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[en] The Kitaev-Heisenberg model is source of a topological quantum spin liquid with Majorana fermions and gauge flux excitations as fractional quasiparticles. The material -RuCl is composed of weakly van der Waals bound honeycomb layers of edge sharing RuCl octahedra which has recently emerged as a prime candidate for realising such physics. We studied -RuCl by means of thermal transport measurements, a valuable tool to probe elementary excitations of systems with low dimensional spin structure. While the in-plane, longitudinal heat transport is governed by heat conduction of phonons that strongly scatter off the magnetic excitations present in the system, studying the thermal Hall effect (Rhighi-Leduc effect) opens up a new path towards detecting a direct contribution of unconventional magnetic excitations to entropy transport. We have observed a sizeable transversal heat conductivity , the agreement of which with the theoretical predictions for the pure Kitaev model being suggestive of heat transport by fractionalised quasiparticles in -RuCl.
[en] Relativistic mean-field (RMF) models with density dependent (DD) couplings have been used successfully to describe finite nuclei and nuclear matter. They usually assume a dependence of the nucleon-meson couplings on the so-called vector density that is derived from the baryon current. A dependence on other densities, in particular the scalar density, was not really explored although suggested in early introductions of the DD-RMF approach. In this contribution, properties of nuclei, the corresponding equations of state (EoS) and symmetry energies of different DD-RMF models are compared using DD couplings of various functional form and dependence on vector and scalar densities. They are obtained by fitting the same set of nuclear observables. The choice of the dependence changes the EoS and the characteristic nuclear matter parameters. Problems of some of the models are identified.
[en] -LiFeSiO is proposed as a promising candidate material for lithium ion batteries. The Fe atoms are tetrahedrally coordinated by Oxygen. Iron has the Fe (S=2) oxidation state and is displaced from the tetrahedron center resulting in a electric field gradient caused by the distorted tetrahedral crystal field. The Mössbauer spectrum of the powder sample shows one dominant site exhibiting magnetic order at 2.1 K and a considerable quadrupole splitting as observed at room temperature. The magnetic hyperfine field of B = 14.7(4) T is oriented orthogonal to the largest principle axis of the electrical field gradient V = -125(3) V/Å. The isomer shift of = 1.1(1) mm/s is consistent with the high spin Fe (S=2) state. We will discuss the implications of these findings on the actual magnetic structure in this system. The observed static order is in agreement with susceptibility measurements showing a transition to antiferromagnetic order below 17 K.
[en] I present quantum refrigerators based on superconducting qubits. In the theoretical part I describe a four-stroke Otto refrigerator, and a model of it in different operation regimes. Experiments on a transmon type qubit coupled to two resonators demonstrate heat transport, where the quantum-classical border is determined by the relative strength of qubit-resonator and resonator-heat bath couplings, respectively.
[en] We present a magnetoelastic investigation of the frustrated triangular-lattice S = 1/2 antiferromagnet CsCuCl by studying the longitudinal modes c, c and c. The measurements were performed in magnetic fields up to 10 T and down to 0.032 K to cover the the long range order and the spin-liquid regime. At the lowest temperatures of our experiment the field dependence of the c mode can be well described using a Landau free energy model which combines the elastic constant with the magnetic susceptibility data, measured independently. From fits to the experimental c data we obtain a very small magnetoelastic coupling constant G/ = 2.8 K for CsCuCl consistent with the results of susceptibility measurements under hydrostatic pressure. Remarkably, we find that the classical approach provides an excellent description of the data at lowest temperatures, i.e., close to the putative quantum critical point at B = 8.5 T of this material. However, at somewhat higher temperatures, there are deviations between the experimental data and the theoretical curves. At these temperatures we also observe anomalies in the ultrasonic attenuation and , the imaginary part of the magnetic susceptibility. We discuss these losses with respect to the peculiarities of the magnetic excitation spectrum for this low dimensional spin system.
[en] The formation of 1D wires of carriers at the edges of MoS nanoribbons (NRs) represents a case study for spontaneous polarization effects. The spatial confinement and the charge accumulation at the edges make MoS-NRs a perfect candidate to investigate interaction effects at the nanoscale, also in view of possible applications in solar-energy devices. While the electronic and optical properties of MoS bulk monolayers have been the focus of an intense research, the investigation of MoS-NRs is at the early stages. We will present a first principle investigation of the electronic structure and optical absorption of MoS-NRs as a function of the NR width, as obtained within the framework of many-body perturbation theory, according to the G0W0 plus Bethe-Salpeter-Equation scheme. We will show that both width-dependent and width-independent mechanisms emerges in the formation of excitonic excitations, and we will explain the relationship between those two mechanisms and the edge-localization of the carriers. Since the investigated MoS-NRs are metallic, the present work deals with the fundamental issue of the presence of excitons in metals.
[en] The semimetals WTe and Td-MoTe have been the first compounds proposed as a type-II Weyl semimetals. While for these parent compounds the Weyl points are far above the Fermi level, it is presumed that mixing the two phases brings them to relevant energies. We have performed magneto-electrical transport measurements on a series of WMoTe single crystals. Zero-field resistivity increases upon substituting Mo for W but keeps metallic properties. The large non-saturating magnetoresistance is reduced in between the two phases, while the Hall resistivity decreases continuously from WTe to MoTe. A simple two-band analysis ascribes this behavior to suppressed charge carrier compensation and declining mobilities. Observed Shubnikov-de-Haas oscillations show no drastic changes of the electronic structure.
[en] Young's archetypal double-slit experiment forms the basis for modern diffraction techniques. We report on an inelastic incarnation of Young's experiment and demonstrate that resonant inelastic X-ray scattering (RIXS) measures interference patterns which reveal the symmetry and character of electronic excited states in the same way as elastic scattering does for the ground state. A prototypical example is provided by the quasi-molecular electronic structure of insulating BaCeIrO with face-sharing IrO octahedra forming structural Ir dimers. The double 'slits' in this resonant experiment are the highly localized core levels of the two Ir atoms within a dimer. The clear double-slit-type sinusoidal interference patterns that we observe allow us to characterize the electronic excitations. The physics is well described by quasi-molecular orbitals. The ground state shows a spin-orbit-entangled j = 0 singlet predominantly built from j = 1/2 moments with the corresponding triplet excitation lying at an extraordinarily large energy of 1.2 eV.
[en] The discovery of materials with 3D linear band-crossing points close to the Fermi level, such as Dirac and Weyl semimetals, has offered the possibility to study relativistic fermions in solid state systems. One manifestation, the 'chiral anomaly', should appear as a reduction of the longitudinal magnetoresistance (LMR), as was quickly observed in TaAs. Subsequent studies found that current inhomogeneities ('current jetting') often induce an apparent negative LMR in semimetals, and that the true LMR is still unknown. In this study, we determine the intrinsic LMR in the TaAs family (TaAs, TaP, NbAs, NbP) of Weyl semimetals. We reduced current jetting effects by trying to achieve a homogeneous current injection and by increasing the aspect ratio of our samples. The results show an absence of the negative LMR in chiral materials and a presence of a negative LMR in non-chiral materials. This suggests a chirality independent effect, which we believe to be weak-localisation physics.