Violation of the Fluctuation-Dissipation Theorem and Heating Effects in the Time-Dependent Kondo Model

The fluctuation-dissipation theorem (FDT) plays a fundamental role in understanding quantum many-body problems. However, its applicability is limited to equilibrium systems and it does in general not hold in nonequilibrium situations. This violation of the FDT is an important tool for studying nonequilibrium physics. In this paper we present results for the violation of the FDT in the Kondo model where the impurity spin is frozen for all negative times, and set free to relax at positive times. We derive exact analytical results at the Toulouse point, and results within a controlled approximation in the Kondo limit, which allow us to study the FDT violation on all time scales. A measure of the FDT violation is provided by the effective temperature, which shows initial heating effects after switching on the perturbation, and then exponential cooling to zero temperature as the Kondo system reaches equilibrium.     

Spin Accumulation in a Rashba Nanoribbon

We investigate theoretically the spin accumulation in a Rashba spin-orbit coupling （SOC） nanoribbon nonadiabatically connected to a normal conductor. Both the nanoribbon and conductor are described by a hard-wall confining potential. Using the scattering matrix approach within the effective free-electron approximation, we have calculated the out-of-plane spin accumulation in the nanoribbon. It is found that the spin accumulation shifts toward the two edges of nanoribbon with the increasing of propagation modes. Specifically, as the Rashba SOC strength increases the spin accumulation in the nanoribbon will be enhanced and this result may suggest us a simple method to control the spin accumulation of the system by Rashba SOC strength.     

Spin-Tunneling Time in Ferromagnetic/Semiconductor/Ferromagnetic Three-Terminal Heterojunction in the Presence of Rashba Spin-Orbit Coupling

We study theoretically the transmission coefficients and the spin-tunneling time in ferromagnetic/semiconductor/ferromagnetic three-terminal heterojunction in the presence of Rashba spin-orbit interaction, in which onedimensional quantum waveguide theory is developed and applied. Based on the group velocity concept and the particle current conservation principle, we calculate the spin-tunneling time as the function of the intensity of Rashba spinrblt coupling and the length of the semiconductor. We find that as the length of the semiconductor increases, the spintunneling time does not increase linearly but shows behavior of slight oscillation, i;brthermore, with the increasing of the soin-orbit coupling, the spin-tunneling time increases.     

Spin-Tunneling Time in Ferromagnetic/Semiconductor/Ferromagnetic Three-Terminal Heterojunction in the Presence of Rashba Spin-Orbit Coupling

We study theoretically the transmission coefficients and the spin-tunneling time in ferromagnetic/semiconductor/ferromagnetic three-terminal heterojunction in the presence of Rashba spin-orbit interaction, in which onedimensional quantum waveguide theory is developed and applied. Based on the group velocity concept and the particle current conservation principle, we calculate the spin-tunneling time as the function of the intensity of Rashba spin-orbit coupling and the length of the semiconductor. We find that as the length of the semiconductor increases, the spintunneling time does not increase linearly but shows behavior of slight oscillation. Furthermore, with the increasing of the spin-orbit coupling, the spin-tunneling time increases.     

Spin Accumulation in a Rashba Nanoribbon

We investigate theoretically the spin accumulation in a Rashba spin-orbit coupling (SOC) nanoribbon nonadiabatically connected to a normal conductor. Both the nanoribbon and conductor are described by a hard-wall confining potential. Using thescattering matrix approach within the effective free-electron approximation, we have calculated the out-of-plane spin accumulation in the nanoribbon. It is found that the spin accumulation shifts toward the two edges of nanoribbon with the increasing of propagation modes. Specifically, as the Rashba SOC strength increases the spin accumulation in the nanoribbon will be enhanced and this result may suggest us a simple method to control the spin accumulation of the system by Rashba SOC strength.     

Inverse Spin Hall Effect in Two-Terminal Device with Rashba Spin-Orbit Coupling

We report a theoretic study on the inverse spin-Hall effect （ISHE） in a two-terminal nano-device that consists of a two-dimensional electron gas （2DEG） with Rashba spin-orbit coupling （RSOC） and two ideal leads. Based on a two-site toy model and Keldysh Green＇s function method, we derive an analytic result of ISHE, which shows clearly that a nonzero transverse charge current stems from the combined effect of the RSOC, the spin bias, and its spin polarization direction in spin space. Our further numerical calculations in a larger system other than two-site lattice model demonstrate that the transverse charge current, dependent on the strength of the RSOC, the Fermi energy of the system, as well as the system size, can exhibit oscillating behavior and even reverse its sign due to Rashba spin precession. These properties may be helpful for eficient detection of the spin current （spin bias） by measuring the transverse charge current in a spin-orbital coupling system.     

自旋轨道耦合引起的电偶极矩

文章介绍了由于自旋轨道耦合导致的电子的电偶极矩在自旋电子学理论中的基本意义.研究发现,该电偶极矩与自旋流张量的反对称部分直接相关.它不仅直接导致可观测的电磁学效应,而且与电子在电场受到的力以及力矩有关,从而为自旋的电子学操控提供了可能.     

Transport polygonal properties in a multi-terminal regular quantum ring with Rashba spin-orbit coupling

<正>Transport properties in a multi-terminal regular polygonal quantum ring with Rashba spin-orbit coupling(SOC) are investigated analytically using quantum networks and the transport matrix method.The results show that conductances remain at exactly the same values when the output leads are located at axisymmetric positions.However,for the nonaxisymmetrical case,there is a phase difference between the upper and lower arm,which leads to zero conductances appearing periodically.An isotropy of the conductance is destroyed by the Rashba SOC effect in the axisymmetric case. In addition,the position of zero conductance is regulated with the strength of the Rashba SOC.     

Spin-dependent transport in four-terminal rings in the presence of the Rashba spin-orbit coupling

Spin dependent transport in one-dimensional four-terminal rings (FTRs) is investigated in the presence of the Rashba spin-orbit coupling (RSOC). In the absence of the RSOC, the conductances as a function of the electron wave vector show resonant behavior for symmetrical configurations. For asymmetrical configurations, the conductances show peculiar zero-conductance resonances, and two kinds of conductance zeros have been found. In the presence of the RSOC, the original conductance zeros disappear as new conductance zeros are generated. Moreover, two kinds of symmetry relations have been found in the conductances, the spin dependent conductances and the spin polarizations. For the FTRs with axial or central symmetry, the phase-locking effect has been found in terminal 2, where there is no spin polarization. Under a weak magnetic field, the Zeeman term is treated by a perturbation, and it is found that the Zeeman effect is obvious for weak RSOC.     

The Effect of Spin-Orbit Coupling and Spin-Spin Coupling of Compact Binaries on Chaos

There are spin-orbit interaction and spin-spin interaction in a generic post-Newtonian Lagrangian formu-lation of comparable mass spinning compact binaries. The spin-orbit coupling or the spin-spin coupling plays a quite important role in changing the evolution of the system and may sometime cause chaotic behavior. How do the two types of couplings exert together any influences on chaos in this formulation? To answer it, we simply take the Lagrangian formulation of a special binary system, including the Newtonian term and the leading-order spin-orbit and spin-spin couplings. The key to this question can be found from a Hamiltonian formulation that is completely identical to the Lagrangian formulation. If the Lagrangian does not include the spin-spin coupling, its equivalent Hamiltonian has an additional term (i.e. the next-order spin-spin coupling) as well as those terms of the Lagrangian. The spin-spin coupling rather than the spin-orbit coupling makes the Hamiltonian typically nonintegrable and probably chaotic when two objects spin. When the leading-order spin-spin coupling is also added to the Lagrangian, it still appears in the Hamiltonian. In this sense, the total Hamiltonian contains the leading-order spin-spin coupling and the next-order spin-spin coupling, which have different signs. Therefore, the chaos resulting from the spin-spin interaction in the Legrangian formulations is somewhat weakened by the spin-orbit coupling.     

Chaotic Cyclotron and Hall Trajectories Due to Spin-Orbit Coupling

It is demonstrated that the synergistic effect of a gauge field, Rashba spin-orbit coupling (SOC), and Zeeman splitting can generate chaotic cyclotron and Hall trajectories of particles. The physical origin of the chaotic behavior is that the SOC produces a spin-dependent (so-called anomalous) contribution to the particle velocity and the presence of Zeeman field reduces the number of integrals of motion. By using analytical and numerical arguments, the conditions of chaos emergence are studied and the dynamics both in the regular and chaotic regimes is reported. The critical dependence of the dynamic patterns (such as the chaotic regime onset) on small variations in the initial conditions and problem parameters, that is the SOC and/or Zeeman constants, is observed. The transition to chaotic regime is further verified by the analysis of phase portraits as well as Lyapunov exponents spectrum. The considered chaotic behavior can occur in solid state systems, weakly relativistic plasmas, and cold atomic gases with synthetic gauge fields and spin-related couplings.     

Two Anderson impurities in the framework of the non-crossing approximation: Spectral functions and spin correlations

Using a novel slave boson mapping, which was recently proposed by us for two Anderson impurities embedded in a metal, we present here spectral functions for all pseudoparticles involved in the mapping. These were numerically calculated in the framework of the Non-Crossing Approximation for different temperatures and inter-impurity distances, both for the degenerate (N_f = 6) and nondegenerate (N_f = 2) models. For the nondegenerate case, temperature dependencies of the partial occupancies and the spin-spin correlation function suggest that an antiferromagnetic (AFM)-like ground state is formed at suitable inter-impurity separation. By contrast, Kondo quenching of the impurity moments takes place for the isotropic N_f = 6 case even when a direct AFM interaction is introduced in order to amplify the indirect (inherent) one. Numerical calculations are in agreement with our previous prediction [Phys. Rev. B 47, 14,297 (1993)] that no AFM-like ground state can appear for two isotropic degenerate impurities.     

Persistent Spin Current in a Hard-Wall Confining Quantum Wire with Weak Dresselhaus Spin-Orbit Coupling

We investigate theoretically the spin current in a quantum wire with weak Dresselhaus spin-orbit coupling connected to two normal conductors. Both the quantum wire and conductors are described by a hard-wall confining potential. Using the electron wave-functions in the quantum wire and a new definition of spin current, we have calculated the elements of linear spin current density j^Ts,xi and j^Ts,yi（i=x, y, z）. We find that the elements j^Ts,xx and j^Ts,yy have a antisymmetrical relation and the element j^Ts,yz has the same amount levelas j^Ts,xx and j^Ts,yy. We also find a net linear spin current density, which has peaks at the center of quantum wire. The net linear spin current can induce a linear electric field, which may imply a way of spin current detection.     

A pure spin-current injector of semiconductor quantum dots with Andreev reflection and Rashba spin-orbit coupling

We propose a four-terminal device consisting of two parallel quantum dots with Rashba spin-orbit interaction (RSOI), coupled to two side superconductor leads and two common ferromagnetic leads, respectively. The two ferromagnetic leads and two quantum dots form a ring threaded by Aharonov-Bohm (AB) flux. This device possesses normal quasiparticle transmission between the two ferromagnetic leads, and normal and crossed Andreev reflections providing conductive holes. For the appropriate spin polarization of the ferromagnetic leads, RSOI and AB flux, the pure spin-up (or spin-down) current without net charge current in the right lead, which is due to the equal numbers of electrons and holes with the same spin-polarization moving along the same direction, can be obtained by adjusting the gate voltage, which may be used in practice as a pure spin-current injector.     

Al0.6Ga0.4N/GaN/Al0.3Ga0.7N/Al0.6Ga0.4N量子阱中的Rashba自旋劈裂

正如人们所知, 可以通过电场或者设计非对称的半导体异质结构来调控体系的结构反演不对称性(SIA)和Rashba自旋劈裂. 本文研究了Al_0.6Ga_0.4N/GaN/Al_0.3Ga_0.7N/Al_0.6Ga_0.4N量子阱中第一子带的Rashba 系数和Rashba自旋劈裂随Al_0.3Ga_0.7N插入层(右阱)的厚度w_s以及外加电场的变化关系, 其中GaN层(左阱)的厚度为40-w_s Å. 发现随着w_s的增加, 第一子带的Rashba系数和Rashba自旋劈裂首先增加, 然后在w_s>20 Å 时它们迅速减小, 但是w_s>30 Å时Rashba自旋劈裂减小得更快, 因为此时k_f也迅速减小. 阱层对Rashba系数的贡献最大, 界面的贡献次之且随w_s变化不是太明显, 垒层的贡献相对比较小. 然后, 我们假w_s=20 Å, 发现外加电场可以很大程度上调制该体系的Rashba系数和Rashba自旋劈裂, 当外加电场的方向同极化电场方向相同(相反)时, 它们随着外加电场的增加而增加(减小). 当外加电场从-1.5×10⁸ V·m^-1到1.5×10⁸ V· m^-1变化时, Rashba系数随着外加电场的改变而近似线性变化, Rashba自旋劈裂先增加得很快, 然后近似线性增加, 最后缓慢增加. 研究结果表明可以通过改变GaN层和Al_0.3Ga_0.7N层的相对厚度以及外加电场来调节Al_0.6Ga_0.4N/GaN/Al_0.3Ga_0.7N/Al_0.6Ga_0.4N量子阱中的Rashba 系数和Rashba自旋劈裂, 这对于设计自旋电子学器件有些启示.     

Influence of Rashba spin-orbit coupling on Josephson effect in triplet superconductor/two-dimensional semiconductor/triplet superconductor junctions

We study theoretically Josephson effect in a planar ballistic junction between two triplet superconductors with p-wave orbital symmetries and separated by a two-dimensional (2D) semiconductor channel with strong Rashba spin-orbit coupling. In triplet superconductors, three types of orbital symmetries are considered. We use Bogoliubov-de Gennes formalism to describe quasiparticle propagations through the junction and the supercurrents are calculated in terms of Andreev reflection coefficients. The features of the variation of the supercurrents with the change of the strength of Rashba spin-orbit coupling are investigated in some detail. It is found that for the three types of orbital symmetries considered, both the magnitudes of supercurrent and the current-phase relations can be manipulated effectively by tuning the strength of Rashba spin-orbit coupling. The interplay of Rashba spin-orbit coupling and Zeeman magnetic field on supercurrent is also investigated in some detail.     

Spin-dependent Kondo effect induced by Rashba spin-orbit interaction in parallel coupled double quantum dots

Electronic transport through parallel coupled double quantum dots (DQD) with Rashba spin-orbit (RSO) interaction is investigated in Kondo regime by means of the slave-boson mean field approximation at zero temperature. By the co-action of the phase factor deduced by RSO interaction and the magnetic flux penetrating the parallel DQD, an interesting spin-dependent Kondo effect emerges. The molecular state representation theory is used to obtain a detailed understanding of the spin-dependent Kondo effect. It is shown that Quantum interference between the bonding Kondo state and antibonding state, which is modulated by the RSO interaction, plays a crucial role to the density of states and the linear conductance. The magnitude of each spin component conductance can be modulated by the RSO interaction strength. The conductance of each spin component exhibits 4π-periodic function with respect to φR. Moreover, the swap operation in the parallel DQD system can be implemented by tuning the RSO interaction.     

Generation and Manipulation of Spin-Orbit Coupling mode via Four-Wave Mixing with Quantum Interference

Recently, the vectorial nonlinear optical processes driven by spin-orbit coupling (SOC) light have come to the fore, leading to striking optical phenomena and important applications both in classical and quantum optics. However, research on the SOC-mediated light-atom interactions is still in its infancy. Here, the generation and manipulation of SOC mode through a vectorial four-wave mixing (FWM) process in ⁸⁵Rb vapor is demonstrated. Under the excitation of two SOC pump beams, multiple FWM paths can be simultaneously established due to the rich atomic Zeeman sublevels coupling with different circularly polarized components of SOC fields. A higher-order cylindrical or more general SOC FWM signal is observed in the vectorial FWM process, which obeys angular momentum conservation and Gouy phase matching. In particular, quantum interference between different FWM paths plays a crucial role in manipulating the SOC mode of the generated FWM signal, revealing that the conversion of SOC mode is intrinsically quantum. This work provides a pathway toward deeper insight into the vectorial nonlinear optical processes and may advance technology for shaping spatially structured light, which is essential in applications such as nonlinear polarization imaging and the optical communication realm.     

Dependence of Spin-orbit Parameters in Al_xGa_(1-x)N/GaN Quantum Wells on the Al Composition of the Barrier

In this paper, we obtain considerable spin-orbit (SO) parameters in AlxGa1-xN/GaN quantum wells (QWs) with sheet carrier concentration Ns=120×1011/cm2 . With increasing Al content (x) of the barrier, the SO parameters increase as a whole, and the two major contributions are found to be the decrease of the expansion region of the envelope functions and the increase of the polarized electric field in the well. Compared with the Rashba parameters for the first two subbands, the intersubband SO parameter is a bit smaller and varies more slowly with x. The results indicate the SO parameters, especially the Rashba parameters can be engineered by the Al composition of the barrier, which may be helpful to the spin manipulation of III-nitride low-dimensional heterostructures.     

Spin current and its heat effect in a multichannel quantum wire with Rashba spin-orbit coupling

Using the perturbation method,we theoretically study the spin current and its heat effect in a multichannel quantum wire with Rashba spin-orbit coupling.The heat generated by the spin current is calculated.With the increase of the width of the quantum wire,the spin current and the heat generated both exhibit period oscillations with equal amplitudes.When the quantum-channel number is doubled,the oscillation periods of the spin current and of the heat generated both decrease by a factor of 2.For the spin current j s,xy,the amplitude increases with the decrease of the quantum channel;while the amplitude of the spin current j s,yx remains the same.Therefore we conclude that the effect of the quantum-channel number on the spin current j s,xy is greater than that on the spin current j s,yx.The strength of the Rashba spin-orbit coupling is tunable by the gate voltage,and the gate voltage can be varied experimentally,which implies a new method of detecting the spin current.In addition,we can control the amplitude and the oscillation period of the spin current by controlling the number of the quantum channels.All these characteristics of the spin current will be very important for detecting and controlling the spin current,and especially for designing new spintronic devices in the future.