GNR coverage on the uniaxial alignment and transfer efficiency will be addressed.
Low-dimensional quantum magnets are of great fundamental importance due to strong quantum fluctuations which can produce novel quantum excitations and ground states. The 2D quantum (S = 1/2) Heisenberg antiferromagnet on a square lattice (2DQHAFSL) is one of the canonical interacting quantum systems. The chiral quantum magnet family A(bO)CU4(PO4)4 shows great promise to be a very exciting system to study novel quantum magnetism, where A-b = Ba-Ti, Pb-Ti, Sr-Ti and Ba-V. Indeed, substitution of A2+ cation controls the strength of the structural chirality in this family and shows a dramatic change in the magnetic interactions. This defines a family of chiral magnets with a tunable crystal structure.
These compounds crystallises in layers of Cu4O16 cupolas that stack along the (00L) direction. Our hope is that it is a good approximation of the square lattice, depending on the chirality of the system.
In this presentation, I will show the results of our neutron scattering measurements on several members of this family. An hamiltonian has been derived based on a Multi Bosonic Wave Theory. I will then include our compounds in the frame of the analysis of the J1-J2 Heisenberg model on the square lattice phase diagram.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
Artificially designed arrays of nanostructures with a microstructure at sub-micrometer length scales can exhibit unique functionality, especially when built from a combination of different classes of materials. We present an overview of a novel magneto-mechanical material, where the coupling between nanoscale magnets embedded in a soft polymer matrix is exploited to control its mechanical properties. In addition, we elaborate on the possible applications unlocked by this new system. Different approaches to the realization of such a material using lithography, 3D laser lithography and nanoparticle dispersions are expanded upon. Finally, we present the most recent results involving fabrication and characterization of magneto-mechanical properties of our proposed material.
The anomalous Nernst effect in a perpendicularly magnetized Ir$_{22}$Mn$_{78}$/Co$_{20}$Fe$_{60}$B$_{20}$/MgO thin film is measured using well defined in-plane temperature gradients. The anomalous Nernst coefficient reaches 1.8 $\mu$V/K at room temperature, which is almost 50 times larger than that of a Ta/Co$_{20}$Fe$_{60}$B$_{20}$/MgO thin film with perpendicular magnetic anisotropy. The anomalous Nernst and anomalous Hall results in different sample structures revealing that the large anomalous Nernst coefficient of the Ir$_{22}$Mn$_{78}$/Co$_{20}$Fe$_{60}$B$_{20}$/MgO thin film is related to the interface between CoFeB and IrMn. Moreover, a possible application of anomalous Nernst effect that takes advantage of the perpendicular magnetization to obtain large voltage, owing to the in-plane geometry of the device has been proposed.
Quasicrystals exhibit long range order but an absence of translational invariance, and therefore, quasicrystals possess non-identical local environment. In our studies we fabricated artificial ferromagnetic quasicrystals (AFQs) made of 19 nm thick CoFeB film with nanoholes in an arrangement of Penrose tiling, and investigated collective spin excitations by means of broadband spin-wave spectroscopy in the few GHz frequency regime. We detected different sets of spin wave eigenmodes, which displayed a ten-fold rotational symmetry in angle dependent studies and reflected the quasicrystalline nature of lattices. Using micromagnetic simulations, we identified characteristics spin-wave motifs within the aperiodic AFQ lattice . The work was supported by SNSF via grant number 163016.
A parametric excitation is an effective way to excite short waved spin waves. We demonstrate an all electrical approach to measure the parametrically excited spin waves in yttrium iron garnet (YIG) films in that we exploited the frequency offset mode of vector network analyzer. The magnon spectra show a very narrow linewidth for the YIG LPE films close to the reported linewidth due to the intrinsic damping in YIG single crystals. In addition, the spectra show complex dependence on the source power. The results provides better understanding on the propagating parametrically excited spin waves. The research was funded by the EPFL COFUND project No. 665667 (EU Framework Programme for Research & Innovation (2014-2020)) and by the Deutsche Forschungsgemeinschaft (DU 1427/2-1).
Magnonic crystals are interesting for spin-wave based data processing. We investigate one-dimensional magnonic crystals (1D MCs) consisting of bistable CoFeB nanostripes separated by 75 nm wide air gaps. By adjusting the magnetic history, we program a single stripe of opposed magnetization in an otherwise saturated 1D MC. Its influence on propagating spin waves is studied via broadband microwave spectroscopy and phase-resolved Brillouin light scattering microscopy. Depending on an in-plane bias magnetic field, we observe spin wave phase shifts of up to π and field-controlled attenuation attributed to the reversed nanostripe. We thank for funding by SNSF via grant 163016.
Extending magnetic structures to the third dimension can lead to new properties such as magnetochi-rality effects and high data storage densities [1]. For the fabrication of three-dimensional magnetic nanostructures, suitable deposition methods need to be developed, as techniques such as sputtering lead to significant shadowing effects [2]. Here, we present the electroless deposition of NiFe on a 3D-printed, non-conductive architecture, where homogeneous layers covering the whole framework are achieved. This new technique represents a step towards the experimental realisation of 3D magnetic nanostructures.
[1] Fernández-Pacheco, A., et al., Nat. Comm., 2017.
[2] Donnelly, C., et al., Phys. Rev. Lett., 2015.
Anomalous magnetic properties in 3d transition metal nanoparticles are often observed, but are still poorly understood. Here, we combine X-ray photo-emission electron microscopy with high-angle annular dark-field scanning transmission electron microscopy in order to correlate magnetism and microstructure of individual nanoparticles. The data are compared to simulated STEM images and atomistic spin dynamic simulations. Besides single crystalline fcc cobalt nanoparticles, we consider nanoparticles containing structural defects such as stacking faults being observed experimentally and evaluate magnetic energy barriers and spontaneous magnetization axes. Experiment and simulation suggest that the magnetic properties of cobalt nanoparticles are determined by a complex competition of shape, surface, and structural contributions. In this contribution we will discuss simulated results with respect to the experimental data.
Spin waves (magnons) are collective spin excitations in magnetically ordered materials. If exhibiting sub-100 nm wavelength they could become the information carrier of future non charge-based information technology. Due to wavelength mismatch interfacing magnons with microwaves is however challenging. We investigate ferrimagnetic nanoparticles as possible magnon nanoemitters. For this we prepared a colloidal suspension of hexaferrite nanopowder with deionized water and deposited it via drop-casting on a yttrium iron garnet (YIG) film. Using integrated microwave antennas we investigate the coupling between nanoparticles (with a diameter of a few 10 nm) and magnons in YIG. Our spectra suggest an inhomogeneous broadening which is enhanced due to scattering at the deposited nanoparticles.
Ferromagnetic nanotubes are promising candidates for high density magnetic storage technology,due to the possibility to define flux-closure remnant states and control their magnetization reversal processes via geometrical parameters such as length,inner and outer diameter.They form interesting nanoelements also in the field of magnonics, allowing for tailored shape-induced spin wave confinement.We report an experimental study of spin-wave excitation in individual Ni80Fe20 nanotubes with lengths in the range of 5 to12μm and outer diameters of about 200nm by means of Brillouin light-scattering(BLS) spectroscopy.A series of spin wave resonances was detected.We explored them both spatially and phase-resolved at several positions along the nanotube.Our results provide microscopic insight into tubular nanocavities for magnons.
This work was funded by DFG GR1640/5-2 in SPP 1538.
Skyrmions are of potential interest for future memory and memristive devices, opening new possibilities for nanoscale magnetism. Neel skyrmions or hedge-hog skyrmions were reported in systems with perpendicular magnetic anisotropy. Here we report the presence of out-of-plane magnetization spin structures (probably Neel skyrmion) in a single layer of Ta/Co/Pt with in-plane anisotropy using X-ray photoelectron emission microscopy (XPEEM). The out-of-plane spin structures have lateral dimension from 300 nm to >1 um. The perpendicular magnetic anisotropy (PMA) of the sample is ~0.25 MJ/m3. Micromagnetic simulations confirm the presence of a Neel wall surrounding the out-of-plane spin structures. Further experiments are required to obtain a quantitative value of Dzyaloshinskii Moriya interaction (DMI).
Two-dimensional XY spin systems have interesting thermodynamic phase diagrams that so far have been mainly explored theoretically. Thermally-active artificial spin systems provide the opportunity to study the phase transitions and thermal behaviour of experimentally difficult to realize two-dimensional spin systems. Here, we experimentally investigate the artificial XY system of lithographically patterned dipolar-coupled nanomagnets arranged on a two-dimensional lattice. Due to their circular shape, the nanomagnets support a single-domain state that can be viewed as macrospins with continuous planar spin degrees of freedom. Employing resonant soft x-ray scattering we characterize the spatial correlations in these artificial XY spin systems and track the temperature-dependent evolution of the magnetic diffuse scattering.
Magnonic crystals (MCs), a metamaterial with artificially introduced periodicity, offer many possibilities to control spin wave propagation within them [1]. Spin waves propagating in opposite directions can have different amplitudes or frequencies, which is known as spin wave nonreciprocity [2]. The aim of this work is to enhance spin wave nonreciprocity in bi-component MCs based on antiferromagnetically coupled ferromagnetic layers [2]. Spin wave nonreciprocity will be measured using a time-resolved magneto-optical Kerr microscope. With first measurements of time-resolved magnetization precession in antiferromagnetically coupled films we observe a decrease of precession frequency, compared with normal ferromagnetic films.
References:
[1] M. Krawczyk et al., J. Phys. Condens. Matter 26. 12, 123202 (2014)
[2] K. Di et al., Sci. Rep. 5, 10153 (2015)
Magnetic skyrmions are topologically non-trivial spin textures, which hold big promise for their use in potential spintronic devices. We have performed real time and real space investigations on skyrmion lattices (SkL) in Cu2OSeO3 using Lorentz microscopy to study magnetic field induced melting of the SkL and its heat-induced rotation. We particularly highlight the methods we use for the analysis of these phenomena, i.e. the algorithms we have developed to routinely identify skyrmion positions in our data in order to study defects in the lattice, spatial and temporal correlation functions and local orientation maps.
We want to look for a spin-dependence in the charge transfer that occurs at the interface between an electrode and an electrolyte. We are using p-GaAs and GaN/AlGaN functionalized with chiral molecules as working electrodes in a methyl-viologen solution. To avoid magneto-hydrodynamic effects, we use electrically detected electron paramagnetic resonance (ED-EPR) to detect a spin effect without changing the magnetic field significantly. For various fields around the resonance condition, we measure the cell current while scanning the working electrode potential through both methyl viologen reduction peaks. We monitor sample-orientation dependent spin resonance for the potential modulated current of the cell when the working electrode is functionalized with chiral-molecules.
Current-induced spin-orbit torque switching of perpendicular magnetization requires the application of an external magnetic field collinear with the current to ensure deterministic reversal. Recently, a number of approaches have been proposed to control the magnetization in absence of the field, for example by employing antiferromagnets or introducing lateral structural asymmetries. Here, using scanning x-ray transmission microscopy, we demonstrate deterministic switching of magnetic nano-dots based on non-uniform distribution of the current. The experimental results are supported by micromagnetic simulations which indicate the origin of the field-free reversal in the interplay between the current inhomogeneity as well as the symmetry of the Dzyaloshinskii-Moriya interaction and spin-orbit torques.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
SwissFEL is PSI’s new FEL facility. It is a 740 m long accelerator which goal is to provide 6-30 fs long pulses of light with wavelength of 1-70 Å at 100 Hz [1, 2]. In order to support flexible beam rates, machine protection assistance and flexible event rates, a reliable and flexible event timing system is required, which is capable to provide event distribution at 142.8 MHz and flexible event sequences running at 100 Hz, In addition to the distribution of events, the timing system has to distribute a unique marker number for each pulse, to support beam synchronous data acquisition [3]. This poster presents solutions for event timing system that were developed in collaboration by PSI and Cosylab.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
The n2EDM experiment being mounted at the Paul Scherrer Institute (PSI) will search for the neutron electric dipole moment (nEDM) with at least an order of magnitude better sensitivity than its predecessor at PSI. With the increment in statistical sensitivity, controls of systematic effects must follow. This study targets to investigate the impact of Johnson-Nyquist noise originating from thermal agitations of electrons in the electric conducting materials in the apparatus. Two possible effects are investigated: systematic sensitivity reduction due to the presence of magnetic noise, and a systematic frequency shift, combined with an applied electric field, which could mimic an nEDM signal. Concepts and methods to calculate the magnetic noise and the magnitude of a potential shift are presented.
Understanding the precise rigidity dependence of the Silicon flux sheds light on the origin, acceleration and propagation of cosmic rays. The Alpha Magnetic Spectrometer (AMS-02) provides measurements of cosmic rays with high accuracy. Data analysis procedures, focusing on charge selection and tracking efficiency calculation, will be presented.
The Beryllium isotopic composition in cosmic rays provides essential information for the study of the propagation of cosmic rays in the Galaxy. The Alpha Magnetic Spectrometer (AMS) installed on the International Space Station (ISS) provides the opportunity to measure this composition in the energy range from ~1 GeV/n to ~10 GeV/n with unprecedented precision. For events selected with a specific nuclear charge, the velocity measured by the Ring Imaging Cherenkov (RICH) detector and the rigidity measured by the silicon tracker give a measurement of the particle mass. A method to extract the relative isotopic abundances from the mass distribution will be presented.
Measurements of decaying charge to decayed charge ratios in cosmic rays, such as Aluminum to Magnesium, allow to constrain the cosmic-ray residence time in the Galaxy, providing complementary information on cosmic-ray propagation with respect to what can be extracted from the Beryllium isotopic composition. With 115 billion cosmic-ray events collected by AMS-02 on the International Space Station, the precision measurement of the Aluminum to Magnesium nuclei ratio in cosmic rays has become feasible. Data analysis procedures, focusing on charge calibration and selection, will be presented.
The LHC will be upgraded to achieve higher instantaneous luminosity. The upgraded machine will be called HL-LHC with the next phase of data taking referred to as Phase II. A novel Monolithic Active Pixel Sensor (MAPS), dubbed MALTA, has been designed and is under investigation to assess its suitability for operation in the outer layers of the Phase II ATLAS pixel detector. A readout system has been developed to cope with the high speeds and asynchronous output of the chip. Test-beam campaigns exposing the prototype to high energy electrons and pions were also conducted for tracking efficiency measurements, radiation hardness and timing studies. An overview of the sensor technology and readout architecture along with preliminary test-beam results are presented.
The motivation for the use of Mott scattered positrons will be shown: By correctly tuning the positron beam, the scattered positrons have similar properties as the expected signal. Thus, they are suitable candidates to fake a signal event in the MEGII Spectrometer to fully characterise the detector response and to extract the positron’s probability density functions used for the search of a $\mu \rightarrow e \gamma$ event.
There are still minor deviations due to momentum spread of the initial beam or the details of the Mott scattering process. These deviations can be taken care of by the so called double turn algorithm discussed as well.
The n2EDM experiment, presently under construction at PSI, aims at searching for the neutron electric dipole moment $d_n$, with at least an order of magnitude higher sensitivity than previous efforts. The systematic uncertainties must be better controlled than in the predecessor experiment. Due to different motional magnetic fields seen by the neutrons and Hg atoms of the co-magnetometer, a false $d_n$ ($d^{false}_{Hg \rightarrow n}$) arises, which is one of the most challenging obstacles. This study focuses on developing a Cs-Magnetometer (CsM) array to measure the magnetic field in the experiment to high enough precision and accuracy, to control the associated systematic in the order of $\Delta d^{false}_{Hg \rightarrow n}<5\times10^{−28}~$e.cm. The requirements, features and limitations of the array are here presented.
The family of decays mediated by $b \to s \ell^+ \ell^-$ transitions ($\ell = \mu, e$) provides a rich laboratory to search for effects of physics beyond the Standard Model. In recent years, LHCb has found hints of deviations from theoretical predictions in branching fraction ratios (\textit{i.e.} $R_{K}$ and $R_{K^{*0}}$) and angular distributions of the muonic channel. More recently, analyses of $B^0 \to K^{*0} \mu^+\mu^-$ and $B^0 \to K^{*0} e^+e^-$ decays by Belle have for the first time investigated lepton flavour universality (LFU) in angular distributions, particularly in the observable $P^{\prime}_{5}$. In this work I will show the current status and prospects for the angular analysis of $B^0 \to K^{*0} e^+e^-$ decays at LHCb, and the sensitivity expected to LFU.
The early neutron electric dipole moment (EDM) experiments were performed using continuous beams. They were replaced in the 1980’s with ultracold neutron storage experiments as they provide better sensitivity due to longer observation times. A new approach, the BeamEDM experiment, is based on the early neutron EDM measurements, however, using a pulse beam. This allows to combine a time-of-flight measurement with the classic neutron Ramsey technique [F.M. Piegsa, Phys. Rev. C 88, 045502 (2013)]. This overcomes the previously limiting relativistic vxE-effect by measuring it directly. I will present preliminary results of a recent beam time during which we demonstrated a first time-of-flight measurement using the Ramsey technique.
Muonic hydrogen (mup) is a bound-state of a negative muon and a proton. Since a muon is 207 times heavier than an electron the energy levels of muonic hydrogen are very sensitive to the nuclear structure. By means of laser spectroscopy, we are aiming at the measurement of the ground-state hyperfine splitting (HFS) to extract the two-photon exchange contribution and the Zemach radius of the proton. This experiment is being conducted at Paul Scherrer Institute (PSI) and it requires designing a detector system capable of measuring the MeV-energy X-rays produced by the muonic atoms. In this talk we will introduce the principle of the HFS measurement and the simulations of the X-ray detection.
As a prequel for the novel pulsed neutron beam electric dipole moment (EDM) experiment, we have designed a smaller scale proof-of-principle experimental apparatus. The experiment represents a novel method to search for a CP-violating neutron EDM which is complementary to the current standard using ultracold neutrons. The setup consists of a set of aluminum cubes on which a neutron beam Ramsey setup and diverse neutron beam optics elements can be mounted. In particular, it features a 2D-CASCADE neutron detector capable of performing high-flux measurements with great time resolution; an actively stabilized homogeneous magnetic field region and, eventually, high-voltage electrodes inside a vacuum chamber. I will present the experimental setup as well as the progress achieved in the construction.
After having observed the first diffuse astrophysical neutrino flux, IceCube has entered a new fascinating phase as it recently saw the first evidence of a point source responsible for a neutrino emission.
It is now crucial to set in place analyses that are able to search for time correlation between flaring objects emitting gamma-rays such as AGNs and neutrino emission and that could be run almost online. Those searches can be either triggered by real observations of an increase in gamma-ray intensity reported by other experiments or can consist in looking for a clustering of neutrinos in time scanning the whole sky. We will present those analyses as well as a framework for monthly monitoring of interesting gamma-ray sources.
The TT-PET project aims at constructing a thin PET scanner with time-of-flight information for concurrent PET-NMR diagnostics. As part of the project a fully monolithic Si-Ge pixel sensor is currently being developed to meet the ambitious goal of a 30ps timing resolution for low energy photons. Lab characterization results of the analogue and digital front-end electronics for a prototype sensor will be shown. Further to this, the hardware and firmware that was developed to make a custom scalable data acquisition (DAQ) system will also be presented.
Currently the n2EDM experiment is under construction at PSI. Its purpose is to measure the neutron electric dipole moment (nEDM) with an order of magnitude better sensitivity than the predecessor experiment. As the experiment is sensitive to magnetic field changes it is important to be able to control the magnetic field in the magnetically challenging environment. A magnetic field mapping campaign of the relevant experimental area has been performed.
The results of this study as well as their influence on the active magnetic shielding system (SFC) planned for n2EDM will be presented.
The study reports on an alternative method to generate transverse Landau damping to suppress coherent instabilities in circular accelerators. The idea is to produce the incoherent tune spread through detuning with longitudinal rather than transverse action. This approach is motivated by the high-brightness, low transverse emittance beams in future colliders where detuning with transverse action is less effective. Detuning with longitudinal action can be introduced with a radio-frequency quadrupole, or using second order chromaticity. The method has been studied in simulations, theory, and experiments. Specifically, second order chromaticity was enhanced in the LHC at CERN, and experimental results on single-bunch stabilisation will be shown. The observations are interpreted from a Vlasov-theory point-of-view including benchmarks against numerical accelerator physics models.
LAr TPCs have been identified as one of the most promising technologies in neutrino physics, due to their excellent performance as high precision calorimeters and their ability to reconstruct in 3D the tracks of ionising particles. Future giant LAr TPCs, at the ten-kilotonne level, are now in the design and prototyping stage in the context of the Deep Underground Neutrino Experiment (DUNE). One of the possible technologies tested is the Dual-Phase LArTPC, which allows for amplification and readout of signals in gas. The first step towards a detector has been the commissioning and operation of the 3x1x1 m^3 prototype with 4.2 tonnes of liquid argon. This poster will give a summary of the performance and results achieved with the detector.
The High Energy cosmic-Radiation Detection (HERD) facility onboard China’s Space Station is planned for operation starting around 2025 for about 10 years. The application of the scintillating fiber tracker with silicon photomultiplier(SiPM) read-out as HERD tracker subsystem is proposed by the UniGE group. Different fiber tracker modules were developed and tested with the CERN SPS beam. An optical simulation of a single fiber mat has also been developed based on the Geant4 toolkit. It takes into account the characteristics of fibers, SiPMs, and electronics, in particular, light yield, SiPM’s photon detection efficiency, thermal noise, cross-talk and the clustering. The simulation was compared to the results of the prototype beamtest, and cross-checks in cluster charge and hit efficiency have been conducted.
Rare semileptonic $b\to s\ell^+\ell^-$ transitions provide some of the most promising framework to search for new physics effects. Recent analyses of these decays have indicated an anomalous behaviour in measurements of angular distributions of the decay $B^0\to K^*\mu^+\mu^-$ and lepton-flavour-universality observables. Unambiguously establishing if these deviations have a common nature is of paramount importance in order to understand the observed pattern. In this talk a novel approach using a simultaneous amplitude analysis of $\bar{B}^0 \to \bar{K}^{*0} \mu^+\mu^-$ and $\bar{B}^0 \to \bar{K}^{*0} e^+e^-$ decays is discussed and prospects for this measurement at LHCb are shown. If current hints of new physics are confirmed, this analysis could allow an early discovery of physics beyond the Standard Model with full LHCb Run-II datasets.
The physics community has been at the forefront of many aspects of open science. With the advent of big science, international collaborations have required tools to share information at scale. The world wide web, preprint servers (arXiv.org) and many other platforms (such as the Worldwide LHC Computing Grid and the CERN data portal) have been developed to allow the rapid and efficient knowledge management and dissemination. While biology and other disciplines only start to adopt best practices that many physicists take for granted, there are probably tricks that physicists could learn from chemists, geneticists or even psychologists. It is time for all disciplines to talk about open science and share their experience on how to make the scientific enterprise more open.
In the 1980s, femtosecond ultrafast lasers enabled optical generation of electric fields at terahertz frequencies. With the recent progress in few-optical-cycle femtosecond and attosecond pulse generation with full electric field control, the frequency regime can be extended into the petahertz. The electron motion under the influence of such a high-frequency electric field ultimately determines the material limit for high-speed device performance. We have started to explore materials in a regime where the quiver energy (or ponderomotive energy Up) of the electrons in such an oscillating electrical field becomes comparable to the photon energy of the driving laser. The system transitions from a more classical (field driven) regime to a more quantum-mechanical (photon driven) regime and we explored these regimes in diamond and GaAs. We also explored how long it takes for an excited electron in a metal to “feel” its effective mass and discovered new electron-localization effects in transition metals. The long-term goal is to explore such electric-field-driven dynamics in strongly correlated materials where we have even less physical understanding today.
Photonic quantum devices based on atomic vapours at room temperature combine the advantages of atomic vapours being intrinsically reproducible and highly nonlinear with scalability and integrability. We show the integration of photonic and electronic components into vapour cells and a first demonstration of an on-demand single-photon source based on four-wave mixing (FWM) and the Rydberg blockade effect. In the future integrated optical and electronic circuits in atomic vapour cells will enable applications in quantum sensing and quantum networks. As examples we discuss microwave detection and trace gas sensing.
Modern cosmology postulates dark matter and dark energy. They may be bypassed with generalised theories of gravity, inevitably introducing new scalar, vector, or tensor fields. The gauge fields of particle physics motivate bosonic vector fields. Promising candidates result in Proca-like theories, which can be generalised to scalar-vector-tensor theories. These will have rich cosmological and astrophysical applications.
In Einstein’s theory, non-metricity and torsion vanish, and gravity is attributed to curvature, while the teleparallel representation of general relativity has torsion only. A third equivalent representation ascribes gravity purely to non-metricity. In this approach, the connection vanishes, giving rise to a flat spacetime.
The Poster Session is held on Wed and Thu. All posters are to be presented on both days. However, due to technical reasons, the contributions are only listed in the timetable of Wed.
I will present examples of metals, which have topologically protected crossings in the phonon spectrum. I will show how the topological invariant of such crossings is defined and will show that the presence of such crossings puts the hosting metals among the best known thermoelectric metallic compounds.
Apart from that I will introduce the notion of non-abelian topological charge, defined in momentum space, and predicted previously in some phases of liquid He. I will show how this phase is realized in simple weakly-interacting crystalline compounds.
Weyl semimetal is a topological nontrivial phase of condensed matter which hosts 2-fold degenerate Weyl nodes formed by unavoidable crossings of linearly dispersive bands. The tilted Weyl cone, Lorentz invariance violation and unusual transport properties distinguish the type-II WSM from its type-I counterpart. By combining ARPES study and first-principles calculations, we present the observation of Weyl nodes in type-II WSM WP2. Different from other type-II WSM, the Weyl nodes with opposite chirality origin from 4-fold degenerate points, which ensure they are robust to external interferences, and which is likely the reason for the novel transport properties in WP2. Most important, our study shows the first observation of 4-fold degenerate points splitting.
Topological materials have attracted great interest in solid state physics due to their ability to support robust quantum states at their boundaries. Recently, it has been predicted that localized zero energy modes can be obtained at junctions of topologically dissimilar graphene nanoribbons (GNR). Within a surface-assisted bottom-up approach using rationally designed molecular precursors, we have experimentally realized such GNR junctions. By realizing well-defined periodic sequences of these topological electronic modes, one-dimensional electronic bands can be created, which are described by the Su-Schrieffer-Heeger (SSH) Hamiltonian. By controlled manipulation of intra- and inter-cell coupling strength we could create SSH analogs with different topological classes, as evidenced by presence, respectively absence of zero energy end states at the junctions between dissimilar GNR segments.
Many topological band structures can be understood as consequences of a quantized Berry phase along phase-space trajectories, which arises from charge polarization. The theory relating topology and polarization has been recently extended to higher-order multipolar moments. Although originally based on the concept of charge polarization, the same theory can also be used to characterize the Bloch bands of neutral bosonic systems such as photonic or phononic crystals. In this talk, we will present an experimental observation of a quadrupole topological insulator in a phononic platform, based on the implementation of a quadrupole tight-binding model in a microfabricated silicon structure. Our measurements confirm the theoretical predictions and provide a route towards protected wave guides in three dimensional materials.
Using super-structures one can effectively change the way that sound waves propagates in space. Topological band theory, known from the description of electrons in solids, provides us with a powerful design-principle for such acoustic metamaterials. Weyl points are robust conical band crossings in three dimensional materials that are monopoles of Berry curvature. When a magnetic field is applied, Weyl points are shifted in momentum space. As a result, in Weyl systems a dispersing chiral channel appear along the direction parallel to the field. The same effect can be realized by strain, which induces an axial gauge field. Here, we present the first observation of chiral channel induced by axial field in a purely classical acoustic metamaterial.
Linear Weyl nodes are classified in type-I and type-II. The Fermi surface, described by a $2^{nd}$ order algebraic surface, has a well define morphology for each type of Weyl point. When the $C_4$ and $C_6$ rotation symmetries forbid linear energy dispersion, as it happens in the composite Weyl nodes, terms with a quadratic or cubic dispersion must be included into the Hamiltonian. Consequently, their Fermi surfaces are given by $4^{th}$ or $6^{th}$ order algebraic surfaces, respectively. We can still classify the composite Weyl nodes in two different types (I and II). However, each type of crossing can exhibit Fermi surfaces with new morphologies. We present the different morphologies that we have found, and illustrate some of them in real materials.
The 6-dimensional Quantum Hall effect offers rich physics that generalize the concepts developed over decades for its 2-dimensional cousin. Using modern technological advances that allow for the study of systems with additional synthetic dimensions, higher-dimensional physics that was previously deemed to be of purely theoretical interest can now be accessed. In this talk, I will show how a 3D-topological pump can be used to probe the 6D Quantum Hall effect, the latter having a quantized bulk response that emerges in systems with six or more dimensions, and is related to a 6D-topological invariant: the 3rd Chern number. Such a 3D-pump could be realized, for example, by extending recent atomic experiments on 1D- and 2D-topological pumps to include 3D optical superlattice.
Iridium oxides with $d^5$ configuration have attracted considerable interest in the last decade due to the realisation of spin-orbit-coupled (SOC) $j_{eff}=1/2$ insulating ground states. Recently, a new class of 5$d^4$ iridates with a singlet ($j_{eff}=0$) ground states have been realised in (Ba/Sr)$_2$YIrO$_6$. Here, we propose a new honeycomb lattice compound Ba$_3$CaIr$_2$O$_9$ in the $j_{eff}=0$ class of materials. Using ab initio methods including many-body wavefunction calculations we characterise the SOC ground and excited states and show that a a $j_{eff}=0$ singlet ground state is realised. Further, we find that the material hosts non-trivial electronic band structure with a well defined $Z_2$ topological invariant. We analyse the effect of electronic correlations on the non-trivial bands using the Gutzwiller wavefunction approach.
Bond-dependent interactions between magnetic moments can lead to strong frustration and nontrivial ground states. In particular, the Kitaev-Heisenberg model has a rich phase diagram and can host a spin liquid state or different frozen states depending on the strength of the additional Heisenberg interactions. Experimentally such phase diagrams can be explored by modifying the relative interaction strengths in materials by applying pressure.
In this presentation I will describe how the muon spin rotation technique can be used to study such materials under applied pressure and what it can reveal about the transitions between different phases. I will then show examples of our recent high-pressure studies in Kitaev candidate materials $\alpha$-Na$_2$IrO$_3$, $\beta$-Li$_2$IrO$_3$ and $\alpha$-RuCl$_3$.
For the last three decades, quantum physics has promised a revolution in information processing - faster computers, better sensors and more secure communication. Today, those promises are becoming reality.
In this work, we experimentally explored various aspects of quantum information processing by encoding the (spin) information into individual ions confined in a Paul trap. Engineered laser fields were used to manipulate those qubits and to generate interactions among them. A large part of this work focuses on quantum simulation of interacting spins, e.g. investigating how entanglement propagates in such a system and exploring new kinds of phase transitions.
Many-body entanglement is an active field of research due to both its fundamental aspects and its potential applications in quantum information processing and metrology. We study correlations between spins in spatially separated regions (A and B) in a 87Rb Bose-Einstein condensate which violate an EPR steering inequality.
Such correlations allow one to predict the results of non-commuting variables in B from identical measurements in A with an inferred uncertainty product smaller than the Heisenberg relation in B. This un-intuitive feature not only provides a stringent test of quantum mechanics but it could also be used to measure spatially dependent quantities with increased sensitivity.
We use tunable cavities with nanoscale defects to create zero-dimensional exciton-polaritons. Data on the strong coupling as well as mode properties are shown as a function of cavity detuning. At ambient conditions, we observe polariton condensation with strong lateral confinement on the wavelength scale.
As a building block towards extended lattices, we realize two coupled cavities by focused ion beam milling and thermal scanning probe lithography. These photonic molecules are investigated by means of atomic force microscopy and optical measurements, to compare both fabrication methods.
These are initial steps towards a quantum simulator with polaritons in arbitrary potential landscapes at ambient conditions.
We demonstrate the experimental creation and detection of a single phonon Fock state at room-temperature using two-color pump-probe excitation and spectrally-resolved time-correlated photon counting [1]. Our scheme is inspired by recent proposals and experiments in cavity quantum optomechanics, but we anticipate that it will be applicable on a broad range of organic and inorganic materials, shedding new light on molecular dynamics in the quantum regime. Perspective on how to probe vibrational entanglement at room temperature will be presented.
[1] M. D. Anderson et al, arXiv:1802.04163 [quant-ph] – https://arxiv.org/abs/1802.04163 (2018)
Motional Sideband Asymmetry is a signature of the quantum regime of mechanical oscillators. It has been studied in several optomechanical systems as self-calibrated thermometry. We present sideband asymmetry measurement of a nano-optomechanical system sideband-cooled close to the ground state probing simultaneously with red- and blue-detuned tones. We show that this can exhibit an artificially modified sideband asymmetry due to Kerr-type nonlinear cavity response. The presence of the sideband cooling tone, creates oscillating intracavity field which leads to scattering between motional sidebands. We develop a theoretical model based on Floquet theory that describes our observations. This has wide-ranging implications for schemes utilizing several probing or pumping tones, as employed in backaction evasion measurements, dissipative squeezing and mechanical squeezing using reservoir engineering.
When measuring the position of a mechanical oscillator, quantum mechanics imposes a strict limit on the attainable precision: Any reduction of imprecision leads to increased quantum backaction of the measuring probe on the oscillator. This quantum limit can be circumvented, in principle allowing to indefinitely reduce imprecision, by monitoring only a single quadrature of the oscillator. Such backaction-evading measurement has been recently demonstrated in electromechanical oscillators coupled to microwave resonant circuits. Here we demonstrate this technique in the optical domain for the first time in a photonic crystal nanomechanical oscillator, cryogenically and optomechanically cooled to a few quanta.
The textbook magnetic domain wall is a very simple object: Its shape and width are described by a few material properties only. In nanostructures, however, the situation is different. By high-resolution imaging we observe complex spin arrangements that strongly deviate from those commonly encountered in magnetic films or bulk ferromagnets, for in-plane as well as perpendicularly magnetized systems. Wall profiles and even wall types can be tailored by geometry, such as wire width or constrictions. Dzyaloshinskii-Moriya exchange interaction is another parameter to modify a domain wall. We will discuss whether and how these extra handles can be exploited for spintronics applications, such as sensors or storage and memory concepts.
In this work, we study how oscillator synchronization can be used to optimize the domain wall (DW) dynamics in magnetic bilayers with perpendicular anisotropy. We show that dipolarly coupled DWs reach larger velocities as compared to isolated walls when the two chiral walls form a magnetic flux closing object. While this situation is easily maintained in a stationary motion regime, the observation of large velocities in the precessional regime imply that the DWs internal oscillations are synchronized, even when their natural frequencies are different. Using an additional in-plane magnetic field, the system can be tuned and synchronization is observed over a finite range of detuning. Using simulations, a rich variety of states is described, from perfect synchronization to complete separation.
The emerging interfacial Dzyaloshinskii-Moriya interaction (iDMI) can lead to fast current-driven domain wall motion and the formation of topological magnetic skyrmions, which is promising for the design of high performance spintronic devices. Here, a nanometric island of Pt/Co/AlOx (Al) was fabricated with patterned regions of varying out-of-plane and in-plane magnetic anisotropies. We observed that, due to iDMI, the magnetization orientation of adjacent out-of-plane and in-plane regions satisfy the chirality. This chiral coupling between out-of-plane and in-plane magnetization might offer a platform to design novel lateral coupled nanomagnetic system.
Recently, current-induced spin-orbit torques(SOT) switching have been demonstrated in ferromagnet(FM) / antiferromagnet(AFM) multilayers with perpendicular anisotropy [1,2]. The use of an AFM results in multiple nonvolatile (memristive) states, akin to neural networks [3]. Past experiments suggest that the non-uniform exchange bias from the AFM causes separate switching of domains, leading to this behavior [4]. This description differs from conventional SOT switching in FM/nonmagnet (NM) cases [5]. A deeper understanding of the switching in FM/AFM structures is thus relevant for SOT research and for AFM applications in spintronic devices. Here we use x-ray photoemission electron microscopy to image the FM domains of [Co/Ni]2/PtMn layers, at intermediate switching states with injected currents. We find that the Co/Ni domains are shaped irregularly and are of the order of 300 nm. Sequential current pulses gradually switch domains by nucleation & expansion, reproducibly; commensurate with electrical measurements. As a 1st visualization of field-free SOT switching, this opens new avenues to understand and optimize this behavior for FM/AFM spintronics devices.
(Work supported by CSTI-ImPACT, JST-OPERA & the Swiss NSF)
[1] S.Fukami et al., Nat.Mater. 15,535 (2016).
[2] A.van den Brink et al., Nat.Comm. 7,10854 (2016).
[3] W. A. Borders et. al., Appl.Phys.Express 10,013007 (2017).
[4] A. Kurenkov et al., Appl.Phys.Lett. 110,092410 (2017).
[5] M.Baumgartner et al., Nat.Nanotech. 12,980 (2017).
We investigated the correlation between structure, oxidation, and magnetic properties of Pt/Co/AlOx heterostructures deposited by magnetron sputtering. We find that the magnetic anisotropy and the spin-orbit torques are both affected by the degree of oxidation of the Co/Al interface. Moreover, we find that oxidation also influences the domain texture and the current-induced domain-wall motion.
Artificial spin ices are composed of geometrically frustrated arrangements of lithographically patterned single-domain nanomagnets. We have fabricated a spin ice based active material, a chiral ice, which converts energy into unidirectional dynamics, thus functioning like a ratchet [1] and demonstrating the potential of spin ices to build functional materials. Measurements combining photoemission electron microscopy with X-ray magnetic circular dichroism show that, following saturation, thermal relaxation proceeds through the rotation of the average magnetization in a unique sense. Micromagnetic simulations demonstrate that this emergent chiral behavior is driven by an asymmetric energy landscape. This opens the possibility of implementing a Brownian ratchet, with applications in nanomotors, actuators or memory cells. [1] Gliga, et al., Nat. Mater. 16, 1106 (2017)
Topological defects in kagome artificial spin ice (KASI)—arrays of interconnected nanomagnets on kagome lattice—appear in the form of monopole-antimonopole pairs and Dirac strings in the switching regime. Experimental determination of spin-precession frequencies of nanobars confined by topological defects has been elusive so far. However their understanding is key towards utilization of KASI as a 2D magnonic crystal. We, for the first time, devise an experimental protocol to systematically study the frequency response of nanobars (810 nm long, 120 nm wide, and 25 nm thick) confined by topological defects in KASI using the combination of magnetic force microscopy and Brillouin light spectroscopy.The work was supported by SNSF via grant number 163016.
The Nitrogen-Vacancy (NV) defect center is a stable, atomic-scale defect containing a single electron spin, which, when integrated into the tip of an atomic force microscope, enables high resolution, quantitative magnetic imaging. Here, we present NV magnetic field imaging and relaxometry to probe static magnetization and GHz spin dynamics in artificial kagome spin ice. Due to the NV center’s non-invasive nature we can probe nanoscale fluctuations even in the superparamagnetic regime. We present spatial magnetic field maps, allowing us to verify local ice rules. We further show relaxation images of spin dynamics, which indicate localized fluctuations in the 3 GHz spectral band.
Frustration denotes that not all interactions can simultaneously be satisfied. In many frustrated magnetic systems, dipolar interactions are of great importance. The dipolar interaction is often intrinsically frustrated due to its inherent anisotropy. Because of this anisotropy, an arrangement of in-plane dipoles on a square lattice is known to exhibit an order-by-disorder transition to a low temperature long-range ordered phase. Using parallel tempering Monte Carlo simulations and newly developed order parameters, we study how either dilution disorder or randomly-displacing sites affects this order. The resulting phase diagrams reveal many similarities between the two types of disorder and can be understood by magnetic flux closure.
The Compact Linear Collider (CLIC) is a mature option for a future electron-positron collider operating at centre-of-mass energies of up to 3 TeV. CLIC will be built and operated in a staged approach where we currently assume three centre-of-mass energy stages, at 380 GeV, 1.5 TeV and 3 TeV. This talk will summarize the status of the CLIC accelerator project, briefly describe the detector envisaged for CLIC, and focus on the high precision results in the domain of Higgs and top expected already at the first energy stage of 380 GeV. High-energy operation also gives access to the Higgs self-coupling, and enhances the potential for direct and indirect discovery of new physics.
Having completed the Standard Model but with several important questions still unanswered, the broadest search for leads towards new physics must be undertaken.
Starting from a very precise, high luminosity Z, W, Higgs and top e+e- factory, the Future Circular Colliders based on the CERN infrastructure of circular tunnels complemented with a 100 km ring passing under Lac Léman, will offer the possibility to continue with 100 TeV pp collisions, but also heavy ions or e-p collisions, and possibly 10-20 TeV µ+µ- collider. The physics highlights of this possible series of powerful physics instruments will be described, within the limits of today's imagination.
We present the status of the Canted-Cosine-Theta (CCT) design of an FCC-hh / HE-LHC dipole, its magnetic and mechanical properties, as well as its protectability. We will develop on the strengths and challenges of this design and lay out the R&D plan at Paul Scherrer Institute to address the challenges, together with first practical results.
The Canted-Cosine-Theta (CCT) type magnet has been proposed for Future Circular Collider (FCC) design. Its unique geometry lowers the coil stress intrinsically. Nevertheless, the former itself is also a barrier for heat to quickly propagate in case of a quench. To succeed in the magnet design and construction, further investigation is required on its electrothermal behavior. The potential detection & protection concepts are studied in both aspects of multiphysics simulations and experiments. The results will allow us to validate the conceptual design and feasibility of the construction of a fast and efficient quench protection system of CCT-type magnets for accelerators.
The Canted-Cosine-Theta (CCT) PSI magnet program aims at demonstrating that the CCT technology has the potential for the development of 16 T dipole magnets, required for the “near” future of circular colliders. The first step in this direction is the implementation of a Nb3Sn 1-m-long, 2-layer CCT single-aperture dipole model, referred to as Canted Dipole One (CD1) which is designed to achieve a peak field in the bore of ~11 T. The in-house manufacturing and assembly of CD1 requires to setup at PSI a number of fabrication steps. In this paper, the authors review the production process of Nb3Sn CCT model magnets at PSI. Thus, particular attention is paid on each manufacturing sub-step describing also the related commissioning phase.
Beam-beam effects are one of the major limitations in past and present hadron colliders. If not understood and well controlled they might result in important particle losses and transverse beam size blow up, with a direct impact on the accelerator performances and discovery potentials. We present in this work the studies of beam-beam effects for the Future Circular Hadron Colliders. We will describe the various aspects of beam-beam interactions (i.e. dynamic aperture, Landau damping, compensation schemes and operational set-up) and their implications to the machine performances are evaluated.
During their passages in the accelerator beam pipe, the charged particles induce electromagnetic wake fields that cause coherent oscillations of the beams. The excited coherent modes may lead to beam instabilities with a consequent limitation of the collider performances. The Landau damping, given by the diversification of oscillation frequencies of the particles in the beams, is a stabilizing mechanisms to mitigate such effects. Motivated by the observed LHC instabilities, we present studies of the Landau damping for the LHC and extrapolations for the Future Circular Collider (FCC).
This study aims to use Machine Learning techniques to build models of the evolution of proton beam losses in the Large Hadron Collider for different operational scenarios. The models are trained on LHC 2017 operational data using a Random Forest supervised learning algorithm. From the models and covariance calculations we extract the parameters most contributing to the beam intensity lifetimes.
In parallel, a similar method has been applied to simulated particle losses. The goal is to train a model on a dataset produced by simulations. The aim is to understand the losses dependency on machine settings to explain the experimental observations, the model will be used to predict the beam lifetimes for the FCC study avoiding computationally expensive simulation campaigns.
XENON1T is the world’s largest dual-phase xenon time-projection chamber, searching for dark matter via the nuclear recoils caused by interactions of weakly interacting massive particles (WIMPs) with xenon nucleons. Here a search for the inelastic WIMP-nucleus scattering is presented, whereby the interaction causes a transition to a low-lying excited nuclear state of either 129Xe or 131Xe. Unlike elastic scattering, inelastic scattering is inherently spin-dependent and so this channel of analysis can give a direct handle on the spin-dependent nature of the interaction.
DARk matter WImp search with liquid xenoN (DARWIN) will be an experiment for direct detection of dark matter using a multi-ton time projection chamber filled with xenon at its core. As a detector searching for rare events, a extremely good understanding of the all possible backgrounds is required, as well as a detailed simulation of all of them to quantify /estimate the expected background levels in the detector. Simulations will be also needed to determine the optimal dimensions of the detector. We discuss here the most important backgrounds for DARWIN and the first simulations of the detector geometry which are being conducted.
The XENON1T experiment searches for Weakly Interacting Massive Particle (WIMP) dark matter candidate with a dual-phase xenon Time Projection Chamber (TPC) located at Laboratori Nazionali del Gran Sasso, Italy, under 3600 m.w.e. overburden. The detector, in operation since summer 2016, employs 3.3 tonnes of liquid xenon, 2 of which are used as target mass. The newest result from 278.8 days of collected data in a inner fiducial volume of 1.3 tonnes (corresponding to 1 tonne-year exposure) will be presented. The focus will be on the experimental apparatus, and on the analysis that led XENON1T to being the currently most sensitive direct detection WIMP experiment.
One of the flagship projects of next-generation dark matter experiments will be DARWIN (DARk matter WImp search with liquid xenoN). With its 50 (40) tons of total (active) xenon target, DARWIN will be able to explore the entire experimentally accessible parameter space for WIMPs as dark matter candidates. Such a large detector, with its low-energy threshold and ultra low background level, will also be sensitive to other rare interactions. It will search for solar axions, axion-like particles, SuperWIMPs and for the neutrinoless double beta decay of $^{136}$Xe as well as measure different neutrino interaction processes. We discuss here the concept of DARWIN, the ongoing R&D, and the expected sensitivities to various physics channels.
The ArDM experiment is a tonne-scale liquid argon detector for direct WIMPs dark matter search which has been running in dual-phase mode since Dec. 2017. Out of its 1.5 tonne total liquid argon volume, 650 kg constitutes the active target of the dual-phase time projection chamber. The scintillation (S1) and ionization (S2) signals has been successfully detected. The detector response and performance have been studied and compared with single-phase operation results. The S1 and S2 signals have been categorized by the boosted-decision-tree-based method. The S1-S2 matching algorithm has been developed and validated by MC simulation. The event 3D position has been reconstructed from the PMT hit pattern and the electron drift time. In addition, the neutron multiple-scattering has been measured.
The First G-APD Cherenkov Telescope (FACT) is performing unbiased monitoring of nearby bright TeV Blazars. Despite being among the best-studied objects, their TeV emission mechanisms and extreme variability are far from understood. Exploiting the excellent temporal coverage of FACT data, we are performing unprecedented long-term periodicity, variability and correlation studies essential to unravel the underlying physics. In this talk, results of multiwavelength studies including data from X-ray satellites and other very-high-energy instruments will be presented, as well as observations of the exceptional outburst of 1ES1959+650 and various flaring activities of Mrk421 and Mrk501.
The First G-APD Cherenkov Telescope (FACT) monitors bright extragalactic gamma-ray sources.
Since summer 2012 it is operated from remote, and the need for human interaction has continuously been reduced to the point where FACT is now running fully robotic.
In case of any problem or scientifically interesting event, automatic phone calls are initiated. This results in hight duty cycle and fast reaction on gamma-ray flares, allowing to send automatic alerts to the community and other observatories.
In this presentation, we will discuss our implementation of the robotic system.
The SST-1M project, a 4 m-diameter Davies Cotton telescope with 9 degrees FoV and a 1296 pixels SiPM camera, is designed to meet the requirements of the next generation of ground based gamma-ray observatory CTA in the energy range above 3 TeV.
In this work, a special emphasis will be given to the commissioning results of the SST-1M camera but also to the latest performance validation tests such as charge resolution, trigger efficiency together with Monte-Carlo comparison. These results will allow to validate the camera prototype in laboratory for the second observation campaign with the telescope prototype foreseen this summer in Kraków.
The high energy cosmic radiation detection (HERD) facility is a space astronomy payload proposed to be installed onboard the future Chinese space station. The University of Geneva is working on the development of a tracking detector made of scintillating fibers read-out by arrays of silicon photomultipliers that could be placed on the four lateral sides of the detector. A mechanical and electronics design has been proposed, and various prototypes have been produced. Two different read-out electronics based on two ASICs (VATA and SIPHRA) are under test. The results of beam tests carried out at CERN, the predictions of a dedicated Monte Carlo simulation and the status of the ongoing space qualification process are presented in this contribution.
Functional processes transform between different material properties, occur on ultrafast timescale, and out of equilibrium. Such can be particularly complex in condensed matter, as the interactions depend on processes on multiple time and length scales. Free Electron Lasers (FELs) provide here new experimental insight with ultrafast X-ray probes for electronic and atomic structures. Science projects from the first years of FEL science show a development from measuring atomic structures to electronic transitions, from measuring kinematic time scales to dynamic processes. They reveal new opportunities and demand for new and specialized instrumentation. Important scientific and technical milestone examples, as well as the motivation and new instrumentation at the SwissFEL facility are discussed.
Titanium Pentaoxide nanocrystals exhibit two stable phases at ambient temperature, a more conductive $\lambda$ phase and a more insulating one ($\beta$) stabilized by pressure. The $\beta\rightarrow\lambda$ transition can be induced by short pulse irradiation. Following preliminary experiments at ID9 (ESRF) beamline, we have performed powder diffraction in Bernina to map the structural dynamics over the entire time course: from the first changes of the unit cells (picosecond time scale) to relaxation (tens of $\mu s$).
Different cases are presented on how ultrafast x-ray pulses can be used to study ultrafast dynamics in correlated electron systems. It is shown how an optical excitation can drive an electronic transition and its perspective to drive it by direct phonon excitation. In addition, it is shown how we can increase a structural order parameter with an electronic excitation in a simple perovskite and how we can upconvert phonons in SrTiO3 by driving its soft mode resonantly.
Identifying the degrees of freedom that lead to the emergence of superconductivity in iron-based materials remains the subject of active research. Amongst spin-driven scenarios, it has also been suggested that electron-electron correlations enhance the electron-phonon coupling in iron chalcogenides and related pnictides, but direct experimental verification has been lacking. We show that the electron-phonon coupling strength in FeSe can be quantified by combining two ultrafast experiments into a “coherent lock-in” measurement in the terahertz regime: x-ray diffraction tracks the light-induced coherent lattice motion, while photoemission monitors the coherent changes in the electronic band structure. Comparison with theory reveals an enhancement of the coupling strength owing to correlation effects, highlighting the importance of a cooperative interplay between electron-electron and electron-phonon interactions.
I will present the results obtained within my PhD. The main goal was to study the femtosecond photo-switching dynamic in bistable molecular materials, using optical and x-ray ultrafast spectroscopies. A part of this work focuses on the study of Prussian Blue Analogues (CoFe), which are prototype bistable metallic cubic crystals. Among the great interests of these systems are their bistable chromic and ferromagnetic properties from the Low-Spin (LS) to High-spin(HS) which can be driven by light excitation. The bistability is associated with a charge transfer between the two metal sites, and a spin-crossover of the Co site. Our studies, optical (at IPR) and XANES (at ESRF and LCLS) of the ultrafast dynamics, revealed a multistep electronic and structural process.
Tuneable THz radiation could be used to selectively excite low-frequency substrate-molecule modes relevant to the catalytic activity. Probing of such motion requires a technique with high structural sensitivity and temporal resolution, such as photoelectron diffraction. Metallic catalyst surfaces strongly modify the electric field of the pump pulse due to dielectric screening. Here we present a pump-probe study of the effective field on Pt(111) thin films exposed to THz radiation. The experiments were carried out at the FLASH (DESY, Hamburg). Photoelectrons emitted by an ultrashort (<80 fs) x-ray pulse were subject to streaking by the THz pulse. Recording all electron momentum components allowed for complete field reconstruction, where we could find distinctive differences between bulk metal and thin film surfaces.
Spin currents generated by ultrafast laser excitation of a magnetic thin film was predicted theoretically [1] and found in magneto-optical measurements using fs-pulsed VUV radiation [2]. Here we show fs time-resolved x-ray magnetic circular dichroism measurements performed at LCLS SXR on similar Ni/Ru/Fe multilayers that were used before [2]. Contrary than in [2], we find in our time- and element-resolved XMCD measurements that the amount of spins transported into the lower magnetic layer is almost negligible, which suggests that here the effectiveness of spin transport is strongly limited.
[1] M. Battiato et al., Phys. Rev. Lett. 105, 027203 (2010).
[2] D. Rudolf et al., Nat. Commun. 3:1037 (2012).
Enforcing the generalized Koopmans’ condition, we construct hybrid density functionals which give band gaps of solids as accurate as state-of-the-art GW calculations and are also capable to correctly describe polaronic distortions. Based on this nonempirical formulation, we address the energetics of native point defects and impurities in GaN. Our results show an amphoteric nature of Mg impurity. It behaves like an acceptor when substitutional to Ga and like a double donor when interstitial. This feature leads to Fermi level pinning and accounts for the observed drop-off of the hole concentration of GaN samples with increasing Mg doping. Importantly, these findings highlight a general effect associated to amphoteric defects which is not limited to the specific case of GaN.
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We investigate the stability of hole polarons at the rutile surface induced by electronegative adsorbates in the intermediate steps of the oxygen evolution reaction through hybrid density functional calculations. Applying the computational hydrogen electrode method, we find that hole polarons reduce the overpotential of the reaction determining step leading to good agreement with experiment. The stability of the polarons is confirmed at the hydrated surface through a free energy study involving the explicit solvent. The occurrence of surface hole polarons is unrelated to the scaling relationships and offers an additional handle in the search for improved catalysts.
High entropy alloys (HEAs) are random alloys with 5 or more components, of near-equi-composition. HEAs exhibit excellent mechanical properties, including high strength, ductility, and fracture toughness. Guiding the design of new HEAs across the composition space requires an ability to compute necessary underlying material parameters accurately. Here, we propose a methodology to compute, via density functional theory, the elemental misfit volumes and other alloy properties, in the fcc noble metal HEA RhIrNiPdPtCu. These properties are then used in a recently developed solute-strengthening model for yield strength, with the prediction 563 MPa in excellent agreement with the experimental value 527 MPa. This methodology links the alloy composition with the strength prediction, indicating a general methodology for exploring new potential high-strength HEAs.
We trained a deep neural network potential [1] based on a set of data generated by well-tempered metadynamics [2] simulations that use a classical potential.
We employed the SCAN exchange-correlation functional [3] and converged the calculations with respect to BZ sampling.
The resulting potential is used to study the nucleation and liquid state properties of silicon, on timescales and system sizes that are beyond the reach of ab-initio molecular dynamics.
[1] Zhang, E et al., P.R.L. (2018)
[2] Barducci, Bussi, and Parrinello, P.R.L. (2008)
[3] Sun, Ruzsinszky, and Perdew, P.R.L. (2015)
GaAs nanowires grown with the vapor-liquid-solid method exhibit complex behavior at the solid-liquid interface, with Wurtzite and Zincblende competing for stability on the As-terminated surface.
Atomistic models can help understanding this phenomenon, but the necessary length and size scales are beyond those accessible by ab initio molecular dynamics (AIMD).
We tested the combination of AIMD and machine learning potentials (MLP) based on multiple-time-stepping integrators, which accelerate AIMD more than 20 times, allows on-the-fly testing of the accuracy of the MLP, and also generates new training configurations to refine the potential.
The stability, surface configurations, and structural properties are investigated to be compared to experiments and previous studies on the same system.
CSEM is developing quantum metrology and sensing devices based on in-house MEMS atomic vapor cells with integrated functionalities [1]. Such cells were successfully integrated in <5mm flat miniature atomic clock physics packages with state-of-the-art performance [2]. In the context of NMR gyroscopes, parameters like relaxation times of noble atoms nuclear spins as a function of cell size and temperature have been characterized [3].
Here, we will present these projects in more details and show how CSEM’s miniaturization and integration competences could benefit various quantum device developments in Switzerland and Europe.
[1] https://doc.rero.ch/record/308907
[2] J. Haesler et al., Proceedings of the 6th international colloquium on scientific and fundamental aspects of GNSS/Galileo, Valencia (2017)
[3] https://doi.org/10.1063/1.5025449
Compact Rb vapor-cell atomic clocks are high-stability frequency references for applications ranging from telecommunication to satellite navigation systems. For achieving state-of-the-art clock operation, stringent requirements apply to the microwave resonator cavity used in the clock. Here we report on a compact microwave cavity produced by additive manufacturing of a polymer followed by metal coating. Using this cavity we demonstrate Rb clock operation in both the continuous-wave and pulsed Ramsey scheme [1]. The microwave field distribution inside the cavity is assessed experimentally using the Rb atomic vapor as a probe inside the cavity.
[1] Applied Physics Letters 112, 113502 (2018).
Sensitive spectroscopic methods for trace gas detection in the mid-infrared range often involve a modulated quantum cascade laser (QCL). Pure frequency modulation (FM) or amplitude modulation (AM) is preferable, while modulating the QCL current results in a combined AM-FM. Here, we demonstrate pure AM or FM realized with a QCL equipped with an integrated resistive heater (IH). By applying modulation signals with proper amplitude and phase to the QCL and IH currents, we show a reduction of the AM by more than 20 dB at two characteristic modulation frequencies of 1 kHz and 10 kHz, resulting in a pure FM, or vice-versa.
Recently, researchers at XFELs have demonstrated the ability to produce two intense femtosecond x-ray pulses with controlled time delay and color. Here we utilize these capabilities to perform X-ray pump/ X-ray probe photoelectron spectroscopy with high temporal resolution. This allows us to observe electronic and nuclear dynamics in core excited states and to follow how an initially local excitation “travels” across a molecule in real time. These experiments on a prototypical molecule, CO, lay the foundation for time-resolved photoelectron spectroscopy following femtoseond energy and charge transfer processes upon photoexcitation in more complex molecules.
This work was supported in part by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), Chemical Sciences, Geosciences, and Biosciences Division
Three-dimensional magnetic systems promise significant opportunities for applications as well as new functionalities. Two main challenges concerning their experimental investigation have been addressed, namely the fabrication of three-dimensional magnetic architectures, and the characterisation of three-dimensional magnetisation configurations. In particular, an artificial magnetic buckyball was fabricated, and a detailed characterisation of the structural and elemental properties performed. In addition, hard X-ray magnetic tomography was developed and the complex magnetic configuration within the bulk of a micropillar was determined. The nanofabrication and characterisation techniques developed in this thesis provide a basis for future investigations of a wide variety of three-dimensional magnetic systems.
Solar cells based on lead halide perovskites have recently emerged. Within few years their power-conversion efficiency (now 22%) has approached values of established photovoltaic technologies. In this contribution, light is shed on some peculiar properties of perovskites. These are their defect tolerance, which allows for high photovoltages and luminescence yields; Furthermore, the mixed electronic-ionic conductivity of this semiconductor, which leads to reversible degradation phenomena during operation. Performance-limiting charge recombination processes are identified and ways toward the theoretical efficiency limit discussed.
Thermodynamics provides a powerful description of energy flow in macroscopic systems. We study its validity upon reducing the number of particles comprising the system down to a single electron. We perform work on an electron residing in a quantum dot coupled to a reservoir and extract the heat it produces when equilibrating with the reservoir. Due to the small system size, the values for heat and work measured in single repetitions of the same experiment fluctuate strongly. These nonequilibrium fluctuations serve to measure an equilibrium quantity, the free energy, demonstrating that information is encoded in an ensemble of random fluctuations.
Hybrid perovskite solar cells have been capturing an enormous research interest in photovoltaics due to their extraordinary performances and ease of fabrication [1]. However, low device lifetime, mainly due to device degradation upon water exposure, challenges their near-future commercialization. Diverse technological approaches have been proposed, but still not sufficient, requesting a radically new solution. In this talk, I will show a new concept by using a different class of perovskites, arranging into a two-dimensional (2D) structure, i.e. resembling natural quantum wells. 2D perovskites show higher stability than the 3D counterparts. Engineering 2D/3D interfaces is the core innovation which I will present as a new way to boost device efficiency and stability [2,3].
[1] http://www.nrel.gov/ncpv/images/efficiency_chart.jpg.
[2] J.-P. Correa-Baena et al., Science, 358 (2017), 739–744.
[3] G. Grancini et al., Nat. Commun., 8 (2017), ncomms15684, Jun. 2017.
The interaction of the proton bunches with its surrounding in the LHC can lead to the production of large amounts of free electrons in the beam pipe. These so-called electron clouds are able to induce coherent beam instabilities by interaction with the proton bunches, effectively limiting the maximal intensity of the beam. We extend a semi-analytical code for the beam transverse oscillation modes by implementing a model based on linearised equation of motion for protons and electrons. The results are compared to a simplified model and to previous studies based on macroparticle simulations. They are found to be in qualitative agreement with the other estimates. The parametric dependance of the instabilities for the LHC at top energy is discussed.
During operation of the Large Hadron Collider (LHC) in 2017, unprecedentedly fast beam instabilities were observed. The instabilities are thought to have been the result of a complex sequence of events leading to a severe local vacuum degradation, resulting in the production of large amounts of free electrons and ions through beam-induced ionization. Simulation tools have been developed in order to numerically model the conditions in the beam chamber. In this contribution the development work will be reviewed and studies of the electron-ion dynamics and their effect on the proton beam will be presented.
At the Paul Scherer Institute, we are developing a novel positive muon beam at low energy with high brightness by compressing the 6-dimensional phase space of a standard surface muon beam.
Muons are stopped in a helium gas target with a density gradient at cryogenic temperature and compressed by making use of complex-shaped B- and E-fields. Compression stages that act along two different (transverse and longitudinal) directions have been developed and tested individually. As a next step we combine both compression stages into a single stage with mixed longitudinal-transverse compression. The feasibility of this mixed scheme has been successfully demonstrated in the 2017 beam test.
Muonium, which is a bound state between an antimuon and an electron, is a promising tool to measure the gravitational interaction of antimatter and a second generation particle. To obtain a high muonium formation rate, a superfluid-helium (SFHe) thin film will be used as a target. As the first step to build the high brightness muonium beam, this talk presents the first experiment of muonium formation in bulk SFHe at temperature down to 0.26 K performed in 2017. The results of the dependence of the muonium formation rate on temperature, magnetic and electric field are discussed.
The Thin-TOF PET (TT-PET) project aims at the construction of a small-animal PET scanner based on silicon monolithic pixel sensors in SiGe BiCMOS technology with 30 ps time-of-flight resolution. The scanner will also measure the photon depth of interaction with 200 $\mu$m precision. This performance will lead to a significant improvement in the image resolution and signal-to-noise ratio with respect to the existing PET scanners. In order to exploit the performance of the scanner, a new reconstruction method based on a maximum likelihood expectation maximization algorithm will be presented.
The first prototype of the monolithic ASIC was tested with a minimum-ionizing beam at CERN. Efficiencies larger than 99% were measured, together with an unprecedented time resolution of approximately 200 ps.
The IceCube detector has observed the first clear detection of a diffuse astrophysical high energy neutrino flux, and it is seeing evidence of the first point source, a flaring AGN. IceCube neutrino source searches involve looking for clustering of neutrinos or a strong correlation with known sources observed by other messengers which are also expected to emit a neutrino flux. The most recent updates will be presented in the search for sources of extraterrestrial neutrinos using an optimized data set of tracks with 10 years of data. Results on the evidence of correlation between a flaring source using time-dependent studies will also be shown.
T2K (Tokai to Kamioka) is a long-baseline neutrino oscillation experiment based in Japan. The off-axis muon neutrino beam with energy peaked at ~0.6 GeV is produced at the J-PARC facilities and directed towards the near detector at 280 meters and the far detector Super-Kamiokande at 295 kilometers. A precise knowledge of neutrino interactions is essential for an accurate estimation of the oscillation parameters. After a description of the effect of interaction uncertainties on the oscillation measurements, various cross-section measurements available at the T2K near detector will be described. The techniques are constantly under development and recent interesting results will be presented.
The GERDA experiment searches for neutrinoless double beta decay of Ge76. The observation of this decay would imply that lepton number is violated in nature. Among the candidate isotopes for neutrinoless double decay, Ge76 is appealing due to its high intrinsic purity and excellent energy resolution. Thanks to the liquid argon veto system along with background discrimination techniques, GERDA has achieved the lowest ever background index for the search of neutrinoless double decay. In early 2018 the projected sensitivity of GERDA surpassed 10^26 yr, which was followed by a data release in May. The latest results from this data release and status of the ongoing upgrade of the experiment will be presented in this talk.
The Liquid Argon Time Projection Chamber (LAr TPC) is currently the most attractive technology for neutrino oscillation studies. Both single phase and dual phase LAr TPCs are now in the design and prototyping phase in the context of the Deep Underground Neutrino Experiment. The dual phase operation allows to amplify the charge signal, offering several advantages over the single phase.
The first large scale dual phase LAr TPC with an active volume of 3x1x1 m^3 has been operated at CERN in 2017. This poster will give a detailed overview of the different reconstruction stages for dual phase LAr TPC data. Furthermore, results on the liquid argon purity, charge readout uniformity and charge-light matching for the 3x1x1 m^3 prototype are presented.
SHiP is a new general purpose fixed target facility. In its initial phase, the 400 GeV proton beam extracted from the SPS will be dumped on a heavy target with the aim of integrating $2\times10^{20}$ pot in 5 years. A dedicated detector, based on a long vacuum tank followed by a spectrometer and particle identification detectors, will allow probing a variety of models with light long-lived exotic particles and masses below $\mathcal{O}(10)$ GeV/$c^{2}$. The main focus will be the physics of the so-called Hidden Portals.
The ultracold neutron (UCN) source at PSI converts fast neutrons from a spallation target into UCN via thermalization in D$_{2}$O, subsequent moderation in solid ortho-deuterium (sD$_{2}$), and finally down-scattering on the sD$_{2}$ lattice. However, during normal operation a decline in UCN output is observed. A conditioning process allows to recover the original performance, and in many cases even increases the UCN output. This process consists of reducing the sD$_{2}$ vessel cooling power and heating its lid for a short period. Investigations to pinpoint the reasons for the decreasing UCN rate and the beneficial impact of the conditioning process, together with monitoring HD and para-deuterium concentrations to exclude UCN losses through these, will be presented.
Atomic parity violation experiments are one attempt to look for physics beyond the standard model. An experiment to measure the atomic parity violation electric dipole contribution to the energy transition 7S1/2 and 6D3/2 in singly ionised Radium-226 is currently ongoing. The extraction of the atomic parity violating signature for the measurement requires precise calculations based on quantities like the indeterminate radius of Radium-226. Muonic atom spectroscopy at PSI enables a precise nuclear charge radius determination. Previous muonic atom spectroscopy experiments at PSI were designed for targets containing at least several grams. Current safety regulations permit only an amount of a few μg of Radium-226. In this contribution, newly developed techniques and preparations for low amount targets will be presented.
The magnetic (Zemach) radius of the proton can be determined from the ground-state hyperfine splitting (HFS) of muonic hydrogen, an atom formed by a muon and a proton. At the Paul Scherrer Institute in Villigen, Switzerland, we aim to measure the HFS at the ppm level by means of laser spectroscopy.
Exciting this electric dipole forbidden transition, requires high laser fluence at an unusual wavelength (6.8 microns). In this talk we will present the development of a thin-disk laser system, insensitive to thermal lens effects and capable of delivering single frequency pulses at 1030 nm with hundreds of mJ. These pulses are successively converted to 6.8 micrometer via non-linear conversion stages to perform the spectroscopy experiment.
The soft x-ray line of SwissFEL, Athos, is currently under construction. This new FEL line will cover the photon energy range from 250 eV to 1900 eV. Athos will include several innovations allowing more complex modes of operation as the standard SASE FEL mode. Small magnetic chicanes between every undulator segments will allow to delay or shift electron bunches in order to better control FEL beam parameters. The novel “APPLE-X” undulator-magnet design will give full control on the X-ray polarization. The production of two colors pulses with adjustable delay will also be part of the operation modes. This variety of options will offer exciting new possibilities to the user community from 2021 on.
Resonant Inelastic X-ray Scattering (RIXS) is a powerful momentum-resolved probe of excitations involving lattice, charge, orbital and spin degrees of freedom for a diversity of materials. With the development of intense pulsed x-ray sources from free electron lasers, it is very attractive to extend this advanced spectroscopy into the time-domain.
After introducing the RIXS technique, we will discuss what new and appealing information time-resolved RIXS can extract about quantum materials. We will elaborate on the special x-ray pulse modes that could enable unique operation of time-resolved RIXS at SwissFEL. We will end this talk by presenting recent examples of time-resolved RIXS studies on correlated materials.
Structure determination is an important activity spanning across disciplines such biology, chemistry and physics. X-ray diffraction is among the powerful techniques used for structure refinements. The emergence of hard x-ray free electron laser facilities including the SwissFEL has open entirely new opportunities. This talk will discuss how synchronization for free electron laser pulses with spikes of extreme sample environment can be used to study matter under extreme conditions. Established results along with potential future experimental work will be presented. The possibility of implementing such experimental activity at the SwissFEL is another topic of this talk.
Extending the methodologies of nonlinear optics to the X-ray regime is a promising and exciting avenue in the light of the recent development of X-ray free electron lasers. For the first time, space, time, and energy resolved XUV-transient grating experiments on S3N4 membranes recorded around the Si L2,3-edge have been realized. The observed signal decays have been assigned to ultrafast charge carrier dynamics driven by Auger recombination and electron diffusion. The increase of the XUV energy above the absorption edge resulted in a shortening of the signal decay, which could be connected to an increase in the initial charge carrier density.
The hard X-ray branch of SwissFEL, Aramis, has successfully completed initial pilot experiments while developments continue of the soft X-ray branch, Athos.
The current status of the commissioned diagnostics measuring temporal, spectral and intensity properties of the FEL pulses are presented.
Plans for photon diagnostics for Athos and also describe. By presenting these details now, it is hoped the future soft X-ray user community will provide feedback and enter into a dialogue to help shape the diagnostics which will be available.
The presentation will describe our aim of fully reconstructing the temporal and spectral profile of complex pulse structures to enable new types of spectroscopy, for example nonlinear spectroscopies. These plans include a high resolution spectrometer and applying machine learning.
The Accelerator on a Chip International Program (ACHIP) is an international collaboration, funded by the Gordon and Betty Moore Foundation, with the goal to demonstrate that laser-driven accelerator on a chip can be integrated to fully build an accelerator based on dielectric structures. PSI will provide access to the high brightness electron beam of SwissFEL to test structures, approaches and methods towards achieving the final goal of the project. In this contribution, we will describe the two interaction chambers installed on SwissFEL to perform the proof-of-principle experiments. In particular, we will present the positioning system for the samples, the magnets needed to focus the beam to sub-micrometer dimensions and the diagnostics to measure beam properties at the interaction point.
Bus departure at the stop "Piccard, just outside the building entrance. For detailed information see the black board at the registration desk.
Only for participants who have registered in advance. On-site registration is not possible.
Fusion reactions hold enormous potential for clean, sustainable energy production from more equitably distributed resources, but a demonstration of technical and economic viability remains to be carried out. The ITER tokamak, now under construction in France, represents an essential step toward a practical technical demonstration of fusion energy. This talk will provide a perspective of where ITER fits in the roadmap to fusion energy, discuss what the present status of the fabrication and construction of the facility, and give an overview of the latest Research Plan, focusing on what questions will be answered through operation of ITER.
The International System of Units (SI) is the modern form of the metric system. It is today the authoritative basis for measurement all over the world.
The definitions of the SI base units require periodic revision in order to take scientific and technical developments into account. This is the only way in which the increasing demands on measurement accuracy can be satisfied.
Work on a fundamental revision of the SI is close to completion. This revision of the SI is expected to be concluded at the General Conference on Weights and Measures in November 2018. In the revised SI all units are defined in terms of a set of seven reference constants, to be known as the "defining constants of the SI", namely the caesium hyperfine splitting frequency, the speed of light in vacuum, the Planck constant, the elementary charge, the Boltzmann constant, the Avogadro constant, and the luminous efficacy of a specified monochromatic source. Starting from these constants, all the units making up the system, both base units as well as derived units, can be realized with the aid of physical laws. This also applies to the unit of mass, the kilogram, which is still defined today through a physical artefact, the international prototype kilogram in Paris.
The Weyl and Dirac semimetals are recently discovered topological quantum states of matter characterized by the unavoidable crossing of two non- or doubly-degenerate energy bands near the Fermi level, respectively. These crossing points (Weyl or Dirac nodes) are the source of exotic phenomena, including the realization of massless Dirac and Weyl fermions as quasiparticles in the bulk and the formation of Fermi arc states on the surfaces. I will show how the Weyl and Dirac semimetals are realized in systems with broken inversion and/or time-reversal symmetry. I shall also address the issue whether the band degeneracy can be retained when parity-time symmetry is broken, which is essential for the emergence of massless Dirac fermions as low-energy excitations in the system.
The theoretical prediction and experimental validation of Na3Bi and Cd3As2 Dirac semimetals, as well as the TaAs-class Weyl semimetals, stimulated considerable research efforts aiming at extending the family of topological semimetals. Several new classes of gapless topological phases such as the topological nodal-line, nodal-chain, and nodal-net systems as well as high-dimensional fermions quasiparticles have been predicted. In the meantime, a large number of candidate materials hosting these novel topological phases have been determined from first principles. In this talk, I will present a new Dirac semimetal MgTa2N3 predicted by the first-principle calculation. The electronic structure and its related topological properties will be discussed. Besides, I will also speak about topological phase transitions in topological semimetals induced by the symmetry breaking.
Cd3As2 is generally considered as archetype of the three-dimensional Dirac semimetal phase, the 3D analogue of graphene with linearly dispersing bulk state. Our reinvestigation of its electronic properties calls for a revision of this simplistic description. Cd3As2 exhibits, in fact, both a surface state and two bulk bands, dispersing across the Fermi level. Hence, these states must all contribute to the unique material electrical transport properties, but with very different effective masses and band velocities.
Hence, Cd3As2 displays a very different behavior in the bulk and at surface, and this has to be carefully considered in order to deepen our comprehension of its transport properties, in the perspective of developing novel devices.
Fundamental research and technological applications of topological insulators are hindered by the rarity of materials exhibiting a robust topologically non-trivial phase, especially in two dimensions. Here, by means of extensive first-principles calculations, we propose a novel quantum spin Hall insulator (QSHI) with a sizeable band gap of ∼0.5 eV that is a monolayer of jacutingaite, a naturally occurring layered mineral first discovered in 2008 in Brazil and recently synthesised. This system realises the paradigmatic Kane-Mele model for quantum spin Hall insulators (QSHIs) in a potentially exfoliable two-dimensional monolayer, with helical edge states that are robust and that can be manipulated exploiting a unique strong interplay between spin-orbit coupling, crystal-symmetry breaking and dielectric response.
Magnetoresistance of both topologically trivial and nontrivial materials was extensively studied during past few years. Different mechanisms were proposed to explain the magnetotransport properties, such as the extremely large non-saturating magnetoresistance observed in a number of materials, without arriving to definitive conclusions. By combining of ab initio calculations based on DFT with the Boltzmann transport theory we systematically investigate magnetoresistance in a large number of materials for magnetic field is applied perpendicular to the applied current. We show that the Fermi surface topology plays very important role in magnetoresistance, especially in explaining its anisotropy. We further focus on selected materials, e.g. copper and bismuth, finding very good agreement with experiment results.
We report a comprehensive study of the low-energy bandstructure of the nodal-line semimetals ZrSiX (X = Se,Te), combining angle-resolved photoemission spectroscopy (ARPES) and first-principle calculations. We discriminate between the existence of bulk and surface states, whose spin texture is revealed by the means of spin-resolved ARPES and confirmed by our calculations. Interestingly, a strong spin polarization of the metallic bulk states is predicted in the surface-projected electronic structure of the centrosymmetric ZrSiTe. This result is investigated experimentally and discussed in terms of the ‘local’ inversion asymmetry of the crystal, which enhances the spin-orbit interaction in the layered crystalline structure of this compound.
At an interface between a topological insulator (TI) and a conventional $s$-wave superconductor, the induced superconductivity in the TI surface state is expected to develop a complex $p$-wave order parameter which may allow to create Majorana Fermions inside vortex cores. These collective excitations are the basic element in a proposal for fault-tolerant quantum computing. Here we present experimental evidence for proximity induced superconductivity in a thin layer of the TI Bi$_2$Se$_3$ grown on top of Nb. From depth-resolved muon spin rotation measurements in the Meissner state, we observe a local enhancement of the magnetic field in Bi$_2$Se$_3$ that exceeds the externally applied field, thus supporting the existence of an intrinsic paramagnetic Meissner effect arising from an odd-frequency superconducting state.
The recent advances in Kerr lens mode locking of thin-disk laser oscillators allowed this laser technology to reach pulse durations below 50 fs at several watts of average power, which made it suitable for broadband THz generation. We demonstrate that using the direct output of the laser oscillator without any additional compression or amplification stage enables generation of broadband THz pulses covering spectrum up to 6 THz by optical rectification in gallium phosphide. This frequency range is of high interest for THz time-domain spectroscopy, which is a widespread tool for investigating the dynamics of complex molecular systems. We believe that thin-disk laser oscillators will become a compact power-scalable source of broadband THz radiation beneficial for the spectroscopy applications.
Most particle accelerators have been developed in laboratories for nuclear and high-energy physics. Here it will be shown which specific requirements apply for the application of accelerators and beam transport systems in radiation therapy. Aspects such as reliability and techniques for an accurate delivery of the dose at the correct position in the patient will be discussed. Also it is essential that operation of the machine can be performed by non-accelerator specialists in a hospital environment.
At the proton therapy facility in PSI, technology is being developed in parallel to a clinical program with patient treatments. An overview will be given on how this situation at PSI has been exploited to convert technology from laboratory into the clinic.
Superconductivity allows gantries to be much lighter and somewhat smaller. The use of superconducting magnets can also improve the treatment process. For our new SC gantry, we have designed specific beam optics with a very large energy acceptance. This allows a wide energy spectrum to be transported without changing the magnetic field. Different energies are needed to cover the tumor thickness. A large energy acceptance, combined with an ultra-fast energy selection system, would potentially lead to a decrease of the treatment time to a fraction of a minute. This opens a way to more efficient organ motion mitigation and possibly to new treatment indications.
Non-thermal plasmas are non-equilibrium ionized gases that can be applied at room temperature and thus be used to treat heat-sensitive biological samples like seeds and plants. When dosed adequately, plasma treatment is considered to be a timely, economical and environmentally friendly technique to improve germination and growth, increase disease resistance, decrease microbial contamination and reduce water consumption. In collaboration with UNIL, we investigate at the SPC the effects of plasmas on seeds and seedlings of a genetically modified Arabidopsis thaliana treated with atmospheric pressure plasma devices. I will present the strategy and first experiments to pinpoint the basic bio-physics mechanisms governing plasma-seed interactions.
As part of an ongoing divertor upgrade of the TCV tokamak it is planned to add gas baffles on the inner and outer wall of the vacuum vessel to form a divertor chamber of variable closure. The baffles promise to increase the compression of neutral particles in the divertor and, thereby extend the divertor research on TCV towards more reactor relevant, highly dissipative divertor regimes.
The thermal analysis of the baffles considers exposure to heat loads expected both during normal operation and off–normal events. The electromagnetic analysis considers Halo currents, which can occur during disruptions, as the worst case scenario.
The obtained results of the thermal, electromagnetic and structural analysis validate the proposed solution for the baffles.
The investigation of turbulence and transport in X-point magnetic geometries is crucial to understand heat and particle deposition on the divertor target plates, one of the main issues in future plasma devices. Basic plasma devices, such as TORPEX at the SPC, play a fundamental role in this endeavour. We present ongoing efforts to achieve localised plasma production in closed field lines and X-point configurations in TORPEX through an internal coil, which locally increases the magnetic field, thus allowing the absorption of externally injected microwaves in the electron cyclotron layer. First characterisation of turbulence in this configuration will also be presented.
Magnetic systems with competing interactions often adopt exotic ground states, which can be relevant to study new physics in quantum matter. The quantum spin ice ground state is expected in certain types of pyrochlore magnets, and theory predicts its low-energy physics to be a lattice analogue of quantum electrodynamics. In Pr$_2$Hf$_2$O$_7$, a ground state with indications of spin ice correlations forms below 0.5 K. We show that the experimental structure factor has pinch points $-$ a signature of a classical spin ice $-$ that are partially suppressed, as expected in the presence of quantum dynamics. Moreover, a continuum of magnetic excitations is observed in inelastic neutron scattering, which relates to the monopoles of spin ices that become quantum-coherent fractionalized excitations.
Different from the situation in conventional paramagnets, spins in the frustrated magnets can fluctuate or dance collectively, leaving a trace in reciprocal space that can be detected using neutron scattering. Here we present our neutron scattering results for two typical spinel compounds: MnSc2S4 and CdEr2X4, where the magnetic ions form the diamond and pyrochlore lattice, respectively. Our results evidence the existence of the spiral spin-liquid state in MnSc2S4, where the spins fluctuate as spirals, and the dipolar spin ice state in CdEr2X4, where the spin fluctuation gives rise to emergent magnetic monopoles.
Quasi one-dimensional (1D) spin ladders host exotic low-energy excitations. Recently, susceptibility measurements suggested that Co doping alters the magnetism in the spin ladder of Sr14Cu24O41. We studied the Co impurity effects of Sr14Cu24O41 using Resonant Inelastic X-Ray Scattering (RIXS) at Cu L3- and O K-edges. Cu L3 RIXS has been shown to be a powerful probe of magnetic excitations in low-dimensional cuprates, and O K RIXS is capable of resolving charge dynamics in the self-hole doped Sr14Cu24O41. We observed a clear hardening and sharpening of the two-triplon continuum in the Co doped Sr14Cu24O41 ladders connected a reduced hole density.
Superconductivity is achieved in T'-Nd2CuO4 by means of oxygen annealing which injects electrons via controlled defect engineering. Here, we use Resonant Inelastic X-Ray Scattering (RIXS) to study the evolution of magnetic and charge excitations in parent and superconducting compounds. Our RIXS study reveals a hardening of the spin waves and the enhancement of charge excitations in the superconducting phase. Remarkably, the hardening of the low energy excitations is strongly anisotropic. A spectral weight redistribution between the antinodal and nodal directions is observed which brings important information on the dynamics of the superconducting state. The evolution of the low energy dynamics is then discussed in comparison with other cuprates as well as Fe-based superconductors.
Superconducting fluctuations above the critical temperature provide valuable insight in the pairing mechanisms of superconductivity. The recent discovery of a pseudogap phase in NbN has opened many new questions. In this study we measure the paraconductivity and the Hall effect response in NbN originating from superconducting fluctuations. These experimental results are compared to a recent theoretical study on the influence of superconducting fluctuations on the Hall response. The presented study demonstrates experimentally how Gaussian fluctuations of superconductivity contribute to the conductivity tensor.
Unconventional superconductivity often emerges from an insulating/bad metallic parent compound phase, such as anti-ferromagnetic, charge density wave or spin density wave. However, the existence and influence of the parent compound correlation on superconductor state are poorly understood. Here we investigate this matter in the bismuthate high-temperature superconductor using angle-resolved photoemission, Raman spectroscopy, and transport measurements. The results indicate that superconductivity is emerging from a unique normal state, where distinct pseudogap signatures are observed. We argue that this is a consequence of crossover from a polaronic liquid state to mixed phase state. In the mixed state, coexistence is observed between the parent compound phase correlations together with a metallic state.
The structural coherence across the ferroelectric transition in improper ferroelectric YMnO3 and other related multiferroic hexagonal manganites is not well understood. Here we reveal the evolution of the local structure in YMnO3 using neutron total scattering and first-principles calculations. We show that, at room temperature, the local and average structures are consistent with the established ferroelectric ground state structure. On heating from~ 800 K and up to above TC, both local and average structural analyses show striking anomalies consistent with increasing fluctuations of the order parameter angle. The fluctuations give an unusual local symmetry lowering into a continuum of structures which coincides well with reported temperatures for which the observable polarization vanishes, persisting into the high-symmetry non-polar phase.
In their textbook „Monte Carlo simulations in statistical physics“ (5th edition 2010), physicists K. Binder and D. W. Heermann claim that computer simulation has emerged as a third branch of physics. This is supposed to question the established dichotomy between experimental and theoretical physics. The aim of this talk to discuss this popular view and to better understand the impact that the method of computer simulation has on physics. I begin with some historical and philosophical reflections about the dichotomy between experimental vs. theoretical and then turn to the epistemology of computer simulations. I argue that simulations do not qualify as experiments, but that they are crucial for the interpretation of some experiments. I then clarify the role of simulations in theoretical research. My conclusion is twofold: On the one hand, computer simulation is a method of its own that requires specific skills and communities. But, on the other hand, it does not form a third pillar that has the same epistemological significance for the aims of physical research as theory and experiment. Throughout my talk, I use examples of computer simulations from particle physics and cosmology.
Today, atomic clocks are among the most emblematic instruments for precision measurement. In the pursuit of the next decimal after the comma, all kinds of cutting edge quantum techniques are mobilized, such as entanglement, atom trapping or laser cooling. Only 70 years ago, improvement of time measurement was predominantly thought of in terms of new astronomical instruments, the perfection of star catalogues or the formulae of celestial mechanics.
In this talk I want to present how atomic physicists gradually arrived in timekeeping during the decades after World War II. What were their motives to get involved with atomic clock research? How was the new technique received by the traditional “Keepers of time” in national observatories? What was the role of atomic clocks in the more general evolution of time and frequency metrology? My goal is to show that the answers to these questions are more complex than a simple, linear story of increasing precision would suggest.
I will focus on two examples from France and Switzerland: the former was seat of the World Time Bureau at the Paris Observatory during the period in question and mobilized renowned physicists like Alfred Kastler (Nobel laureate of 1966) to work on atomic clocks. Despite this accumulated prestige, it was not the French Republic but the small Republic and Canton of Neuchâtel that managed to build and operate the first atomic clocks on the European mainland in the late 1950s.
A la fin du 18e siècle, le physicien et astronome genevois Marc-Auguste Pictet (1752-1825), ancien assistant et élève du fameux naturaliste Horace-Bénédict de Saussure (1740-1799), débute à Genève des cours de physique expérimentale qui rencontrent très vite un grand succès. Destinés à un large public (aisé) aussi bien féminin que masculin, ces cours, divisés en une trentaine de séances parcourant tous les domaines en vogue de la physique de l’époque, consacrent une large place aux démonstrations effectuées avec des instruments (plus de 500 !) issus en majorité du cabinet de Pictet.
Dans un souci pédagogique, Pictet accompagne ses cours d’un Syllabus, premier ouvrage du genre publié à Genève, véritable manuel de travaux pratiques qui fait correspondre plus de 500 instruments scientifiques avec les phénomènes physiques qu’ils sont censés démontrer, dans la meilleure tradition des livres de physique du 18e siècle.
Pédagogue sur le tard, mais aussi physicien de renom, Pictet a acquis tout au long de sa carrière des instruments fort variés pour mener à bien ses recherches dans les domaines de la chaleur, la géodésie ou encore la météorologie.
Les instruments de physique du cabinet Pictet sont une des collections fondatrices du premier Musée académique de Genève créée en 1818. Une centaine d’entre eux existent encore et sont conservés au Musée d’histoire des sciences de Genève. Un grand travail d’analyse est en cours sur cette collection pour établir un catalogue raisonné, et pour tenter de restaurer certains d’entre ou encore pour procéder à des réplications.
The greek astronomer Hipparchos of Nikaia (194-120) BC is said to have compiled a catalogue with some hundreds of stars, although this catalogue has not survived history. Nevertheless this caused the European Space Agency ESA to call a catalogue of 120 000 stars High Precision Parallax Collecting Satellite, i.e. Hipparcos, measured by satellites in the 1990s with an accuracy of 1 milli-arcsec. We show how the catalogue data are applied to track stars in astronomy, mention the Farnese celestial globe with its star configuration, which is considered by some experts as proof for the lost Hipparchos catalogue, and finally show how we can use the star data to calibrate optical telescopes for space applications.
Taking opportunity of this year’s celebration of the 250th anniversary of Joseph Fourier, I analyse his approach to the study of heat exposed in his celebrated treatise Théorie analytique de la chaleur (1822). In a way of counterpoint, I take as comparison the renowned memoire of Sadi Carnot, Réflexions sur la puissance motrice du feu published almost simultaneously in 1824. Both bring a major innovation in the study of heat concentrating on its effects rather than on its ultimate nature. But their aims are radically different in what concerns the heat phenomena considered. Both treatises will deeply change the physical sciences: Fourier’s will become a model for all subsequent work in mathematical physics and a major source of mathematical innovation while Carnot’s will pave the way to the foundation of Thermodynamics.
CMS is a multi-purpose detector constructed to study high-energy particle collisions of the LHC at CERN. An all-silicon pixel tracker system provides CMS with excellent resolution for charged tracks and efficient tagging of b-jets. At the beginning of 2017, a new pixel detector has been installed anticipating the increase of instantaneous luminosity of the LHC to up to $2\times10^{34}\textrm{cm}^{-2}\textrm{s}^{-1}$, well surpassing the rate capabilities of the previous detector. The new pixel detector features four central barrel layers and three end-cap disks in forward/backward directions for robust tracking and tagging performance. This contribution gives an overview of the design of the CMS Phase-1 pixel detector, with a special focus on the tracker performance during 2017 data taking.
For operation at the High Luminosity LHC, the ATLAS detector will be upgraded in 2024-2026. Its Inner Tracker will be able to handle pile-up conditions of $\left<\mu\right>=200$ at a trigger rate of 1 MHz. This increases the digital data output to up to 5.12 Gbps per pixel module. The optical to electrical conversion stage, the optoboard, needs to be upgraded in order to cope with this bandwidth requirement.
The versatile transceiver (VTRx) is the main component of the optoboard. The talk will present a first integration test of the VTRx4, coupling it to the FELIX readout and measuring the eye diagram and the bit error rate. Furthermore, the design of the optoboard is discussed, addressing questions of modularity.
The multi-dimensional energy correction for jets arising from bottom-quarks is presented. The study is performed on a simulated dataset of jets produced in 13 TeV proton-proton collisions. The energy correction is computed through a regression based on a deep neural network. The b-jet regression is trained on jet properties and jet composition information. The b-jet energy correction and jet resolution estimator are output simultaneously by the neural network, providing information that can be used to improve the sensitivity of several CMS analyses with b-jets in the final state.
At the High Luminosity LHC, the ATLAS trigger and data acquisition (TDAQ) system necessary to select in real-time interesting data will have to deal with severe reconstruction challenges due to due to the unprecedented rates of particle collisions. The Hardware-based Tracking for the Trigger (HTT) is a critical element to deal with these data-taking conditions. It implements track reconstruction using fast and highly-parallel Associative Memories ASICs and FPGAs, but many are the challenges that we are facing in order to meet the demanding requirements.
In this work we present the HTT basic principles for track reconstruction and track fitting, the HTT design and its data-flow. We also demonstrate the challenges to implement such system.
In 2017 the first evidence in CMS data of the Higgs boson (originally discovered in 2012 by ATLAS and CMS collaborations) decaying into a pair of b-bbar quarks has been observed with a significance of 3.3 sigma for associated Higgs production with a vector boson. This analysis is dominated by a large background of V+Jets events and therefore multivariate methods are used to classify signal and background. A better discrimination of signal and background will increase the significance and can contribute to raise it over the threshold needed for an observation. Therefore, new methods, e.g. deep neural networks are explored and their performance in this analysis will be presented together with a general overview of the analysis strategy and results.
The two photon decay channel is among the most sensitive for Higgs physics at the LHC. In particular, it provides the best precision for the measurement of many Higgs boson properties. The ability to interpret LHC data in this channel relies on accurately modeling the detector response to prompt photons. Fine-tuning the particle detector simulation through the exploitation of clean standard candles, such as the dielectron decay of the Z boson, is essential for this goal. We present a method that allows correcting the simulated CMS detector response to isolated prompt photons, differentially in the photon’s kinematics and detector occupancy. The method employs machine learning algorithms and dedicated likelihood models to tune the reconstructed cluster shape, and associated isolation sums.
The Minimal Supersymmetric Standard Model (MSSM) is today one of the most credited theories for physics Beyond the Standard Model (BSM). However, despite its success in providing a solution to many cosmological and High Energy Physics (HEP) observations, MSSM particles are still elusive today at the Large Hadron Collider (LHC). In this context, the zero lepton SUSY Multijets analysis has a remarkable sensitivity to supersymmetric particles due to the large jet multiplicity and the low Missing Transverse Momentum (MET) characterising its signature. This talk will focus on the possible reconstruction improvements and model extensions of the ATLAS Multijets analysis in view of the results that will be released after the end of the full Run II LHC data taking.
Hadronic jets coming from the fragmentation of b-quarks are crucial tools for a number of physics channels at the CERN LHC, ranging from the Higgs physics to searches for physics beyond the Standard Model. We present a technique that allows tuning the simulated response of the CMS detector at the LHC to b-jets. Machine learning algorithms and likelihood fits are used to obtain finely-grained correction factors, based on samples of b-jets from ttbar decays. Specifically, we employ multivariate classifiers and non-linear multi-dimensional quantile regression models to tune the detector response to b-jets with different properties and compositions.
We will report on the first pilot experiments at the Alvra experimental station of SwissFEL employing X-ray spectroscopy methods in the tender X-ray regime. The experiments allowed testing the main instrumental components of the beamline with application to pump-probe X-ray emission spectroscopy on applied (charge transfer in OLED and TiO2 systems) and fundamental research (UV damage of DNA). We will discuss the achieved beamline performance and design parameters together with near-future goals and description of the foreseen beamline capabilities. Finally we will describe a number of experiments in the field of nonlinear X-ray spectroscopy and time-resolved studies to be executed when SwissFEL will achieve its optimal operation parameters.
With the advent of X-ray Free Electron Lasers, it is now possible to combine novel experimental strategies, based on ultrafast element-selective core-level spectroscopies and scattering techniques. This unique capability can be harnessed to study the interplay of electronic and structural degrees of freedom in several photo-excited systems. Here, I will provide examples how this approach can be applied to the study of spin and conformational changes in Myoglobin. I will also provide a perspective on future projects on metal oxides at SwissFEL and other XFEL facilities.
Myoglobin(Mb) is a small protein consisting of a single polypeptide chain and a heme as its active center. The photodissociation of the NO ligand from low spin MbNO leads its recombination to the high spin, domed and unligated porphyrin, which occurs over multiple timescales (from sub-ps to 100s ps) through an intermediate state that is presumed to be a high spin domed ligated form. We will present combined ultrafast X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS) and X-ray diffuse scattering (XDS) results obtained at the European XFEL and SACLA and correlate the spin to the conformational changes.
Free-Electron Lasers (FELs) are capable of producing intense and ultrashort X-ray pulses, which enable femtosecond time-resolved diffractive imaging experiments. This allows to initiate chemical reactions in molecules using an optical pump pulse and probing the induced changes in the nuclear structure by X-ray scattering using a delayed FEL pulse. Here, results on the strong-field induced dynamics of C$_{60}$ molecules probed by X-ray scattering will be presented. Furthermore, transient absorption experiments on the UV-induced dynamics of small carbon containing molecules, probed with a broadband soft X-ray pulse from a High Harmonics source, will be shown. An outlook on experiments with the new Athos AMO endstation at SwissFEL will be given.
The novel x-ray free-electron laser sources, including the upcoming SwissFEL, offer new opportunities for the investigation of ultrafast phenomena in the chemical, materials, and biological sciences. The high number of photons (10^12) per pulse in combination with femtosecond pulse length allows new approaches in x-ray based imaging and spectroscopy, including single-shot imaging of single nanometer-sized particles. New approaches with multiple scatterers in the focuse enable the recording of holographic information in the gas phase. Combining optical with x-ray lasers enables time-resolved experiments. Calculations show that ideal imaging conditions are only reached for pulses shorter than the Auger lifetime which can be uniquely produced at SwissFEL.
This work is performed with multiple collaborators worldwide.
This documentary on fusion energy research uses the form of interviews of physicists and engineers, either from the large international project ITER at the halfway pint of its construction in the south of France and involving many partner countries, or other smaller projects mainly in North America. Directed by two Canadian filmmakers, Mila Aungh-Thwin and Van Royko, it follows researchers in their daily lives. Thus, not only does it show us large experimental facilities while recalling the important milestones in the history of fusion, but it also leads us to the heart of what motivates researchers to dedicate their lives to see the realization of fusion energy. There is a good balance between the scientific and the social aspects in this film, which was one of the top 10 Canadian films in 2017. This film represents a solid introduction to the subject and a useful update on the state of the art for those already familiar with the subject, but it is primarily intended for a large non-scientific audience.