存在自旋轨道耦合的介观小环中的持续自旋流

文章作者研究了存在自旋轨道耦合的介观小环的平衡态性质.此前人们已经知道,在有磁通穿过的介观小环中,绕环运动的电子会产生一附加的Berry相位而导致持续电流;同样地,在仅有自旋轨道耦合的体系中,电子绕环运动也应当会产生附加的自旋Berry相位,进而驱动持续自旋流.文章作者通过对一个有正常区和自旋轨道耦合区的复合小环的计算,结果表明,无电流伴随的纯持续自旋流的确存在.文章作者指出,这持续自旋流描述真实的自旋运动,并且它能被实验观测.     

Persistent current in a magnetized Rashba ring

We theoretically study the persistent currents flowing in a Rashba quantum ring subjected to the Rashba spin-orbit interaction. By introducing uniform or nonuniform magnetization into the ring, we find that a nonzero persistent charge current circulates in the ring, which stems from the original equilibrium spin current due to the Rashba spin-orbit interaction. Because of broken time reversal symmetry, the two oppositely flowing spin-up and spin-down charge currents of the equilibrium spin current are no longer equal, and so a net persistent charge current can flow in the system. It is also found that the persistent current can be modulated by the Fermi energy, the Rashba spin-orbit interaction strength and the magnetization in the ring. Moreover, the magnetization perpendicular to the ring plane can optimize the current. The persistent current flowing in the ring is a manifestation of the nonzero equilibrium spin current existing in the ring.     

Nonadiabatic Phase and Persistent Currents for System of Spin-1/2 Particles in Presence of Electromagnetic Fields and Spin-Orbit Interaction

We present a comprehensive view and details of calculations on Aharonov-Anandan phase for the charged particles in the external electric and magnetic fields for a nonadiabatic process. We derive, with consideration of a spin-orbit interaction and Zeemann Splitting, the persistent currents as a response to an Aharonov-Casher topological interference effect in one-dimensional mesoscopic ring. We also establish a connection to Berry adiabatic phase with deduced dynamical-nature dependence in the nonadiabatic process. The second quantization representation has also been employed in exhibition of persistent currents in the many-body case.     

Anomalous magnetic response of a quasi-periodic mesoscopic ring in presence of Rashba and Dresselhaus spin-orbit interactions

We investigate the properties of persistent charge current driven by magnetic flux in aquasi-periodic mesoscopic Fibonacci ring with Rashba and Dresselhaus spin-orbitinteractions. Within a tight-binding framework we work out individual state currentstogether with net current based on second-quantized approach. A significant enhancement ofcurrent is observed in presence of spin-orbit coupling and sometimes it becomes orders ofmagnitude higher compared to the spin-orbit interaction free Fibonacci ring. We alsoestablish a scaling relation of persistent current with ring size, associated with theFibonacci generation, from which one can directly estimate current for any arbitrary flux,even in presence of spin-orbit interaction, without doing numerical simulation. Thepresent analysis indeed gives a unique opportunity of determining persistent current andhas not been discussed so far.     

Aharonov-Bohm oscillations with spin: evidence for Berry's phase

We report a study of the Aharonov-Bohm effect, the oscillations of the resistance of a mesoscopic ring as a function of a perpendicular magnetic field, in a GaAs two-dimensional hole system with a strong spin-orbit interaction. The Fourier spectra of the oscillations reveal extra structure near the main peak whose frequency corresponds to the magnetic flux enclosed by the ring. A comparison of the experimental data with results of simulations demonstrates that the origin of the extra structure is the geometric (Berry) phase acquired by the carrier spin as it travels around the ring.     

Effects of geometry and linearly polarized cavity photons on charge and spin currents in a quantum ring with spin-orbit interactions

We calculate the persistent charge and spin polarization current inside a finite-width quantum ring of realistic geometry as a function of the strength of the Rashba or Dresselhaus spin-orbit interaction. The time evolution in the transient regime of the two-dimensional (2D) quantum ring connected to electrically biased semi-infinite leads is governed by a time-convolutionless non-Markovian generalized master equation. The electrons are correlated via Coulomb interaction. In addition, the ring is embedded in a photon cavity with a single mode of linearly polarized photon field, which is polarized either perpendicular or parallel to the charge transport direction. To analyze carefully the physical effects, we compare to the analytical results of the toy model of a one-dimensional (1D) ring of non-interacting electrons with spin-orbit coupling. We find a pronounced charge current dip associated with many-electron level crossings at the Aharonov-Casher phase ΔΦ = π, which can be disguised by linearly polarized light. Qualitative agreement is found for the spin polarization currents of the 1D and 2D ring. Quantitatively, however, the spin polarization currents are weaker in the more realistic 2D ring, especially for weak spin-orbit interaction, but can be considerably enhanced with the aid of a linearly polarized electromagnetic field. Specific spin polarization current symmetries relating the Dresselhaus spin-orbit interaction case to the Rashba one are found to hold for the 2D ring, which is embedded in the photon cavity.     

Quantum interference of electrons in a ring: tuning of the geometrical phase

We calculate the oscillations of the dc conductance across a mesoscopic ring, simultaneously tuned by applied magnetic and electric fields orthogonal to the ring. The oscillations depend on the Aharonov-Bohm flux and of the spin-orbit coupling. They result from mixing of the dynamical phase, including the Zeeman spin splitting, and of geometric phases. By changing the applied fields, the geometric phase contribution to the conductance oscillations can be tuned from the adiabatic (Berry) to the nonadiabatic (Ahronov-Anandan) regime. To model a realistic device, we also include nonzero backscattering at the connection between ring and contacts, and a random phase for electron wave function, accounting for dephasing effects.     

Spin-polarized current in a Rashba ring pumped by a microwave field

We report a theoretical study on generation of a spin polarized charge current with arbitrary spin polarization, including the fully-spin-polarized current. In a two-terminal mesoscopic ring device, the Rashba spin-orbit coupling (RSOC) is considered as well as a microwave field applied on one of arms of the ring. It is shown that at zero external bias a spin current can be produced in addition to the usual charge current pumped by the microwave field, which is attributed to the the quantum interference effect of the RSOC induced spin precession phase. By varying the system parameters such as the microwave frequency and the RSOC strength, not only the magnitude but also the direction of the spin current can be efficiently controlled, moreover, the spin-polarization degree of the charge current can readily be tuned by these system parameters in the range [-1,1]. Since all the parameters can be controlled electrically in our study, the proposed device may shed light on the possibility of an all-electrical generation and tuning of a spin-polarized current in the field of the spintronics.     

Characteristics of persistent spin current components in a quasi-periodic Fibonacci ring with spin–orbit interactions: Prediction of spin–orbit coupling and on-site energy

In the present work we investigate the behavior of all three components of persistent spin current in a quasi-periodic Fibonacci ring subjected to Rashba and Dresselhaus spin–orbit interactions. Analogous to persistent charge current in a conducting ring where electrons gain a Berry phase in presence of magnetic flux, spin Berry phase is associated during the motion of electrons in presence of a spin–orbit field which is responsible for the generation of spin current. The interplay between two spin–orbit fields along with quasi-periodic Fibonacci sequence on persistent spin current is described elaborately, and from our analysis, we can estimate the strength of any one of two spin–orbit couplings together with on-site energy, provided the other is known.     

Mesoscopic effects in adiabatic spin pumping

We show that temporal shape modulations (pumping) of a quantum dot in the presence of spin-orbital coupling lead to a finite dc spin current. Depending on the strength of the spin-orbit coupling, the spin current is polarized perpendicular to the plane of the two-dimensional electron gas, or has an arbitrary direction subject to mesoscopic fluctuations. We analyze the statistics of the spin and charge currents in the adiabatic limit for the full crossover from weak to strong spin-orbit coupling.     

特殊结构的多臂量子环的持续电流

提出了中臂弯曲的多臂量子环模型,且是上臂最短和下臂最长的不等臂量子环.研究发现：总磁通为零时,持续电流随半导体环增大发生非周期性振荡,并与电极的磁矩方向及隧穿电子的自旋方向相关,下臂因为最长而获得最小的平均持续电流.AB磁通增强时,持续电流会发生周期性振荡,各臂之间明显出现相互制约的现象.各臂持续电流之间的差异与臂长和磁通分布相关,Rashba自旋轨道耦合具有改变持续电流相位和相位差的效应.在一定条件下,两种波函数所对应的持续电流是可分离的. 关键词： 多臂量子环 持续电流 Rashba自旋轨道耦合     

The persistent current in an Aharonov－Bohm ring with a side-coupled quantum dot

We have investigated the persistent current in a mesoscopic ring with a side-coupled quantum dot. The problems are probed by using the one-impurity Anderson Hamiltonian and are treated with the slave boson mean field theory. It is shown that the persistent current in this system has the spin fluctuations, and the charge transfers between the two subsystems are suppressed in the limit of Δ/T_K^0《1. The minimum value of the persistent current for ξ_K/L=5 of the odd parity system provides an opportunity to detect the Kondo screening cloud.     

Charge current driven by spin dynamics in disordered Rashba spin-orbit system

Pumping of charge current by spin dynamics in the presence of the Rashba spin-orbit interaction is theoretically studied. Considering a disordered electron, the exchange coupling and spin-orbit interactions are treated perturbatively. It is found that the dominant current induced by spin dynamics is interpreted as a consequence of the conversion from spin current via the inverse spin Hall effect. We also find that the current has an additional component from a fictitious conservative field. The results are applied to the case of a moving domain wall.     

Effective period potential in a hybrid mesoscopic ring with Rashba spin–orbit interaction

We investigate theoretically the persistent charge current (PCC) and pure spin current (PSC) in a hybrid mesoscopic ring with Rashba spin–orbit interaction (RSOI). The PCC and PSC surviving in the ring would experience a periodic potential formed by the band offset of the constituent materials. Similarly, an effective tunnel barrier can be introduced by a region with different RSOI strength. This provides us a convenient way to manipulate the periodic potential by changing the RSOI strength through an electric field. With the increment of the RSOI strength, the PCC is suppressed, while the PSC presents an oscillatory pattern changing from negative to positive.     

Optical out-of-plane spin polarization and charge conductivities in spin-orbit-coupled systems in the presence of an in-plane magnetic field

Out-of-plane spin and charge responses to the terahertz field for a clean two-dimensional electron gas with a Rashba spin-orbit interaction in the presence of an in-plane magnetic field are studied. We show that the characteristic optical spectral behavior is remarkably different from that of the system in the absence of in-plane magnetic fields. It is found that the optical spin polarization normal to the plane is nonzero even for this clean system, in sharp contrast to the static case. Due to the combined effect of spin-orbit coupling and in-plane magnetic field, both diagonal and off-diagonal components of optical charge conductivity tensor are nonvanishing. It is indicated that one can control the spin polarization and the optical current by adjusting the optical frequency. In addition, the out-of-plane spin polarization and conductivities strongly rely on the direction of the external magnetic field. Nevertheless, they meet different angle-dependent relations. This dynamical out-of-plane spin polarization could be measured by the time-resolved Kerr rotation technique.     

Spin Hall current driven by quantum interferences in mesoscopic Rashba rings

We propose an all-electrical nanostructure where pure spin current is induced in the transverse voltage probes attached to a quantum-coherent ballistic one-dimensional ring when unpolarized charge current is injected through its longitudinal leads. Tuning of the Rashba spin-orbit coupling in a semiconductor heterostructure hosting the ring generates quasiperiodic oscillations of the predicted spin-Hall current due to spin-sensitive quantum-interference effects caused by the difference in the Aharonov-Casher phase accumulated by opposite spin states. Its amplitude is comparable to that of the spin-Hall current predicted for finite-size (simply connected) two-dimensional electron gases, while it gets reduced gradually in wide two-dimensional rings or due to spin-independent disorder.     

一维介观环中持续电流的电子-声子相互作用非经典效应

基于声子相干态功效和计及声子压缩态非经典效应,研究了电子-磁振子和电子-声子相互作用对一维介观环持续电流的影响. 与自由环比较,由于电子-磁振子相互作用,持续电流的振幅呈现指数减小. 对于正常态电子,电子-声子相互作用导致持续电流以Debye-Waller(D-W)因子衰减.但是计入跳步电子-单声子相干态关联效应导致系统本征态能量大幅度下降,从而持续电流I_n有大幅度增加.另一方面计入双声子相干态行为,由于声子压缩态效应压缩电子-相干(态)声子弹性散射行为,导致电子绕环运 关键词： 持续电流 电子-声子相互作用 声子相干态 声子压缩态效应     

Controllable Spin-Polarized Transport in a Three-Terminal Model

By means of the Keldysh Green's function method, we investigate the spin-polarized electron transport in a three-terminal device, which is composed of three normal metal leads and two serially-coupled quantum dots (QDs). The Rashba spin-orbit interaction (RSOI) is also considered in one of the QDs. We show that the spin-polarized charge current with arbitrary spin polarization can be obtained because of the quantum spin interference effect arising from the Rashba spin precession phase, and it can be modulated by the system parameters such as the applied external voltages, the RSOI strength, the QD levels, as well as the dot-lead coupling strengths. Moreover, a fully spin-polarized current or a pure spin current without any accompanying charge current can also be controlled to flow in the system. Our findings indicate that the proposed model can serve as an all-electrical spin device in spintronics field.     

Probing entanglement and nonlocality of electrons in a double-dot via transport and noise

Addressing the feasibility of quantum communication with electrons we consider entangled spin states of electrons in a double-dot which is weakly coupled to leads. We show that the entanglement of two electrons in the double-dot can be detected in mesoscopic transport and noise measurements. In the Coulomb blockade and cotunneling regime the singlet and triplet states lead to phase-coherent current and noise contributions of opposite signs and to Aharonov-Bohm and Berry phase oscillations. These oscillations are a genuine two-particle effect and provide a direct measure of nonlocality in entangled states. We show that the ratio of zero-frequency noise to current is equal to the electron charge.     

Geometric correlations and breakdown of mesoscopic universality in spin transport

We construct a unified semiclassical theory of charge and spin transport in chaotic ballistic and disordered diffusive mesoscopic systems with spin-orbit interaction. Neglecting dynamic effects of spin-orbit interaction, we reproduce the random matrix theory results that the spin conductance fluctuates universally around zero average. Incorporating these effects into the theory, we show that geometric correlations generate finite average spin conductances, but that they do not affect the charge conductance to leading order. The theory, which is confirmed by numerical transport calculations, allows us to investigate the entire range from the weak to the previously unexplored strong spin-orbit regime, where the spin rotation time is shorter than the momentum relaxation time.