We study nonlocal resistance in an H-shaped two-dimensional HgTe/CdTe quantum well consist of injector and detector, both of which can be tuned in the quantum spin Hall or metallic spin Hall regime. Because of strong spin-orbit interaction, there alw
ays exist spin Hall effect and the nonlocal resistance in HgTe/CdTe quantum well. We find that when both detector and injector are in the quantum spin Hall regime, the nonlocal resistance is quantized at $0.25frac{h}{e^2}$, which is robust against weak disorder scattering and small magnetic field. While beyond this regime, the nonlocal resistance decreases rapidly and will be strongly suppressed by disorder and magnetic field. In the presence of strong magnetic field, the quantum spin Hall regime will be switched into the quantum Hall regime and the nonlocal resistance will disappear. The nonlocal signal and its various manifestation in different hybrid regimes originate from the special band structure of HgTe/CdTe quantum well, and can be considered as the fingerprint of the helical quantum spin Hall edge states in two-dimensional topological insulator.
We report on the injection of quantized pure spin current into quantum conductors. In particular, we propose an on demand single spin source generated by periodically varying the gate voltages of two quantum dots that are connected to a two dimension
al topological insulator via tunneling barriers. Due to the nature of the helical states of the topological insulator, one or several {it spin pair}s can be pumped out per cycle giving rise to a pure quantized alternating spin current. Depending on the phase difference between two gate voltages, this device can serve as an on demand single spin emitter or single charge emitter. Again due to the helicity of the topological insulator, the single spin emitter or charge emitter is dissipationless and immune to disorders. The proposed single spin emitter can be an important building block of future spintronic devices.
We study the nature of the disorder-induced quantized conductance, i.e., the phenomena of topological Anderson insulator (TAI) induced in HgTe/CdTe semiconductor quantum well. The disorder effect in several different systems where anomalous Hall effe
ct exist, is numerically studied using the tight-binding Hamiltonian. It is found that the TAI phenomena also occur in the modified Dirac model where the quadratic corrections $k^2sigma_z$ is included and electron-hole symmetry is kept. It also occurs in the graphene system with the next nearest-neighbor coupling and staggered sublattice potential. Comparison between the localization lengths of the 2D ribbon and 2D cylinder clearly reveals the topological nature of this phenomena. Furthermore, analysis on the local current density in anomalous quantum Hall systems where the TAI phenomena can or can not arise reveals the nature of TAI phenomena: the bulk state is killed drastically and only the robust edge state survives in a moderate disorder. When the edge state is robust enough to resist the strong disorder that can completely kills the bulk state, TAI phenomena arise.
It is known that the parity of reflection amplitude can either be even or odd under the mirror operation. Up to now, all the parities of reflection amplitude in the one-mode energy region are even under the mirror operation. In this paper, we give an
example of odd parity for Andreev reflection (AR) in a three-terminal graphene-supercondutor hybrid systems. We found that the parity is even for the Andreev retroreflection (ARR) and odd for specular Andreev reflection (SAR). We attribute this remarkable phenomenon to the distinct topology of the band structure of graphene and the specular Andreev reflection involving two energy bands with different parity symmetry. As a result of odd parity of SAR, the SAR probability of a four-terminal system with two superconducting leads (two reflection interfaces) can be zero even when the system is asymmetric due to the quantum interference of two ARs.
