Robin Steinigeweg: Constructing nonequilibrium steady states from equilibrium correlation functions
State-of-the-art approaches to extract transport coefficients of many-body quantum systems broadly fall into two categories: (i) they target the linear-response regime in terms of equilibrium correlation functions of the closed system; or (ii) they consider an open-system situation typically modeled by a Lindblad equation, where a nonequilibrium steady state emerges from driving the system at its boundaries. While quantitative agreement between (i) and (ii) has been found for selected model and parameter choices, also disagreement has been pointed out in the literature. Studying magnetization transport in the spin-1/2 XXZ chain, we here demonstrate that at weak driving the nonequilibrium steady state in an open system, including its buildup in time, can remarkably be constructed just on the basis of correlation functions in the closed system. We numerically illustrate this direct correspondence of closed-system and open-system dynamics, and show that it allows the treatment of comparatively large open systems, usually only accessible to matrix product state simulations. We also point out potential pitfalls when extracting transport coefficients from nonequilibrium steady states in finite systems.
Rok Zitko: Richardson model description of spin-orbit coupling in superconducting islands
Richardson model, first introduced in nuclear physics as a simplified model of nucleon pairing, is also an appropriate description of a small superconducting island with fixed charge. Complex systems composed of interconnected superconducting islands and interacting quantum dots can be modelled using Hamiltonians that can be transformed into the matrix-product-operator form with small matrices that can be efficiently solved using the density matrix renormalization group. This approach allows to include without any approximations the effects of both the exchange interaction (Kondo screening and Yu-Shina-Rusinov subgap states) and the charge repulsion (Coulomb blockade, capacitive coupling) and thereby provide reference results for this family of Hamiltonians that are more general than regular quantum impurity problems. The theory results match well the experimental measurements on hybrid semi-super devices.I will describe how this approach can be extended to incorporate two further phenomena, the spin-orbit coupling and the proximity effect leading to level-dependent pairing strength. The combination of the two leads to a degeneracy of even and odd-parity ground states in the regime where the external magnetic field becomes strong enough to generate an increasing number of quasiparticles in the superconducting levels with the weakest pairing strength. This manifests as equal spacing of even and odd states in the charge stability diagrams.
Simon Gerber: Direct characterization of photo- and electrically-induced “hidden” state switching of 1T-TaS2
Brilliant, ultrashort, and coherent X-ray free-electron laser (FEL) pulses allow for investigation of dynamics at the inherent time and length scale of atoms. I will illustrate this capability at the example of recent time-resolved X-ray diffraction data taken in the “hidden” phase of the Van der Waals material 1T-TaS2, hinting that out-of-plane restacking suppresses the optically-induced hidden state. Furthermore, I will also present preliminary static micro-beam X-ray diffraction data of electrically switched 1T-TaS2 cryomemory cells which indicate that also from a structural point of view the photo- and electrically-induced “hidden” states are closely related.
Corinna Burri: Spatial mapping of the electronic structure of 1T-TaS2
As a step towards resolving the spatially-resolved electronic structure of the layered transition metal dichalcogenide 1T-TaS2 – and eventually electronically switched cryomemory devices – we have performed angle-resolved photoemission spectroscopy (ARPES). Using a photon energy of 72 eV and the micrometer spot size available at the spectromicroscopy beamline of the Elettra synchrotron, we measured the band structure and high-statistics Fermi surface of 11, 500 and 600 nm thick flakes. Also, the first-order phase transition from the nearly-commensurate to the commensurate CDW state leads to a prominent splitting of the Ta 4f core levels which we have mapped spatially. In addition, using 400 eV soft X-ray ARPES at the ADRESS beamline of the Swiss Light Source synchrotron, we established the kz dependence of the band structure of a 110-nm thick flake, which reveals a two dimensionality of the electronic structure.
