Effects of electric and magnetic fields on electronic states and transport of a Hall bar

We study the edge-state band and transport property for a HgTe/CdTe quantum well Hall bar under the combined coupling of a transverse electric field and a perpendicular magnetic field. It is demonstrated that a weak magnetic field can protect one of the two edge states, open or enlarge a gap of the other edge state in the Hall bar. However, an appropriate electric field can remove the gap, restoring the quantum spin Hall effect. Using the scattering matrix method, we study the electronic transport of the system. We find that the electric field can not only make the switch from pure spin-up to spin-down current, but also open or close the edge-state channels in a narrow Hall bar under a weak magnetic field, which provides us with a new way to construct a topological insulator-based spin switch and charge switch.     

A Theoretic Approach to SU(4) Kondo Effect in Carbon Nanotube Quantum Dots

We propose a mean field approach to the transport properties of carbon nanotube quantum dots. Quantum interaction between spin and orbital pseudo-spin degrees of freedom results in an SU(4) Kondo effect at low temperatures. By calculating the chemical potentials and the tunnelling strengths, and hence the spectral functions for different coupling constants and applied magnetic fields, we find that this exotic Kondo effect manifests as a four-peak splitting in the non-linear conductance when an axial magnetic field is applied.     

Spin-flip mesoscopic transport through a quantum dot coupled to carbon nanotube terminals

We investigate mesoscopic transport through a system that consists of a central quantum dot (QD) and two single-wall carbon nanotube (SWCN) leads in the presence of a rotating magnetic field. The spin-flip effect is induced by the rotating magnetic field, and the tunnelling current is sensitively related to the spin-flip effect. We present the calculations of charge and spin current components to show the intimate relations to the SWCN leads. Zeeman effect is important when the applied magnetic field is strong enough. The current characteristics are quite different when the source-drain bias is zero (eV=0) and nonzero (eV≠0). The asymmetric peak and valley of spin current versus gate voltage exhibit Fano resonance. Multi-resonant peaks of spin current versus photon energy ħω reflect the structure of CN quantum wires, as well as the resonant photon absorption and emission effect. The matching-mismatching of channels in the CN leads and QD results in novel spin current structure by tuning the frequency.     

Spin edge helices in a perpendicular magnetic field

We present an exact solution to the problem of the spin edge states in the presence of equal Bychkov-Rashba and Dresselhaus spin-orbit fields in a two-dimensional electron system, restricted by a hard-wall confining potential and exposed to a perpendicular magnetic field. We find that the spectrum of the spin edge states depends critically on the orientation of the sample edges with respect to the crystallographic axes. Such a strikingly different spectral behavior generates new modes of the persistent spin helix-spin edge helices with novel properties, which can be tuned by the applied electric and magnetic fields.     

Generating spin currents in semiconductors with the spin Hall effect

We investigate electrically induced spin currents generated by the spin Hall effect in GaAs structures that distinguish edge effects from spin transport. Using Kerr rotation microscopy to image the spin polarization, we demonstrate that the observed spin accumulation is due to a transverse bulk electron spin current, which can drive spin polarization nearly 40 microns into a region in which there is minimal electric field. Using a model that incorporates the effects of spin drift, we determine the transverse spin drift velocity from the magnetic field dependence of the spin polarization.     

Spin configurations of a carbon nanotube in a nonuniform external potential

We study, theoretically, the ground state spin of a carbon nanotube in the presence of an external potential. We find that when the external potential is applied to a part of the nanotube, its variation changes the single electron spectrum significantly. This, in combination with Coulomb repulsion and the symmetry properties of a finite length armchair nanotube, induces spin flips in the ground state when the external potential is varied. We discuss the possible application of our theory to recent measurements of Coulomb-blocked peaks and their dependence on a weak magnetic field in armchair carbon nanotubes.     

Charge and spin currents tunnelling through a toroidal carbon nanotube side-coupled with a quantum dot

We have investigated the mesoscopic transport through the system with a quantum dot (QD) side-coupled to a toroidal carbon nanotube (TCN) in the presence of spin-flip effect. The coupled QD contributes to the mesoscopic transport significantly through adjusting the gate voltage and Zeeman field applied to the QD. The compound TCN-QD microstructure is related to the separate subsystems, the applied external magnetic fields, as well as the combination of subsystems. The spin current component I_z^s is independent on time, while the spin current components I_x^s and I_y^s evolve with time sinusoidally. The rotating magnetic field induces novel levels due to the spin splitting and photon absorption procedures. The suppression and enhancement of resonant peaks, and semiconductor-metal phase transition are observed by studying the differential conductance through tuning the source-drain bias and photon energy. The magnetic flux induces Aharonov-Bohm oscillation, and it controls the tunnelling behavior due to adjusting the flux. The Fano type of multi-resonant behaviors are displayed in the conductance structures by adjusting the gate voltage V_g and the Zeeman field applied to the QD.     

Study on the competition between density waves, singlet, and triplet pairing superconductivity in organic conductors (TMTSF)2X

We study the effect of dimerization of TMTSF molecules and the effect of magnetic field (Zeeman splitting) on the phase competition in quasi one-dimensional organic superconductors (TMTSF)₂X by applying the random phase approximation method. As for the dimerization effect, we conclude that due to the decrease of the dimerization, which corresponds to applying the pressure and cooling, spin and charge density wave states are suppressed and give way to a superconducting state. As for the magnetic field effect, we find generally that spin-triplet pairing mediated by a coexistence of 2k_F spin and 2k_F charge fluctuations can be strongly enhanced by applying magnetic field rather than triplet pairing due to a ferromagnetic spin fluctuations. Applying the above idea to (TMTSF)₂X compounds, a magnetic field induced singlet-triplet transition is consistent with above mechanism in (TMTSF)₂ClO₄.     

Spin pump effects on the spin current through two coupled quantum dots

We theoretically study the spin pump effects of the rotating magnetic field on the spin current through two coupled quantum dots. Owing to the interdot coupling, two molecular states with different bands can be formed, resulting asymmetric spin current peaks. The possibility of manipulating the spin current is explored by tuning the strength, the frequency, and the direction of the rotating magnetic field. The number and location of the spin current peaks can be controlled by making use of various tunings. Furthermore, the normal 2π period of the spin current with respect to the magnetic flux can be destroyed by the interdot coupling.     

磁场中的拓扑绝缘体边缘态性质

用数值方法研究了拓扑绝缘体薄膜体系在外加垂直磁场 作用下其边缘态的性质. 磁场的加入通过耦合k+eA, 即Peierls势替换关系和 该作用导致的Zeeman交换场体现在哈密顿量中. 考虑窄条圆环状结构的二维InAs/GaSb/AlSb薄膜量子阱材料, 当其处于拓扑非平庸状态, 即量子自旋霍尔态时, 会出现受时间反演对称性保护的两支简并边缘态, 而在垂直磁场的作用下, 时间反演对称性被破坏, 这时能带将形成一条条的朗道能级, 原来简并的两支边缘态也会分开到朗道能级谱线的两侧, 从电子态密度的空间分布情况则可以看到边缘态分别局域在材料的两个边界. 随着磁场的增大, 位于同一边界上的不同 自旋极化的边缘态将出现分离: 一支仍然局域在边缘, 另一支则随外加磁场的增加而有逐渐演化到材料内部的趋势. 文中还计算了同一边界上的两支边缘态之间的散射, 结果表明由于两个边缘态在空间发生分离, 相互之间的散射被很大的压制, 得到了其散射随磁场增加没有明显变化的结论, 所以磁场并不会增强散射过程, 也没有破坏体拓扑材料的性质, 说明了量子自旋霍尔态在没有时间反演对称的情况下也可以有较强的稳定性.     

Magnetoresistance due to edge spin accumulation

Because of spin-orbit interaction, an electrical current is accompanied by a spin current resulting in spin accumulation near the sample edges. Due again to spin-orbit interaction this causes a small decrease of the sample resistance. An applied magnetic field will destroy the edge spin polarization leading to a positive magnetoresistance. This effect provides means to study spin accumulation by electrical measurements. The origin and the general properties of the phenomenological equations describing coupling between charge and spin currents are also discussed.     

Spin-polarization and x-ray magnetic circular dichroism in GaAs

The combination of angular spin momentum with electronics is a promising successor to charge-based electronics. The conduction bands in GaAs may become spin-polarized via optical spin pumping, doping with magnetic ions, or induction of a moment with an external magnetic field. We investigated the spin populations in GaAs with x-ray magnetic circular dichroism for each of these three cases. We find strong anti-symmetric lineshapes at the Ga L₃ edge indicating conduction band spin splitting, with differences in line width and amplitude depending on the source of spin polarization.     

Experimental observation of the spin-Hall effect in a spin–orbit coupled two-dimensional hole gas

Electrically induced ordering and manipulation of electron spins in semiconductors has a number of practical advantages over the established techniques using circularly polarized light sources, external magnetic fields and spin injection from a ferromagnet. The spin-Hall effect utilizes spin–orbit coupling to induce edge spin accumulation in response to a longitudinal electric field which can be applied locally and lead to low energy consumption devices. We study spin accumulation near the edge of a weakly disordered two-dimensional hole gas (2DHG) in a GaAs/AlGaAs heterostructure where the magnitude of the transverse spin current approaches the intrinsic, disorder independent value, in contrast to the impurity dominated regime observed in 3D electron doped systems. In our experiment, the induced spin polarization is detected by the electroluminescence resulting from two p–n junctions bordering the 2DHG channel. When an electric field is applied across the 2DHG channel, a non-zero out-of-plane component of the spin is optically detected. The sign of the spin depends on the direction of the field and is opposite for the two edges, consistent with theory predictions. We also report and analyze an in-plane spin-polarization effect induced in the device by asymmetric electron–hole recombination.     

Coherent electron transport in a helical nanotube

The quantum dynamics of carriers bound to helical tube surfaces is investigated in a thin-layer quantization scheme. By numerically solving the open-boundary Schrödinger equation in curvilinear coordinates, geometric effect on the coherent transmission spectra is analysed in the case of single propagating mode as well as multimode. It is shown that, the coiling endows the helical nanotube with different transport properties from a bent cylindrical surface. Fano resonance appears as a purely geometric effect in the conductance, the corresponding energy of quasibound state is obviously influenced by the torsion and length of the nanotube. We also find new plateaus in the conductance. The transport of double-degenerate mode in this geometry is reminiscent of the Zeeman coupling between the magnetic field and spin angular momentum in quasi-one-dimensional structure.     

Spin Nernst effect in the absence of a magnetic field

We study the spin Nernst effect of a mesoscopic four-terminal cross-bar device with the Rashba spin–orbit interaction (SOI) in the absence of a magnetic field. The interplay between the spin Nernst effect and the seebeck coefficient is investigated for a wide range of the Rashba SOI. When no peaks appeared in the seebeck coefficient, an oscillatory spin Nernst effect still occurs. In addition, the disorder effect on the spin Nernst effect is also studied. We find that the spin Nernst effect can be enhanced up to threefold by disorder. Besides, due to the interface effect, the counter propagating of the charge current to the direction of the temperature gradient is possible for a nonuniform system.     

Sudden Death, Birth and Stable Entanglement in a Two-Qubit Heisenberg XY Spin Chain

Taking the decoherence effect due to population relaxation into account, we investigate the entanglement properties for two qubits in the Heisenberg XY interaction and subject to an external magnetic field. It is found that the phenomenon of entanglement sudden death (ESD) as well as sudden birth (ESB) appear during the evolution process for particular initial states. The influence of the external magnetic field and the spin environment on ESD and ESB are addressed in detail. It is shown that the concurrence, a measure of entanglement, can be controlled by tuning the parameters of the spin chain, such as the anisotropic parameter, external magnetic field, and the coupling strength with their environment. In particular, we find that a critical anisotropy constant exists, above which ESB vanishes while ESD appears. It is also notable that stable entanglement, which is independent of different initial states of the qubits, occurs even in the presence of decoherence.     

Proposal for measuring mechanically equilibrium spin currents in the rashba medium

We demonstrate that an equilibrium spin current in a 2D electron gas with Rashba spin-orbit interaction (Rashba medium) results in a mechanical torque on a substrate near an edge of the medium. If the substrate is a cantilever, the mechanical torque displaces the free end of the cantilever. The effect can be enhanced and tuned by a magnetic field. Observation of this displacement would be an effective method to prove the existence of equilibrium spin currents. The analysis of edges of the Rashba medium demonstrates the existence of localized edge states. They form a 1D continuum of states. This suggests a new type of quantum wire: spin-orbit quantum wire.     

The family of topological Hall effects for electrons in skyrmion crystals

Hall effects of electrons can be produced by an external magnetic field, spin–orbit coupling or a topologically non-trivial spin texture. The topological Hall effect (THE) – caused by the latter – is commonly observed in magnetic skyrmion crystals. Here, we show analogies of the THE to the conventional Hall effect (HE), the anomalous Hall effect (AHE), and the spin Hall effect (SHE). In the limit of strong coupling between conduction electron spins and the local magnetic texture the THE can be described by means of a fictitious, “emergent” magnetic field. In this sense the THE can be mapped onto the HE caused by an external magnetic field. Due to complete alignment of electron spin and magnetic texture, the transverse charge conductivity is linked to a transverse spin conductivity. They are disconnected for weak coupling of electron spin and magnetic texture; the THE is then related to the AHE. The topological equivalent to the SHE can be found in antiferromagnetic skyrmion crystals. We substantiate our claims by calculations of the edge states for a finite sample. These states reveal in which situation the topological analogue to a quantized HE, quantized AHE, and quantized SHE can be found.     

Rectification effect about vacuum separating carbon nanotube bundle predicted by first-principles study

For the molecular spintronics transport systems, we propose that the spin current rectifier can be constructed using the nonmagnetic lead. The proposal is confirmed according to the first-principles study of the transport characteristics of a vacuum separating (15,0)$(15,0)$ carbon nanotube bundle where only one zigzag edge is hydrogenated. The strong rectification effect for spin (charge) current is obtained in the case of the magnetic parallel (anti-parallel) configuration of two zigzag edges. Our investigations indicate that such device can be used as the spin filter and the counterpart of the p–n$p\text{–}n$ junction in the field of molecular electronics.     

Voltage-controllable spin-polarized transport through parallel-coupled double quantum dots with Rashba spin-orbit interaction

A spin device, consisting of parallel-coupled double quantum dots and three normal metal leads, is proposed to realize spin-polarized current without the help of magnetic field and magnetic material. Based on the Keldysh nonequilibrium Green function technique and equation of motion method, the spin-dependent current formula in each lead is derived. It is shown that not only a fully polarized current but also a tunable pure spin current can be obtained by modulating the structure parameters, strength of Rashba spin-orbit interaction and bias voltages properly. It further demonstrates the dependence of the spin-polarized current on the strength of the Rashba spin-orbit interaction.