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

Abstract: Exploring the interplay between topological phases and photons opens new avenues for investigating novel quantum states. Here we show that superconducting resonators can serve as sensitive probes for properties of topological insulator nanowires (TINWs) embedded within them. By combining a static, controllable magnetic flux threading the TINW with an additional oscillating electromagnetic field applied perpendicularly, we show that orbital resonances can be generated and are reflected in periodic changes of the Q-factor of the resonator as a function of the flux. This response probes the confinement of the two-dimensional Dirac orbitals on the surface of the TINW, revealing their density of states and specific transition rules, as well as their dependence on the applied flux. Our approach represents a promising cross-disciplinary strategy for probing topological solid-state materials using state-of-the-art photonic cavities, which would avoid the need for attaching contacts, thereby enabling access to electronic properties closer to the pristine topological states.

Abstract: We theoretically consider the inverse Rashba-Edelstein effect (IREE) induced by spin pumping from a ferromagnetic insulator (FI) into a two-dimensional electron gas (2DEG) in which the Rashba and Dresselhaus spin-orbit interactions coexist. We clarify that the magnetization and current in the 2DEG generated by the IREE depend on the resonant frequency of the ferromagnetic resonance (FMR) and azimuth angle of the spontaneous spin polarization of the FI. We further show that the magnetization and current increase substantially as the ratio of magnitudes of Rashba and Dresselhaus spin-orbit interactions approaches unity.

Abstract: We report on the low temperature measurements of the magnetotransport in Si-doped InAs quantum wire in the presence of a charged tip of an atomic force microscope serving as a mobile gate, i.e. scanning gate microscopy (SGM). By altering the carrier concentration with back gate voltage, we transfer the wire through several transport regimes: from residual Coulomb blockade to nonlinear resonance regime, followed by linear resonance regime and, finally, to almost homogeneous diffusion regime. We demonstrate direct relations between patterns measured with scanning gate microscopy and spectra of universal conductance fluctuations. A clear sign of fractal behavior of magnetoconductance dependence is observed for non-linear and linear resonance transport regimes.

Abstract: This paper focuses on investigating high-order harmonic generation (HHG) in graphene quantum dots (GQDs) under intense near-infrared laser fields. To model the GQD and its interaction with the laser field, we utilize a mean-field approach. Our analysis of the HHG power spectrum reveals fine structures and a noticeable enhancement in cutoff harmonics due to the long-range correlations. We also demonstrate the essential role of Coulomb interaction in determining of harmonics intensities and cutoff position. Unlike atomic HHG, where the cutoff energy is proportional to the pump wave intensity, in GQDs the cutoff energy scales with the square root of the field strength amplitude. A detailed time-frequency analysis of the entire range of HHG spectrum is presented using a wavelet transform. The analysis reveals intricate details of the spectral and temporal fine structures of HHG, offering insights into the various HHG mechanisms in GQDs.

Abstract: Gate-defined quantum dots in silicon-germanium heterostructures have become a compelling platform for quantum computation and simulation. Thus far, developments have been limited to quantum dots defined in a single plane. Here, we propose to advance beyond planar systems by exploiting heterostructures with multiple quantum wells. We demonstrate the operation of a gate-defined vertical double quantum dot in a strained germanium double quantum well. In quantum transport measurements we observe stability diagrams corresponding to a double quantum dot system. We analyze the capacitive coupling to the nearby gates and find two quantum dots accumulated under the central plunger gate. We extract the position and estimated size, from which we conclude that the double quantum dots are vertically stacked in the two quantum wells. We discuss challenges and opportunities and outline potential applications in quantum computing and quantum simulation.

Abstract: Spin currents driven by spin-orbit coupling are key to spin torque devices, but determining the proper spin current is highly non-trivial. Here we derive a general quantum-mechanical formula for the intrinsic proper spin current showing that it is a topological quantity, and can be finite even in the gap. We determine the spin-Hall current due to the bulk states of topological insulators both deep in the bulk, where the system is unmagnetized, and near the interface, where a proximity-induced magnetization is present, as well as for low-dimensional spin-3/2 hole systems.

Abstract: The electronic properties of a hybrid system made of single-bilayer graphene structures subjected to a perpendicular magnetic field are studied for the zigzag boundaries of the junction, zigzag-1 (ZZ1) and zigzag-2 (ZZ2). These later examples exhibit different behaviors that have been investigated using the continuum Dirac model. Our results reveal that the conductance depends on the width of bilayer graphene for ZZ1 and shows maxima for ZZ2 as a function of the magnetic field, in contrast to ZZ1. It is found that interfaces have significant impacts on the transmission probability, with the confinement of the ZZ1 boundary being more substantial than that of ZZ2

Abstract: We theoretically study gate-defined one-dimensional channels in planar Ge hole gases as a potential platform for non-Abelian Majorana zero modes. We model the valence band holes in the Ge channel by adding appropriate confinement potentials to the 3D Luttinger-Kohn Hamiltonian, additionally taking into account a magnetic field applied parallel to the channel, an out-of-plane electric field, as well as the effect of compressive strain in the parent quantum well. Assuming that the Ge channel is proximitized by an $s$-wave superconductor (such as, e.g., Al) we calculate the topological phase diagrams for different channel geometries, showing that sufficiently narrow Ge hole channels can indeed enter a topological superconducting phase with Majorana zero modes at the channel ends. We estimate the size of the topological gap and its dependence on various system parameters such as channel width, strain, and the applied out-of-plane electric field, allowing us to critically discuss under which conditions Ge hole channels may manifest Majorana zero modes. Since ultra-clean Ge quantum wells with hole mobilities exceeding one million and mean-free paths on the order of many microns already exist, gate-defined Ge hole channels may be able to overcome some of the problems caused by the presence of substantial disorder in more conventional Majorana platforms.

Abstract: The work of Ahn derives the noise power spectrum of a two-level fluctuator (TLF) in the case that it interacts only with a subregion of a full electron bath and thus is subject to a fluctuating temperature. However, Eq.~(1), which gives the variance of the subbath temperature in terms of the heat capacity, in that work carries the implicit assumption that the heat capacity of this subbath may be taken to be a constant, which is a good approximation at higher temperatures, but breaks down at lower temperatures. We thus extend this work to the case in which the fact that the electronic heat capacity of a two-dimensional electron gas (2DEG) $C_V\propto T$, rather than constant in temperature, is fully taken into account. We show that, at low temperatures, the resulting power spectrum of the noise $S(\omega)\propto e^{-C/T^{3/8}}$, in contrast to $S(\omega)\propto e^{-C'/T^{1/3}}$ as found previously, where $C$ and $C'$ are constants. We also compare the numerical results that one would obtain from the two models and find that our results for $S(\omega)$ can differ from those of Ahn by several orders of magnitude at low temperatures.