Xing-Tao An

Spin-polarized current induced by a local exchange field in a silicene nanoribbon

A mechanism to generate a spin-polarized current in a two-terminal zigzag silicene nanoribbon is predicted. When a weak local exchange field that is parallel to the surface of silicene is applied on one of edges of the silicene nanoribbon, a gap is opened in the corresponding edge states but another pair of gapless edge states with opposite spin are still protected by the time-reversal symmetry. Hence, a spin-polarized current can be induced in the gap opened by the local exchange field in this two-terminal system. What is important is that the spin-polarized current can be obtained even in the absence of Rashba spin–orbit coupling and in the case of the very weak exchange field. That is to say, the mechanism to generate the spin-polarized currents can be easily realized experimentally. We also find that the spin-polarized current is insensitive to weak disorder.

Quantum spin Hall effect induced by electric field in silicene

We investigate the transport properties in a zigzag silicene nanoribbon in the presence of an external electric field. The staggered sublattice potential and two kinds of Rashba spin-orbit couplings can be induced by the external electric field due to the buckled structure of the silicene. A bulk gap is opened by the staggered potential and gapless edge states appear in the gap by tuning the two kinds of Rashba spin-orbit couplings properly. Furthermore, the gapless edge states are spin-filtered and are insensitive to the non-magnetic disorder. These results prove that the quantum spin Hall effect can be induced by an external electric field in silicene, which may have certain practical significance in applications for future spintronics device.

Hydrogenic impurities in parabolic quantum-well wires in a magnetic field

The properties of a hydrogenic impurity in a parabolic GaAs quantum-well wire in the presence of the magnetic field are investigated using the finite-difference method within the quasi-one-dimensional effective potential model. The magnetic effects on the energies and binding energies of the ground and lowest excited states of a hydrogenic impurity in a parabolic GaAs quantum-well wire are studied for various parabolic potentials. The calculated results indicate that the interplay of the spatial confinement and the magnetic confinement of electrons in the quantum-well wires leads to complicated binding energies of the hydrogenic impurity, and high magnetic fields significantly increase the binding energies of the hydrogenic impurity in the case of weak spatial confinement.

Interplay between edge and bulk states in silicene nanoribbon

We investigate the interplay between the edge and bulk states, induced by the Rashba spin-orbit coupling, in a zigzag silicene nanoribbon in the presence of an external electric field. The interplay can be divided into two kinds, one is the interplay between the edge and bulk states with opposite velocities, and the other is that with the same velocity direction. The former can open small direct spin-dependent subgaps. A spin-polarized current can be generated in the nanoribbon as the Fermi energy is in the subgaps. While the later can give rise to the spin precession in the nanoribbon. Therefore, the zigzag silicene nanoribbon can be used as an efficient spin filter and spin modulation device.

Aharonov–Bohm ring with a side-coupled quantum dot array as a spin switch

We study the spin polarization and the spin accumulation in an Aharonov–Bohm ring structure, in which a quantum dot (QD) array is side-coupled to one arm of the ring and the Rashba spin-orbit interaction exists in the other. The device can be as a spin switch by decreasing the tunneling coupling between the QD array and the ring. Moreover, we find that the spin polarization and the spin accumulation in the QD are affected by the number of the QDs in the QD array and can be controlled by the strength of the Rashba spin-orbit interaction and the bias on the Aharonov–Bohm ring.We study the spin polarization and the spin accumulation in an Aharonov–Bohm ring structure, in which a quantum dot (QD) array is side-coupled to one arm of the ring and the Rashba spin-orbit interaction exists in the other. The device can be as a spin switch by decreasing the tunneling coupling between the QD array and the ring. Moreover, we find that the spin polarization and the spin accumulation in the QD are affected by the number of the QDs in the QD array and can be controlled by the strength of the Rashba spin-orbit interaction and the bias on the Aharonov–Bohm ring.

Valley polarization in graphene-silicene-graphene heterojunction in zigzag nanoribbon

Considering the difference of energy bands in graphene and silicene, we put forward a new model of the graphene-silicene-graphene (GSG) heterojunction. In the GSG, we study the valley polarization properties in a zigzag nanoribbon in the presence of an external electric field. We find the energy range associated with the bulk gap of silicene has a valley polarization more than 95%. Under the protection of the topological edge states of the silicene, the valley polarization remains even the small non-magnetic disorder is introduced. These results have certain practical significance in applications for future valley valve.

Electron delocalization in gate-tunable gapless silicene

The application of a perpendicular electric field can drive silicene into a gapless state, characterized by two nearly fully spin-polarized Dirac cones owing to both relatively large spin-orbital interactions and inversion symmetry breaking. Here we argue that since intervalley scattering from nonmagnetic impurities is highly suppressed by time-reversal symmetry, the physics should be effectively single-Dirac-cone like. Through numerical calculations, we demonstrate that there is no significant backscattering from a single impurity that is nonmagnetic and unit-cell uniform, indicating a stable delocalized state. This conjecture is then further confirmed from a scaling of conductance for disordered systems using the same type of impurities.

Even-odd parity oscillations in spin polarization of a quantum dot array

We propose a quantum dot (QD) array device which contains a bridge coupling between two leads. Due to the quantum interference and the Rashba spin-orbit interaction, the spin of the electrons through the device is polarized. Moreover, we find odd-even parity oscillations of spin polarization of the electrons through the system. We study the spin accumulations in every QD and find that there is difference between the spin accumulations of the QDs directly and indirectly coupling to leads. These results demonstrate that the bridge coupling is a flexible and feasible way to manipulate the electron spin of the QD array.

Measurable spin-polarized current in two-dimensional topological insulators

We propose a simple method for generating a spin-polarized current in a two-dimensional topological insulator. As z-component magnetic impurities exist on one edge of the Kane-Mele model, a subgap is opened in the corresponding pair of edge states, but another pair of gapless edge states is still protected by the time reversal symmetry. Thus the conductance plateau with the value e(2)/h in the subgap corresponds to a single-edge and spin-polarized current. We also find that the spin-polarized current is insensitive to weak non-magnetic disorder. This mechanism for generating spin-polarized currents is independent of the concrete theoretical model and can be generalized to two-dimensional topological insulators, such as HgTe/CdTe quantum wells and silicene nanoribbons.

Shot noise of spin-dependent currents in ferromagnetic/semiconductor/ferromagnetic heterojunctions

Using the scattering matrix method, we investigate the spin-dependent conductance and the shot noise of the multichannel ferromagnetic/semiconductor/ferromagnetic heterojunctions in the presence of the spin-orbit coupling (SOC). We find that spin-up and spin-down electrons have different contributions to the conductance and the shot noise. The rounded quantum plateaus of the conductance appear when the length of the semiconductor is made shorter. As the number of conducting channels in the system increases, the shot noise power oscillates and the Fano factor is increasingly suppressed. We also find that interband mixing due to the SOC brings significant effects on the spin-dependent conductance and the shot noise.