Despite theoretical predictions for the Cherenkov radiation of spin waves (magnons) by various propagating magnetic perturbations, it has not been observed so far. Our recent experiments [arXiv:2103.10156][1] evidence the Cherenkov radiation of magnons in a Co-Fe magnonic conduit by fast-moving magnetic flux quanta (Abrikosov vortices) in an adjacent Nb-C superconducting strip. The radiation...
Magnons, the quanta of spin waves, could be used to encode information in beyond-Moore computing applications. Here, we report a nano-scale magnonic directional coupler based on yttrium iron garnet , which can function as circuit building blocks. The coupler consists of single-mode waveguides with a width of 350 nm. We use the amplitude of a spin wave to encode information and to guide it to...
Spin waves, and their quanta magnons, are of great interest as potential data carriers in future low-energy computing devices. Here, we will present the method of inverse-design magnonics, in which any functionality can be specified first, and a feedback-based computational algorithm is used to obtain the device design. Our proof-of-concept prototype is based on a rectangular ferromagnetic...
Long-lived coherences, emerging under periodic pulse driving in the disordered ensembles of strongly interacting spins, offer immense advantages for future quantum technologies, but the physical origin and the key properties of this phenomenon remain poorly understood. We theoretically investigate this effect in ensembles of different dimensionality, and predict existence of the long-lived...
In this contribution we present direct imaging of spin-wave dynamics in Ni$_{80}$Fe$_{20}$ rectangular microstrips ($5\times1\times0.03$ µm$^3$) under uniform excitation. Both a single strip and two strips of the same size oriented perpendicular to each other are investigated. For FMR and time-resolved STXM measurements a static magnetic field is applied in plane, aligned parallel or...
We present a microscopic study of a doped quantum spin liquid candidate, the Hyperkagome Na3Ir3O8 compound, by using 23Na NMR (1). We determine the intrinsic behavior of the uniform q = 0 susceptibility via shift measurements and the dynamical response by probing the spin-lattice relaxation rate. Throughout the studied temperature range, the susceptibility is consistent with a semimetal...
Metastable skyrmions with long lifetimes are attractive for various applications. However, the physics behind the non-equilibrium topological phases is far from being fully understood. We report the creation of a new hidden skyrmion phase in a Cu2OSeO3 lamella achieved by femtosecond laser excitation of the material at 5 K and a magnetic field of 15 mT. The formation of the skyrmion lattice...
The insulating multiferroic material Cu2OSeO3 holds the possibility to study and manipulate topological magnetic order and skyrmion dynamics in a current free environment, purely under the influence of magnons or electric field. Here we drive and control ratchet-like skyrmion rotational motion via femtosecond pulses of mid-infrared (1 eV) light, far below the bandgap of the material. We image...
In a superconductor with inversion, spin-singlet and spin-triplet order parameters are distinct by symmetry and can be distinguished experimentally through their magnetic response. In non-centrosymmetric systems, however, the two order parameters mix and the magnetic response is inconclusive. In our work, we examine the situation, where inversion is broken locally in a sublattice, but retained...
With the availability of internet, social media, etc., the interconnectedness of people within most societies has increased tremendously over the past decades. Across the same timespan, an increasing level of fragmentation of society into small groups has been observed. With a simple model of societies, in which the dynamics of opinion formation is integrated with social balance, we show that...
Optical frequency combs refer to the emission of perfectly periodic waveforms of light. These waveforms can be formed due to optical nonlinearities that provide the coherent coupling of the amplitude and phase of the light. We show that Bloch gain serves as the physical origin of the linewidth enhancement factor and that it plays an essential role in the formation of quantum cascade laser...
Mid-infrared dual-comb spectroscopy is emerging as powerful tool for broadband and high-speed molecular spectroscopy. Chip-scale frequency combs based on quantum cascade lasers (QCLs) have become an invaluable technology, because they are electrically pumped, have a small footprint and offer an unrivalled power per mode. However, the mutual drift of both combs over time limits the averaging...
Dual comb spectroscopy using Quantum Cascade Laser (QCL) frequency combs, is a widely applied technique for identification of optical absorption features in the radio-frequency (RF) domain. Temperature fluctuations and electronic noise lead to often highly unstable heterodyne beating signals, hindering reproducible evaluation and analysis. We present a simple, yet reliable phase locking...
The linewidth enhancement factor (LEF) is known as an important property of semiconductor lasers. Recently, it is gaining more interest due to its key role in frequency comb operation. However, as of yet existing techniques to measure the LEF are limited to sub-threshold bias or single-mode operation. Here, we introduce a novel and universally applicable method to directly obtain the...
We present our recent results on the impact of intersubband absorption in the valence band on the performance of interband cascade lasers (ICLs). We observe a clear performance dependence on the thickness and composition of the Ga$_{1-x}$In$_x$Sb hole-quantum well (QW), reflecting in the characteristic temperature $T_\mathrm{0}$ as well as the threshold current density $J_\mathrm{th}$. By...
We present a planarized Y-coupled double metal waveguide THz quantum cascade laser. The low-area waveguide geometry is obtained via inverse design, and the device features broadband emission over 1 THz, a low threshold current, and frequency comb operation. Far-field measurements reveal characteristic interference patterns, a signature of equal power splitting and broadband phase locking of...
The large ZnO LO-phonon energy reduces the thermally activated LO-phonon scattering, showing great potential for improving the temperature performance of THz quantum cascade lasers. Here, we report the observation of THz intersubband electroluminescence from ZnO/MgxZn1–xO quantum cascade structures grown on a nonpolar m-plane ZnO substrate up to room temperature. The electroluminescence...
By engineering the dispersion of the cavity, we observe the formation of bright dissipative Kerr solitons in the mid-infrared range. The soliton formation appears after an abrupt symmetry breaking between the two lasing directions of the ring cavity. The pump field of the soliton is generated by direct electrical driving and closely resembles the soliton Cherenkov radiation observed in passive...
Quantum Cascade Detectors (QCDs) utilize the conduction band offset (CBO) of materials to create quantum confined electron states in the well material. The lattice-matched InAs/AlAs0.16Sb0.84 material system shows a large CBO of 2.1 eV at the Γ-point. This material system is therefore a candidate for the design and growth of short-wavelength mid-infrared QCDs. However, InAs has a narrow...
Replacing 193 nm UV-radiation, extreme ultraviolet (EUV) radiation of 13.5 nm wavelength has entered commercial microelectronics production. Further progress and novel applications like microscopy of biological specimens require a further reduction of the wavelength to the sub-10-nm range (“Beyond EUV” - BEUV), e.g. the water window (2.33 nm to 4.4 nm). In this wavelength range multilayer...
Charge order (CO) is established in most known hole underdoped cuprates and considered as universal property on equal footing with the pseudogap phase and superconductivity. In La-based cuprates, several studies on the charge and spin order have recently lead to controversial results in the overdoped regime. To address open questions on the origin of CO, its connection to the tentative...
Multilayered cuprates possess not only the highest superconducting temperature transition but also offer a unique platform to study the interplay between competing and intertwined orders with superconductivity. Here, we study the underdoped trilayer cuprate HgBa2Ca2Cu3O8 and we report the first quantum oscillation measurements in magnetic field up to 88 T. A careful analysis of the spectra of...
A major difficulty in understanding cuprate superconductors is the presence of strong correlations which give rise to the rich phase diagrams of these systems. I will discuss the charge density wave (CDW) order, which was demonstrated to be intrinsic to cuprates. This modulation is observed in the intermediate carrier concentration range, below the optimal doping. While resonant X-ray...
Cuprates superconductors undergo various charge states as electron or hole carries doping into the parent charge-transfer insulators. The characters of charge dynamics are thus of great importance to understand the underlying physics behind the complex phase diagram. Using O K-edge resonate inelastic X-ray scattering, we studied the low-energy charge excitations in hole-doped superconducting...
Exploring strongly interacting engineered vortex patterns in copper-oxide superconductors requires vortex distances smaller than the London penetration depth. With the focused beam of a helium ion microscope, we fabricate pinning sites with spacings down to 40 nm in YBa2Cu3O7−𝛿 thin films and investigate vortex commensurability effects at unprecedented high magnetic fields at the order of 1 T....
Understanding of the ordering tendencies exhibited by the cuprates can give valuable insight into the origin of superconductivity in these complex oxides. Thus, I will present the study of the charge density wave (CDW) order and discuss its connection with the electron-phonon coupling in Nd$_{2-x}$Ce$_x$CuO$_4$. Recent studies suggested that the CDW order can cause an anomalous softening of...
The normal state of cuprate high-temperature superconductors exhibits a plethora of unusual behaviors that hinder the understanding of the superconducting phenomenon. Despite this complexity, the behavior of the scattering rate was demonstrated to be surprisingly simple. It is doping and compound independent and exhibits a quadratic temperature dependence, like a Fermi liquid. A distinct...
Cuprates are prototypical for high-temperature superconductivity and host a vast family of com-
pounds. Recently, the new Ba$_2$CuO$_{3+y}$ cuprate superconductor has been discovered, which challenges the previous physics picture in other cuprate superconductors by (1) exhibiting superconducting only in a narrow ’highly overdoped’ doping region ($y$=0.2) (2) hosting a different Fermi-surface;...
The idea that unconventional superconductivity (SC) in Sr2RuO4 is a solid-state analogue to superfluid 3He-A has been recently overturned. Here we use 17O NMR spectroscopy to probe the SC state in Sr2RuO4 in the limit T→0 down to B/Bc2< 0.2. While the NMR Knight shift K includes...
Exploration of novel materials in search for unconventional superconductivity can lead not only to the synthesis of compounds with important technological applications but also contributes to understanding the mechanism of this phenomenon. One such compound is murunskite (K2FeCu3S4), a material isostructural to iron-based superconductors. I will discuss our synthesis efforts and...
This talk will be about the rich set of possibilities provided by graphene-based moiré superlattices to create and study interesting many-body physics at the intersection of strong correlations and topology. Initially driven by experimental findings in twisted bilayer graphene, related systems have been realized experimentally more recently and found to exhibit similar properties; examples are...
Understanding nanoscale energy dissipation is nowadays among few priorities in solid state systems. Pendulum geometry Atomic Force Microscope (pAFM), oscillating like a tiny pendulum over the surface, is perfectly suited to measure minute energy loss.
Here we report on low temperature energy dissipation measurements on twisted bilayer graphene (TBG) at magic angle twist - a system with flat...
An imminent doubt has always been around whether angle-resolved photoelectron spectroscopy (ARPES) of high-Tc superconductors, visualizing the superconducting gap in k-space, can truly represent the intrinsic bulk spectral function whose response is distorted by energy- and k-dependent matrix elements and small photoelectron escape depth. We address this fundamental question with...
When colloidal nanocrystals self assemble into ordered superstructures they form functional solids that inherit the electronical properties of the single nanocrystals (NCs). To what extent these properties are enhanced depends on the specific ordering of the NCs within the superstructure.
Here, the formation of supercrystals using faceted nanocrystals as building blocks was investigated by...
With recent advances in quantum cascade (QC) lasers and detectors the mid-IR spectral region - often called fingerprint region, due to the characteristic fundamental absorption features of molecules in this range – becomes more and more accessible for various spectroscopic applications. Here, we present a compact liquid sensor featuring QC lasers and detectors coupled by plasmonic waveguides...
Recently, it has been shown that disordered dielectrics can show a photonic band gap in the presence of structural correlations, but 30 years after John's seminal proposal on the interplay between the photonic pseudo band gap in disordered photonic crystals and Anderson localization, a controlled experimental study of the transport properties in between ordered and disordered states is still...
We study coupling of electromagnetic waves to magnetization dynamics. Magnon-polaritons are intensively explored in ferromagnetic materials at gigahertz frequencies. Antiferromagnets have resonance frequencies at in the terahertz band, thus, there are only a few reports of light-matter coupling in that case. We report strong magnon-photon coupling in hematite alpha-Fe2O3. A cube of hematite...
Heterostructures based on AlxGa1-xN are the building blocks of state-of the-art high-power and optoelectronic devices working in the visible and ultra-violet range. It was recently found, that the self-assembly of Mn-Mgk complexes in epitaxial GaN:(Mn,Mg) and AlxGa1-xN:(Mn,Mg) allows extending the emission spectra of these...
Chiral topological semimetals are a new class of topological matter that host chiral multifold fermions in a chiral crystal structure. These new fermionic quasiparticles can be viewed as a higher spin generalization of Weyl fermions without equivalence in elementary particle physics. Their large topological charge has been predicted to give rise to unusual phenomena, such as giant quantized...
Time-periodic Floquet drive is a powerful method to engineer quantum phases of matter, including fundamentally nonequilibrium states that cannot be realized in static Hamiltonian systems. A characteristic example is the anomalous Floquet topological insulator, which exhibits a unique topologically protected nonequilibrium transport phenomenon: nonadiabatic quantized charge pumping. We study...
We investigate triple nodal points, i.e., three-fold degeneracies of energy bands in the momentum space of three-dimensional crystalline solids. First, based on the symmetries required for their stability, we develop a classification of triple nodal points in weakly spin-orbit-coupled materials. Second, by combining the derived classification with symmetry indicators for corner charges, we...
We enrich the notions of stable and fragile topology by introducing delicate topological insulators: band structures possessing topological invariants that can be trivialized through an addition of a trivial conduction band. We find that although delicate topological insulators are Wannier representable with exponentially-localized symmetry-preserving Wannier functions, they can possess a...
We employ the Ensemble Geometric Phase (EGP) - a generalisation of the Zak phase to mixed states - to analyse the topology of an open Su-Schrieffer-Heeger model involving both unitary Hamiltonian dynamics and dissipative coupling. For dissipation described by the Lindblad formalism, we discover regimes where the EGP is quantised to zero or pi, and relate the quantisation to the existence of an...
Recent theoretical works unveiled that crystalline symmetries can stabilize topologically fragile Bloch bands that challenge our very notion of topology: one can trivialize these bands through the addition of trivial Bloch bands. Here, we show via auxiliary-field Monte Carlo simulations how fragile topology enhances the superfluid weight and hence the superconducting critical temperature. This...
We present control over the emitted state of quantum cascade laser frequency combs through strong radio-frequency modulation close to their repetition frequency. In particular, coherent broadening of the spectrum from about 20 cm$^{-1}$ to 60cm$^{-1}$ can be achieved throughout the DC-current dynamical range. Close to the free-running beatnote frequency, tuning of the modulation frequency...
Hydrogen (H2) is widely considered as an ideal CO2-free energy carrier. Physisorption in nanoporous carbons is a promising way for reversible H2 storage at pressure below 100 bar. The gravimetric amount of stored H2 strongly depends on the nanopore structure, notably the mean pore size being typically below one nanometer. Yet the mechanisms which determine the storage capacity are still not...
As wireless near-IR telecommunications through air and space are reaching their performance-limitations in terms of bandwidth and transmission under turbulent conditions, solutions for low-atmospheric attenuation data transmission are sought. Quantum-cascade-based systems offer such capabilities, i.e. intrinsically high GHz-modulation properties and robust free-space transmission by addressing...
The longwave-infrared holds various applications ranging from sensing and imaging to optical free-space communication. The increasing demand for miniaturized systems requires the development of compact photonic networks between on-chip optoelectronic components such as lasers, detectors and modulators. To resolve this challenging task, we introduce and experimentally demonstrate a novel type...
There exists a growing interest in the properties of the light generated by hybrid systems as a means of providing nanoscale sources of quantum light. Specifically, plasmonic nanocavities have been shown to retain and enhance excitonic nonlinearities even when the number of emitters is large. In these platforms, photon antibunching is generally achieved by coupling one or several emitters to a...
Quantum Cascade Random Lasers (QCRLs) feature a broadband gain medium as well as a broadband cavity, which makes them ideal for spectroscopic applications. However, due to mode competition and spatial hole-burning, only discrete modes arise in reality.
The full potential of QCRLs can be reached by perturbing them optically. We present such an optical perturbation scheme, which by the help...
The project has aimed to investigate digital models of photovoltaic (PV) modules and to describe PV modules’ behavior under different failure modes. The modeling of the modules and other components is mainly performed by physical models. The model approach considers the integration of artificial failures representing equivalent classes of failure types in a real diode, enabling a better...
For a long time, the sole function of a luminaire was to illuminate its surrounding. Nowadays, with the rise of light-emitting diode (LED) based luminaires also functionalities beyond illumination become more and more of relevance. Recent attempts in this regard focus on communication, localization and, most recently, backscattered visible light sensing. Here we demonstrate and discuss system...
The anharmonicity of atomic couplings, responsible e.g. for finite heat conductivity in crystals due to phonon-phonon scattering, is most fundamentally accessible as broadenings of phonon dispersions or finite lifetimes.
Here we will compare inelastic neutron scattering on fcc-Al up to the melting point to ab initio calculations of q-dependent line broadenings [1]. Further, an analysis of...
In low-dimensional semiconducting nanostructures, strong confinement leads to quantization of charges allowing to investigate and control their individual physical properties. Particulary, one-dimensional semiconducting nanowires have attracted a lot of attention as hosts of spin-qubits or, in combination with superconducting leads, as hosts of Andreev levels and Majorana bound states. Here,...
We describe the discovery of ice XIX based on neutron diffraction, Raman spectroscopy, calorimetry and dielectric spectroscopy and study transitions in its hydrogen sublattice. The high-pressure ice polymorph crystallises in a √2 x √2 x 1 supercell with respect to the parent ice VI phase in space group P-4, where the water molecules are partially antiferroelectrically ordered. At ambient...
Long lived quasi-stationary states (QSSs) are a signature characteristic of long-range interacting systems both in the classical and in the quantum realms. Despite their ubiquity, the fundamental mechanism at their root remains unknown. Here, we show that the spectrum of systems with power-law decaying couplings remains discrete up to the thermodynamic limit. Then, several traditional results...
The spin-1/2 Heisenberg model on the pyrochlore lattice is debated to possess a spin-liquid ground state. We contest this hypothesis with a numerical investigation using exact diagonalization and variational techniques: RVB-like Monte Carlo ansatz and convolutional neural network for (variational) calculations up to $4×4^3$ spins. We determine the phase transition between the putative...
A longstanding goal is to utilize the spin of an electron in electronic devices, both, in applications summarized as spintronics, as well as in fundamental research, for example to demonstrate spin correlations in superconducting electronic elements. To this end, we have introduced ferromagnetic split-gates (FSGs) that allow to individually polarize the electron spin states of semiconductor...
We perform large scale quantum Monte Carlo simulations of the Hubbard model at half filling with a single eight component Dirac cone close to the relativistic quantum critical point. We discuss the implementation of a single Dirac cone in the SLAC formulation for eight Dirac components and its reliability upon the introduction of interactions. The finite size scaling properties of the Hubbard...
There is a huge renewed interest in magentotransport phenomena in solids, especially in relation with topology, Berry curvature, and Weyl points. The responses of interest include nonlinear anomalous Hall effect, crystal Hall effect, planar Hall effect, unidirectional magnetoresistance, and electrical magnetochiral anisotropy, among others. In this talk. I will show how to classify all these...
Non-Hermitian topological phases of open quantum many-body systems are fundamentally dynamical in nature. This poses a challenge to identify robust signatures of non-Hermitian topology and to characterize critical behavior at topological phase transitions. We show that non-Hermitian topology of a driven-dissipative Kitaev chain becomes manifest in crossings in the entanglement spectrum after a...
Rare-earth nickelates, RNiO$_{3}$, are negative charge-transfer materials with electronic configuration of Ni-3$d^8\underline{L}$ ($\underline{L}$ = oxygen ligand hole) in their metallic state. Most RNiO$_{3}$ undergo low-temperature metal-to-insulator transition (MIT) accompanied by breathing distortion in their crystal structure, where neighboring expanded NiO$_{6}$ octahedra (Ni-3$d^8$...
