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

Abstract: We study a dynamical spin response of surface Majorana fermions in a topological superconductor-magnet hybrid under microwave irradiation. We find a method to toggle between dissipative and non-dissipative Majorana Ising spin dynamics by adjusting the external magnetic field angle and the microwave frequency. This reflects the topological nature of the Majorana fermions, enhancing the Gilbert damping of the magnet, thereby, providing a detection method for the Majorana Ising spins. Our findings illuminate a magnetic probe for Majorana fermions, paving the path to innovative spin devices.

Abstract: The quantum anomalous Hall effect (QAHE) is a topological state of matter with a quantized Hall resistance. It has been observed in some two-dimensional insulating materials such as magnetic topological insulator films and twisted bilayer graphene. These materials are insulating in the bulk, but possess chiral edge states carrying the edge current around the systems. Here we discover a metallic QAHE in a topological insulator film with magnetic sandwich heterostructure, in which the Hall conductance is quantized to $e^{2}/h$, but the longitudinal conductance remains finite. This effect is attributed to the existence of a pair of massless Dirac cones of surface fermions, with each contributing half of the Hall conductance due to quantum anomaly. It is not characterized by a Chern number and not associated to any chiral edge states. Our study offers novel insights into topological transport phenomena and topological metallic states of matter.

Abstract: We measure electron transport through point contacts in an electron gas in AlGaAs/GaAs heterostructures and graphene for a range of temperatures, magnetic fields and electron densities. We find a magnetoconductance peak around B = 0. With increasing temperature, the width of the peak increases monotonically, while its amplitude first increases and then decreases. For GaAs point contacts the peak is particularly sharp at relatively low temperatures $T\approx$1.5 K: the curve rounds on a scale of few tens of $\mu$T hinting at length scales of several millimeters for the corresponding scattering processes. We propose a model based on the transition between different transport regimes with increasing temperature: from ballistic transport to few electron-electron scatterings to hydrodynamic superballistic flow to hydrodynamic Poiseuille-like flow. The model is in qualitative and, in many cases, quantitative agreement with the experimental observations.

Abstract: The interplay between strong correlations and topology can lead to the emergence of intriguing quantum states of matter. One well-known example is the factional quantum Hall effect, where exotic electron fluids with fractional charge excitations form in partially filled landau levels. The emergence of topological moir\'e flat bands provides exciting opportunities to realize the lattice analogs of both the integer and fractional quantum Hall states without the need for an external magnetic field. These states are known as the integer and fractional quantum anomalous Hall (IQAH and FQAH) states. Here, we present direct transport evidence of the existence of both IQAH and FQAH states in twisted bilayer MoTe2 (AA stacked). At zero magnetic field, we observe well-quantized Hall resistance of h/e2 around moir\'e filling factor {\nu} = -1 (corresponding to one hole per moir\'e unit cell), and nearly-quantized Hall resistance of 3h/2e2 around {\nu} = -2/3, respectively. Concomitantly, the longitudinal resistance exhibits distinct minima around {\nu} = -1 and -2/3. The application of an electric field induces topological quantum phase transition from the IQAH state to a charge transfer insulator at {\nu} = -1, and from the FQAH state to a generalized Wigner crystal state, further transitioning to a metallic state at {\nu} = -2/3. Our study paves the way for the investigation of fractional charge excitations and anyon statistics at zero magnetic field based on semiconductor moir\'e materials.

Abstract: The tunneling process, a prototypical phenomenon of nonperturbative dynamics, is a natural consequence of photocarrier generation in materials irradiated by a strong laser. Common treatments for Zener tunneling are based on a one-body problem with a field-free electronic structure. In a literature (Ikemachi et al., Phys. Rev. A 98, 023415 (2018)), a characteristic of gap shrinking or excitation can occur due to the electron-hole interaction for slow and strong time-varying electric fields. We have developed a theoretical framework called the quasi-Hartree-Fock (qHF) method to enable a more flexible imitation of the electronic structures and electron-hole attraction strength of materials compared to the original Hartree-Fock method. In the qHF framework, band gap, reduced effective mass, and electron-hole interaction strength can be independently selected to reproduce common crystals. In this study, we investigate the effect of electron-hole attraction on Zener tunneling subjected to a DC electric field for four different systems using the qHF method. Our findings demonstrate that the electron-hole attraction promotes the tunneling rates in all four material systems assumed as examples. Specifically, systems that have a strong electron-hole interaction show a few factor enhancements for tunneling rates under DC fields, while systems with a weak interaction show higher enhancements of a few tens of percent.