Quantum dots are confined semiconductor nanostructures. The confinement leads to a discrete energy spectrum, similar to the spectrum of atoms. We use the discrete energy spectrum to create single photons and cascades of photons. Quantum dots can be epitaxially grown and thus, they are an on chip photon source. They can be embedded into p-i-n junctions and can be pumped electrically, although I...
We theoretically investigate different aspects of dynamic mesoscopic conductors with the ultimate goal of contributing to the development of quantum technologies. Key results include: demonstrating that a single-electron partitioned at a beam-splitter is entangled and therefore possibly useful for quantum computation, developing a theory of joint electron waiting times and using it to describe...
The bit of a magnetic storage device is composed of hundreds of thousands of atoms. Although this number was continuously reduced with advancing miniaturization, the physical limit of one single atom per bit has remained a mere though experiment. Here we show that a single Holmium atom on magnesium oxide represents this fundamental limit. We read the single atom magnet via tunnel-magneto...
We present novel model systems for catalysis that have been developed using state-of-the-art top-down nanofabrication techniques, such as extreme ultraviolet lithography and electron beam lithography, to achieve nanometer precision over particle size and its positioning. This is combined with X-ray photoemission electron microscope at the Swiss Light Source to study catalytic metal...
Jumping from the expanse of galactic scales to land in the laboratory might seem a gargantuan task. Common to both, however, is the concept of symmetry breaking and in particular the formation of topological defects. This work looks at the formation of topological defects in multiferroic hexagonal manganites whose ferroelectric behavior enables the direct imaging of these defects. I also show...
Recently, topological classification of quantum phases has been extended from non-interacting insulators to strongly correlated insulators, and further to semimetals. In this talk, I will introduce our recent results on direct visualizations of new topological quantum states with angle-resolved photoemission spectroscopy at SLS, including:
• Direct observations of the metallic surface state...
The transition metal dichalcogenide 1$T$-TiSe$_2$ is a quasi-two-dimensional layered material that undergoes a commensurate 2 $\times$ 2 $\times$ 2 charge density wave (CDW) transition with a weak periodic lattice distortion (PLD) below $T_{\text{CDW}}$ $\approx$ 200 K. Scanning tunneling microscopy (STM) combined with intentionally introduced interstitial Ti atoms allow, for the first time,...
The transition-metal dichalcogenide TiSe$_2$ is a layered material with a charge density wave (CDW) transition temperature around 200 K. Its origin is still a matter of debate. Using angle-resolved photoemission spectroscopy (ARPES) and scanning tunneling microscopy and spectroscopy (STM/STS), the influence of sulfur concentration in TiSe$_{2-x}$S$_x$ is studied. While TiS$_2$ does not show a...
We measured and analyzed the optical response of 1T-TiSe2 at temperatures above the CDW transition. Separate responses of electron and hole subsystems are identified and followed in temperature. We show that neither semiconductor nor semimetal pictures apply in their generic forms, as the carrier scattering is very strong, with the related energy scale $\hbar/\tau$ being of the...
It is widely believed that the integer quantum Hall effect (IQHE) is dominated by single-particle interaction. In a recent paper we show that quite oppositely the IQHE regime behaves similarly to a non-interacting single particle system — not because of the absence, but rather due to the dominance of many-body effects [1]. We utilize a fully self-consistent Hartree-Fock implementation and a...
Recently, the quantum anomalous Hall effect has been observed in Cr and V doped (Sb,Bi)$_2$Te$_3$ [1]. Its eventual technological applications require a homogeneous magnetization without any contribution from dopant bands to the conduction of the system. We combine low energy muon spin rotation and soft-X-ray angle-resolved photoelectron spectroscopy (SX-ARPES) to study the magnetic and...
Due to the absence of a seizable magnetization in antiferromagnetic spintronics their direct control is difficult and alternative means as, e.g., coupling to a ferromagnet via exchange bias [1], are needed. Antiferromagnetic Co:ZnO was presented as a model system in which uncompensated spins lead to a vertical exchange bias like shift [2] connected to the Co-doping level [3]. We studied...
The method of Roman-Perez and Soler [Phys. Rev. B 103, 096102 (2009)], which allows for a fast implementation of the nonlocal van der Waals (vdW) functionals, has contributed in making the vdW functionals popular. However, the Roman-Perez-Soler method relies on a plane-wave expansion of the electron density, therefore it can not be applied to all-electron densities for which an unaffordable...
This work shows that it is possible to detect phase transitions in condensed matter systems using a novel approach based on machine learning. A neural network is trained on purposefully (mis)labeled data, after which the transition point can be identified from the network's performance. This technique is capable of identifying thermodynamic and topological transitions as well as other...
The Ba$_3$MIr$_2$O$_9$ family offers a tremendous playground to study the influence of spin orbit coupling (SOC) in the stabilization of a spin liquid ground state. We present here the zero field $\mu$SR study of the $J=1/2$ compounds (M=Y,Sc,In) synthesized in the 6H lattice.
For the Y and Sc ones we found an homogeneous ordered ground state, with transitions at 4.5 K and 10 K respectively,...
Due to the unique topology, defects in one-dimensional systems have a profound effect on the physics of quantum systems. In this contribution we will present an extensive experimental inquiry into the effect of disorder in Heisenberg antiferromagnetic S = 1/2 chain. By combining muon and neutron experiments, we have found drastic changes and suppression of magnetism in both static and dynamic...
We apply field theory methods to $\mbox{SU}(3)$ symmetric Heisenberg chains in the fully symmetric representation, with p boxes in the Young tableau, mapping them into a $\mbox{SU}(3)/(\mbox{U}(1)\times\mbox{U}(1))$ non-linear $\sigma$-model with a non-trivial topological term and a topological angle $\theta =2\pi p/3$. Based on this mapping we argue that $\mbox{SU}(3)$ spin chains are...
Quantum spin liquids (QSL) exhibit large sensitivity to perturbations,such as the Dzyaloshinskii-Moriya (DM) interaction.Using the combination of Electron Spin Resonance (ESR) and Muon Spin Rotation ($\mu$SR),we studied the DM interaction in QSL candidates.In the triangular-lattice QSL,$\kappa$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$,our ESR measurements found a staggered moment of...
We report results on the unconventional finite-temperature properties of the localized singlet-triplet excitations in the 2D frustrated magnet $SrCu_2(BO_3)_2$. Performing inelastic neutron scattering, singlet-triplet modes were separated in energy with an applied magnetic field, so that their damping could be studied independently. The $Q$-dependence of the damping mechanism was determined by...
Progress in low emittance electron storage ring design necessitates an upgrade of the Swiss Light Source (SLS) in order to stay competitive in the future.
For the period 2021-24 it is planned to replace the storage ring by a new one providing 30-40 times lower emittance and correspondent higher brightness and coherent fraction. A novel type of lattice was developed to provide an emittance of...
Atomic scale X-Ray Photon Correlation Spectroscopy (aXPCS) is a powerful technique to study the dynamics on an atomic scale in hard condensed matter by using coherent high flux X-ray radiation. It is shown that using such beams to probe the dynamics in hard condensed matter also induce dynamics in the material. Measurements of the influence of X-rays on the dynamics in a wide range of...
It is well known that in some cases powerful synchrotron X-ray sources are not only probing but at the same time also influencing the structure
of the sample.
Recently it was shown that coherent X-rays with a high flux as used in aXPCS (atomic scale X-ray Photon Correlation Spectroscopy) measurements influence the dynamics on some kind of materials like oxide glasses.
On the basis of aXPCS...
In situ small angle X-ray scattering (SAXS) is a suitable technique to analyze global ion fluxes and local ion re-arrangement in nanoporous carbon-based supercapacitor electrodes upon charging and discharging.[1,2] Recently, we have extended our studies to include volumetric changes of the electrodes during operation. Therefore, a novel hierarchical carbon material with ordered mesoporosity...
Transient absorption is a widely used technique in ultrafast spectroscopy. In this contribution we show how the correlation of the noise on the employed light source can be used to reduce the measurement time by two orders of magnitude without need to increase the repetition rate of the laser system or the light exposure of the sample. Besides being an important progress in itself, the gain in...
The extreme ultraviolet (EUV) is of particular interest for future lithography replacing 193 nm UV by, e.g., 13.5 nm radiation from Sn plasma. Here, instead of lenses Mo/Si multilayer structures are considered. These structures are one-dimensional photonic crystals (1D-PhCs).
In this work, the EUV reflectivity of more complex PhCs are studied. For example, by replacing every 4th Mo layer...
Diffuse scattering from thermally populated phonons contains important details on elastic and thermodynamic properties. I will show that high-precision measurements of thermal diffuse scattering together with a rigorous data analysis allow for the determination of the full elasticity tensor in a single crystal diffraction experiment [1].
The new approach allows the measurement of elastic...
Quantum state tomography is an approach to reconstruct a quantum state but involves a lot of computational post-processing. So in 2011 a novel more direct method was established, without the post processing, using so called weak measurements. However, because of this weakness the information gain is very low. Now we managed to combine these two methods and got the benefits from both. Our...
