Thermoelectric transport and spin density of graphene nanoribbons with Rashba spin–orbit interaction

In the present paper, we have theoretically investigated thermoelectric transport properties of armchair and zigzag graphene nanoribbons with Rashba spin–orbit interaction, as well as dephasing scattering processes by applying the nonequilibrium Green function method. Behaviors of electronic and thermal currents, as well as thermoelectric coefficients are studied. It is found that both electronic and thermal currents decrease, and thermoelectric properties been suppressed, with increasing strength of Rashba spin–orbit interaction. We have also studied spin split and spin density induced by Rashba spin–orbit interaction in the graphene nanoribbons.     

Spin Hall and spin Nernst effects in graphene with intrinsic and Rashba spin-orbit interactions

The spin Hall and spin Nernst effects in graphene are studied based on Green’s function formalism.We calculate intrinsic contributions to spin Hall and spin Nernst conductivities in the Kane-Mele model with various structures.When both intrinsic and Rashba spin-orbit interactions are present,their interplay leads to some characteristics of the dependence of spin Hall and spin Nernst conductivities on the Fermi level.When the Rashba spin-orbit interaction is smaller than intrinsic spin-orbit coupling,a weak kink in the conductance appears.The kink disappears and a divergence appears when the Rashba spin-orbit interaction enhances.When the Rashba spin-orbit interaction approaches and is stronger than intrinsic spin-orbit coupling,the divergence becomes more obvious.     

Thermoelectric properties of magnetic configurations of graphene-like nanoribbons in the presence of Rashba and spin–orbit interactions

In this paper we investigate the influence of spin–orbit interaction and two types of Rashba interaction (intrinsic and extrinsic) on magnetic and thermoelectric properties of graphene-like zigzag nanoribbons based on the honeycomb lattice. We utilize the Kane-Mele model with additional Rashba interaction terms. Magnetic structure is described by the electron-electron Coulomb repulsion reduced to the on-site interaction (Hubbard term) in the mean field approximation. We consider four types of magnetic configurations: ferromagnetic and antiferromagnetic with in-plane and out-of plane direction of magnetization. Firstly, we analyze the influence of extrinsic Rashba coupling on systems with negligible spin–orbit interaction, e.g. graphene of an appropriate substrate. Secondly, we discuss the interplay between spin–orbit and intrinsic Rashba interactions. This part is relevant to materials with significant spin–orbit coupling such as silicene and stanene.     

Spin Transport in the Presence of Rashba Interaction

A Gaussian type spin-polarized electronic wave packet is constructed to investigate the spin transport behaviour in an infinite two-dimensional electron gas system with Rashba spin--orbit (SO) interaction by solving the Schrödinger equation exactly. In the presence of Rashba SO interaction, the spin-dependent force induces a momentum dependent splitting of the two spin directions, the average spin current indicates the corresponding spin accumulation clearly. Furthermore, the coherence of the injected spin-polarized wave packet, as well as the transverse force, decays during the motion in the Rashba SO regime.     

The nondissipative spin Hall current in a nonuniform Rashba effect system

The spin Hall current in a two-dimensional electron system with nonuniform Rashba spin–orbit interaction (SOI) is investigated by means of the lattice Green's function method. Large electric and spin Hall currents are produced by this nonuniform Rashba SOI, while the electric Hall current vanishes in the uniform Rashba SOI system. A nondissipative spin Hall current is also produced, without any longitudinal voltage bias, any external magnetic field and any special class of band insulators.     

Transport through a triple quantum dot system: Formation of resonance band and its application as a spin filter

A three-quantum-dot spin filter based on nonequilibrium Green?s function technique is proposed with external magnetic flux, Rashba spin orbit interaction, and intradot coulomb interaction taken into consideration. Numerical results indicate a spin filter can be made efficient by adjusting external magnetic flux and Rashba spin orbit interaction. Moreover, the formation of a resonance band is discussed through calculation. It is observed that the possibility of transition from one peak to other three peaks in the conductance spectrum increases with increasing interdot coupling strength.     

Spin Hall effect and spin current generation in two-dimensional systems with random Rashba spin–orbit coupling

We consider a new effect induced by spin–orbit coupling in a two-dimensional electron gas confined in a semiconductor quantum well, i.e. the possibility of spin current generation by fluctuating random Rashba spin–orbit interaction, with the corresponding mean value of the interaction being equal to zero. Our main results suggest that – in contrast to the spatially uniform Rashba spin–orbit interaction – the spin Hall effect does not vanish for typical disorder strengths. We also point out some other possibilities of using such a random Rashba coupling for the generation of spin density and spin current in two-dimensional nonmagnetic structures.     

Possible polaron formation of zigzag graphene nano-ribbon in the presence of Rashba spin–orbit coupling

In this article we study the role of Rashba spin–orbit coupling and electron–phonon interaction on the electronic structure of zigzag graphene nanoribbon with different width. The total Hamiltonian of nanoribbon is written in the tight binding form and the electron–electron interaction is modeled in the Hubbard term. We used a unitary transformation to reach an effective Hamiltonian for nano ribbon in the presence of electron–phonon interaction. Our results show that small Rashba spin orbit coupling annihilates the anti-ferromagnetic phase in the zigzag edges of ribbon and the electron–phonon interaction yields small polaron formation in graphene nano ribbon. Furthermore, Rashba type spin–orbit coupling increases (decreases) the polaron formation energy for up (down) spin state.     

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.     

Rashba spin-orbit effect on dwell time of electrons tunneling through semiconductor barrier

A study on characteristics of electrons tunneling through semiconductor barrier is evaluated, in which we take into account the effects of Rashba spin-orbit interaction. Our numerical results show that Rashba spin-orbit effect originating from the inversion asymmetry can give rise to the spin polarization. The spin polarization does not increase linearly but shows obvious resonant features as the strength of Rashba spin-orbit coupling increases, and the amplitudes of spin polarization can reach the highest around the first resonant energy level. Furthermore, it is found that electrons with different spin orientations will spend quite different time through the same heterostructures. The difference of the dwell time between spin-up and spin-down electrons arise from the Rashba spin-orbit coupling. And it is also found that the dwell time will reach its maximum at the first resonant energy level. It can be concluded that, in the time domain, the tunneling processes of the spin-up and spin-down electrons can be separated by modulating the strength of Rashba spin-orbit coupling. Study results indicate that Rashba spin-orbit effect can cause a nature spin filter mechanism in the time domain.     

Third harmonic generation in quantum dot with Rashba spin orbit interaction

Here we have investigated the influence of magnetic field and confinement potential on nonlinear optical property, third harmonic generation (THG) of a parabolically confinement quantum dot in the presence of Rashba spin orbit interaction. We have used density matrix formulation for obtaining optical properties within the effective mass approximation. The results are presented as a function of confining potential, magnetic field, Rashba spin orbit interaction strength and photon energy. Our results indicate that an increase of Rashba spin orbit interaction coefficient produces strong effect on the peak positions of THG. The role of confinement strength and spin orbit interaction strength as control parameters on THG have been demonstrated.     

Nonlocal Andreev reflection with Rashba spin-orbital interaction in a triple-quantum-dot ring

We theoretically studied the nonlocal Andreev reflection with Rashba spin-orbital interaction in a triple-quantumdot(QD) ring,which is introduced as Rashba spin-orbital interaction to act locally on one component quantum dot.It is found that the electronic current and spin current are sensitive to the systematic parameters.The interdot spin-flip term does not play a leading role in causing electronic and spin currents.Otherwise the spin precessing term leads to shift of the peaks of the the spin-up and spin-down electronic currents in different directions and results in the spin current.Moreover,the spin-orbital interaction suppresses the nonlocal Andreev reflection,so we cannot obtain the pure spin current.     

Spin and charge transport in the presence of spin-orbit interaction

We present the study of spin and charge transport in nanostructures in the presence of spin-orbit (SO) interaction. Single band tight binding Hamiltonians for Elliot-Yafet and Rashba SO interaction are derived. Using these tight binding Hamiltonians and spin resolved Landauer-Büttiker formula, spin and charge transport is studied. Specifically numerical results are presented for a new method to perform magnetic scanning tunneling microscopy with non-magnetic tip but in the presence of Elliot-Yafet SO interaction. The spin relaxation phenomena in two-dimensional electron gas in the presence of Rashba SO interaction are studied and contrary to naive expectation, it is shown that disorder helps to reduce spin relaxation.     

Current-induced spin accumulation in lateral superlattices with Rashba and Dresselhaus spin–orbit interaction

The current-induced spin accumulation is calculated for a 1D lateral semiconductor superlattice with spin–orbit interaction of the Rashba and Dresselhaus type. Due to its particular symmetry, the Rashba interaction alone only leads to an in-plane component of the magnetization transverse to the applied electric field. When in addition a Dresselhaus contribution is present, this symmetry is lifted, and all components of the magnetization are induced by the electric field. Based on the density-matrix approach, the induced spin polarization is determined as a function of external in-plane electric and magnetic fields.     

Rashba coupling induced spin susceptibility and magnetic phase transition of conduction electrons in monolayer graphene

Isolated graphene cannot be obtained by the known synthesis processes and it should be placed on a substrate. This substrate introduces a new type of spin–orbit interaction known as Rashba coupling. Using the Kubo formalism, the magnetic properties of the system in the linear regime have been investigated. Mainly the effect of non-magnetic substrate on the spin susceptibility is calculated. Results show that the Rashba coupling has a central role in the magnetic response function of the system and it is really remarkable since this type of spin orbit coupling can be effectively controlled by an external gate voltage. Most importantly, it was shown that, in the presence of the Rashba interaction a magnetic phase transition could be observed. This magnetic phase corresponds to a magnetic order of conduction electrons that takes place at some special frequencies of external magnetic field.     

Rashba效应影响下量子点中弱耦合束缚极化子的性质

采用改进的线性组合算符和幺正变换的方法研究了Rashba效应影响下量子点中弱耦合束缚极化子的性质,导出了Rashba效应影响下量子点中弱耦合束缚极化子的振动频率、有效质量、基态分裂能和相互作用能。数值计算结果表明随Rashba自旋-轨道耦合常数的增加,由于声子作用产生的附加能量能对零磁场时自旋分裂能的影响占有绝对优势。库仑势对束缚极化子的基态能量的影响同时也占有绝对优势。所以,研究Rashba自旋轨道相互作时声子的影响不可忽略。     

Detection of spin bias in a four-terminal quantum-dot ring

In this work, via introducing local Rashba spin-orbit interaction in a four-terminal quantum-dot (QD) ring, we show that the spin bias in the transverse terminals induces apparent charge currents in the longitudinal terminals, accompanied by the similar magnitude and opposite directions of them. The reason is that the Rashba interaction destroys the space-reversal symmetry of this structure and causes the spin dependence of the quantum interference. Then the opposite-spin currents driven by the spin bias present different magnitudes, which gives rise to the emergence of finite charge currents in the longitudinal terminals. Via these results, we suggest that the spin bias can be measured by observing the longitudinal charge currents, which provides an electrical but practical scheme to detect the spin bias (or spin current).     

Rashba效应影响下量子点中弱耦合束缚极化子的性质

采用改进的线性组合算符和幺正变换的方法研究了Rashba效应影响下量子点中弱耦合束缚极化子的性质,导出了Rashba效应影响下量子点中弱耦合束缚极化子的振动频率、有效质量、基态分裂能和相互作用能。数值计算结果表明随Rashba自旋-轨道耦合常数的增加,由于声子作用产生的附加能量能对零磁场时自旋分裂能的影响占有绝对优势。库仑势对束缚极化子的基态能量的影响同时也占有绝对优势。所以,研究Rashba自旋轨道相互作时声子的影响不可忽略。     

Spin transfer torque in the semiconductor/ferromagnetic structure in the presence of Rashba effect

Spin transfer torque in magnetic structure occurs when the transverse component of the spin current that flows from the nonmagnetic medium to ferromagnetic medium is absorbed by the interface. In this paper, considering the Rashba effect on the semiconductor region, we discuss the spin transfer torque in semiconductor/ferromagnetic structure and obtain the components of spin-current density for two models:(i) single electron and(ii) the distribution of electrons. We show that no matter whether the difference in Fermi surface between semiconductor and Fermi spheres for the up and down spins in ferromagnetic increases, the transmission probability decreases. The obtained results for the values used in this article illustrate that Rashba effect increases the difference in Fermi sphere between semiconductor and Fermi sphere for the up and down spins in ferromagnetic. The results also show that the Rashba effect, brings an additional contribution to the components of spin transfer torque, which does not exist in the absence of the Rashba interaction. Moreover, the Rashba term has also different effects on the transverse components of the spin torque transfer.     

Diffusive spin dynamics in ferromagnetic thin films with a Rashba interaction

In a ferromagnetic metal layer, the coupled charge and spin diffusion equations are obtained in the presence of both Rashba spin-orbit interaction and magnetism. The misalignment between the magnetization and the nonequilibrium spin density induced by the Rashba field gives rise to Rashba spin torque acting on the ferromagnetic order parameter. In a general form, we find that the Rashba torque consists of both in-plane and out-of-plane components, i.e., T=T(perpendicular)y^×m^+T(parallel)m^×(y^×m^). Numerical simulations on a two-dimensional nanowire consider the impact of diffusion on the Rashba torque and reveal a large enhancement to the ratio T(parallel)/T(perpendicular) for thin wires. Our theory provides an explanation for the mechanism driving the magnetization switching in a single ferromagnet as observed in the recent experiments.