Mesoscale and Nanoscale Physics (cond-mat.mes-hall)

Abstract: Two metals at different temperatures separated by large gaps exchange heat under the form of electromagnetic radiation. When the separation distance is reduced and they approach contact (nanometer and sub-nanometer gaps), electrons and phonons can tunnel between the bodies, competing and eventually going beyond the flux mediated by thermal photons. In this transition regime the accurate modeling of electronic current and heat flux is of major importance. Here we show that, in order to quantitatively model this transfer, a careful description of the tunneling barrier between two metals is needed and going beyond the traditional WKB approximation is also essential. We employ analytical and numerical approaches to model the electronic potential between two semi-infinite jellium planar substrates separated by a vacuum gap in order to calculate the electronic heat flow and compare it with its radiative counterpart described by near-field radiative heat transfer. We demonstrate that the results for heat flux and electronic current density are extremely sensitive to both the shape and height of the barrier, as well as the calculation scheme for the tunneling probability, with variations up to several orders of magnitude. Using the proximity force approximation, we also provide estimates for tip-plane geometries. The present work provides realistic models to describe the electronic heat flux, in the scanning-thermal-microscopy experiments.

Abstract: We develop a theory of anomalous Hall effect in ultraclean channels with two-dimensional electron gas. In ultraclean systems, the electron mean free path due to static disorder and phonons is considered to be much larger compared to the channel width. The electron momentum relaxation and conductivity are mainly determined by the diffusive scattering at the channel edges and electron-electron collisions making electron magnetotransport highly nontrivial. The anomalous Hall electric field and Hall voltage are generated owing to the skew scattering of electrons, side-jump, and anomalous velocity effects that appear as a result of the spin-orbit coupling. We study both ballistic and hydrodynamic transport regimes which are realized depending on the relation between the electron-electron mean free path and the channel width. Compact analytical expressions for the anomalous Hall field and voltage are derived. We demonstrate that both in the ballistic and hydrodynamic regime the electron momentum relaxation in the bulk of the channel due to impurities or phonons is required for the anomalous Hall effect in contrast to the normal Hall effect and conductivity that can appear due to the electron-electron collisions combined with momentum relaxation at the channels edge scattering.

Abstract: Violation of the Wiedemann-Franz (WF) law in a 2D topological insulator due to Majorana bound states (MBS) is studied via the Lorenz ratio in the single-particle picture. We study the scaling of the Lorenz ratio in the presence and absence of MBS with inelastic scattering modeled using a B\"uttiker voltage-temperature probe. We compare our results with that seen in a quantum dot junction in the Luttinger liquid picture operating in the topological Kondo regime. We find that the scaling of the Lorenz ratio in our setup corresponds to the scaling in the Luttinger-liquid setup only when both phase and momentum relaxation occur, but not when only phase relaxation occurs. This suggests that the interplay between the presence of Majorana bound states and the type of inelastic scattering process, can have a significant impact on the violation of the Wiedemann-Franz law in 2D topological insulators.

Abstract: We report on the dependence of curvilinear shaped coplanar waveguides on the near-field diffraction patterns of spin waves propagating in perpendicularly magnetized thin films. Implementing the propagating spin waves spectroscopy techniques on either concentrically or eccentrically shaped antennas, we show how the link budget is directly affected by the spin wave interference, in good agreement with near-field diffraction simulations. This work demonstrates the feasibility to inductively probe a magnon interference pattern with a resolution down to 1$\mu$m$^2$, and provides a methodology for shaping spin wave beams from an antenna design. This methodology is successfully implemented in the case study of a spin wave Young's interference experiment.

Abstract: Hexagonal boron nitride (h-BN), a wide bandgap, two-dimensional solid-state material, hosts pure single-photon emitters that have shown signatures of optically-addressable electronic spins. Here, we report on a single emitter in h-BN exhibiting optically detected magnetic resonance at room temperature, and we propose a model for its electronic structure and optical dynamics. Using photon emission correlation spectroscopy in conjunction with time-domain optical and microwave experiments, we establish key features of the emitter's electronic structure. Specifically, we propose a model that includes a spinless optical ground and excited state, a metastable spin-1/2 configuration, and an emission modulation mechanism. Using optical and spin dynamics simulations, we constrain and quantify transition rates in the model, and we design protocols that optimize the signal-to-noise ratio for spin readout. This constitutes a necessary step toward quantum control of spin states in h-BN.

Abstract: The dynamical properties of skyrmions can be exploited to build devices with new functionalities. Here, we first investigate a skyrmion-based ring-shaped device by means of micromagnetic simulations and Thiele equation. We subsequently show three applications scenarios: (1) a clock with tunable frequency that is biased with an electrical current having a radial spatial distribution, (2) an alternator, where the skyrmion circular motion driven by an engineered anisotropy gradient is converted into an electrical signal, and (3) an energy harvester, where the skyrmion motion driven by a thermal gradient is converted into an electrical signal, thus providing a heat recovery operation. We also show how to precisely tune the frequency and amplitude of the output electrical signals by varying material parameters, geometrical parameters, number and velocity of skyrmions, and we further prove the correct device functionality under realistic conditions given by room temperature and internal material defects. Our results open a new route for the realization of energy efficient nanoscale clocks, generators, and energy harvesters.