Rashba Spin-orbit Effect Research Articles

Study of the properties of strong-coupling magnetopolaron in quantum disks induced by the Rashba spin-orbit interaction

On the basis of Lee-Low-Pines unitary transformation, the properties of strong-coupling magnetopolarons in quantum disks (QDs) induced by the Rashba spin-orbit interaction are studied using the Tokuda improved linearly combined operator method. Results show that the state properties of magnetopolarons are closely linked with the sign of the interaction energy Eint, and the Eint of magnetopolarons changes with the transverse confinement strength 0, the cyclotron frequency of the external magnetic field c, the electron-LO phonon coupling strength , and the thickness L of QDs. The average number N of phonons increases with increasing c, 0 and , but the oscillation decreases with increasing thickness L of QDs. The effective mass m0* of magnetopolarons splits into two (m+*, m-*), induced by the Rashba spin-orbit interaction, and the values of them increase with increasing c, 0 and , but the oscillation decreases with increasing thickness L of QDs. For the ground state of magnetopolarons in QDs, the electron-LO phonon interaction plays a significant role, meanwhile, the Rashba spin-orbit coupling effect cannot be ignored. Only for the lower volocity of the electrons, can the polaron effect and the Rashba spin-orbit interaction effect on the magnetopolaron be obvious.

Asymmetric Spin Accumulation Induced by the Rashba Spin-Orbit Effect in a Domain Wall Inside a Magnetic Nanowire

We have investigated the spin accumulation effect in a Bloch-type magnetic domain wall in the presence of the Rashba spin-orbit interaction. The investigation has been done to study the excess voltage drop due to spin accumulation across the domain wall using the kinetic equation in Wigner space. It has been demonstrated that the Rashba interaction extremely increases the spin accumulation effect in the domain wall which in turn results in up to two orders of magnitude enhancement in the excess voltage drop.

Group theoretical and topological analysis of the quantum spin Hall effect in silicene

Silicene consists of a monolayer of silicon atoms in a buckled honeycomb structure. It was recently discovered that the symmetry of such a system allows for interesting Rashba spin–orbit effects. A perpendicular electric field is able to couple to the sublattice pseudospin, making it possible to electrically tune and close the band gap. Therefore, external electric fields may generate a topological phase transition from a topological insulator to a normal insulator (or semimetal) and vice versa. The contribution of the present paper to the study of silicene is twofold. Firstly, we perform a group theoretical analysis to systematically construct the Hamiltonian in the vicinity of the K points of the Brillouin zone and find an additional, electric field induced spin–orbit term, that is allowed by symmetry. Subsequently, we identify a tight-binding model that corresponds to the group theoretically derived Hamiltonian near the K points. Secondly, we start from this tight-binding model to analyze the topological phase diagram of silicene by an explicit calculation of the topological invariant of the band structure. To this end, we calculate the topological invariant of the honeycomb lattice in a manifestly gauge invariant way which allows us to include Sz symmetry breaking terms—like Rashba spin–orbit interaction—into the topological analysis. Interestingly, we find that the interplay of a Rashba and an intrinsic spin–orbit term can generate a non-trivial quantum spin Hall phase in silicene. This is in sharp contrast to the more extensively studied honeycomb system graphene where Rashba spin–orbit interaction is known to compete with the quantum spin Hall effect in a detrimental way.

Controllable spin polarization in an organic molecule

The spin transport properties of an organic molecule (1,4-2-phenyl-dithiolate) in the presence of the Rashba spin–orbit effect as well as a transverse (in-plane) electric field are calculated. We show that, for an appropriate amount of Rashba spin–orbit coefficient, the spin polarization of the outgoing electrons can be easily controlled by changing the transverse electric field. Also, by changing the transverse electric field periodically, the molecule can work as a “spin clock” which can potentially have a very high switching speed because the capacitance of the molecule is very small. The results can be used for designing molecular spintronic devices.

Spin torque due to non-uniform Rashba spin orbit effect

Following the early theoretical descriptions of the spin-orbit-induced spin torque [S. G. Tan et al., arXiv:0705.3502 (2007); S. G. Tan et al., Ann. Phys. 326, 207 (2011)]10.1016/j.aop.2010.11.014, the first experimental observation of such effect was reported by L. M. Miron et al., Nature Mater 9, 230 (2010). We present in this article three additional spin torque terms that arise from the non-uniformity in magnetization space of the Rashba spin-orbit effect. We propose a simple Rashba gradient device consisting of a heterogeneous trilayer structure, which could potentially lower switching current by n orders of magnitude, where n = −log10(|ΔM|/|M|) is a measure of the magnetization change across the device.

The impact of quantum dots magnetization on spin separation and spin current in a multiple quantum-dot ring in the presence of Rashba spin-orbit coupling

The influence of quantum dot magnetization on electronic spin-dependent transport is investigated through a triple-quantum-dot ring structure in which one of the quantum dots is non-magnetic subjected to the Rashba spin-orbit interaction and the two other ones possess magnetic structure. Evaluated results, based on single particle Green’s function formalism, indicate that the presence of magnetic moment on the quantum dots leads to additional spin-dependent phase factor which affects electronic transport through the system. For both antiferromagnetic and ferromagnetic quantum dots, the system can operate as a spin-splitter but differently; by tuning Rashba spin-orbit strength and in the presence of magnetic flux, respectively. Besides, in the absence of one of the outgoing leads, spin current in the output is calculated and demonstrated that magnetization of quantum dots leads to spin current even in the absence of Rashba spin-orbit effect. Moreover, it is shown that in the presence of Rashba spin orbit interaction, magnetic quantum dots, and magnetic flux, the two terminal system produces a completely tunable spin current.

Rashba spin-orbit effect on tunneling time in graphene superlattice

Based on transfer-matrix and stationary phase methods, we have investigated the tunneling time (group delay time) through monolayer graphene superlattice in the presence of Rashba spin-orbit interaction. It is found that the tunneling time has an oscillatory behavior with respect to Rashba spin-orbit interaction strength. Furthermore, the tunneling time for normal incident angle is independent of spin state of electron, while for oblique incident angles, it depends on the spin state of electron. It is also shown that, for normal incident, the Hartman effect vanishes, while for oblique incident, the Hartman effect appears whenever the x (the growth direction of superlattice) component of the electron wave vector inside the barriers is imaginary.

Spin-dependent resonant tunneling in a periodic non-magnetic heterostructure with spin–orbit effects

We theoretically investigate the electron transport in a periodic non-magnetic heterostructure with both Dresselhaus and Rashba spin–orbit effects. We show that the transport properties obviously depend on the number of periods and the large spin polarization can be achieved in such a structure. We also show that for m > 1 , the resonance splitting occurs in the transmission curves of both spin-up and spin-down electrons when the transmission curves are plotted as a function of the electron energy or the well width.

Spin Filter Based on an Aharonov–Bohm Interferometer with Rashba Spin–Orbit Effect

We propose a spin filter based on both the quantum interference and the Rashba spin–orbit (RSO) effects. This spin filter consists of a Aharonov–Bohm (AB) interferometer with two quantum dots (QDs) inserted in its arms. The influences of a magnetic flux φ threading through the AB ring and the RSO interaction inside the two QDs are taken into account by using the nonequilibrium Green's function technique. Due to the existence of the RSO interaction, the electrons flowing through different arms of the ring will acquire a spin-dependent phase factor in the linewidth matrix elements. This phase factor, combined with the influence of the magnetic flux, will induce a spin-dependent electron transport through the device. Moreover, we show that by tuning the magnetic flux, the RSO strength and the inter-dot tunnelling coupling strength, a pure spin-up or spin-down conductance can be obtained when a spin-unpolarized current is injected from the external leads, which can be used to filter the electron spin.

Spin accumulation and pure spin current in a three-terminal quantum dot ring with Rashba spin-orbit effect

Based on the Keldysh nonequilibrium Green’s function technique, we study the spin accumulation and the pure spin current in a three-terminal quantum ring device, in which a quantum dot (QD) is inserted in one arm of the ring and the Rashba spin-orbit interaction (RSOI) exists in the other. We find that under certain circumstances, the pure spin current in the middle lead and the spin accumulation in the QD emerge simultaneously. The sign and the magnitude of them can be modulated by adjusting the bias voltages, the RSOI-induced phase factor and the magnetic flux penetrating the ring. The origin of the spin accumulation and the pure spin current is interpreted in terms of the spin-dependent total effective coupling strengths between the leads and the QD.

Generation of dc spin current in a narrow channel with Rashba and Dresselhaus spin-orbit interactions

The authors consider a finite range ac-biased front gate acting on a quantum channel with Rashba and Dresselhaus spin-orbit interaction effects. The ac-biased gate, giving rise to a dynamical Rashba coupling, causes spin-resolved coherent resonant inelastic scattering. A pure dc spin current is subsequently generated without accompanying charge current. In the presence of Dresselhaus effect, the dc spin current is suppressed in the regime but is assisted in the high kinetic energy regime.

Rashba spin-orbit effect on the zero conductance and the magnetoresistance of a quantum ring

Using a mode matched scattering matrix method, we study the effect of Rashba spin-orbit coupling and a tunnel barrier on the zero conductance and the tunnel magnetoresistance (TMR) in a one-dimensional conducting Aharonov–Bohm ring symmetrically coupled to two ferromagnetic leads. It is found that the conductances of spin-up and spin-down electrons show periodic variations as the ring’s length changes. The conductance amplitude in antiferromagnetic alignment is very different from that in ferromagnetic alignment. The lifting of the real zero conductance is changed by the spin-orbit interaction and the tunnel barrier. The numerical results also show that this structure can provide a large TMR. When the ring’s length is increased, the TMR varies its period.

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

Abstract 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.

Spin injection in quantum wells with spatially dependent rashba interaction

We consider Rashba spin–orbit effects on spin transport driven by an electric field in semiconductor quantum wells. We derive spin diffusion equations that are valid when the mean free path and the Rashba spin–orbit interaction vary on length scales larger than the mean free path in the weak spin–orbit coupling limit. From these general diffusion equations, we derive boundary conditions between regions of different spin–orbit couplings. We show that spin injection is feasible when the electric field is perpendicular to the boundary between two regions. When the electric field is parallel to the boundary, spin injection only occurs when the mean free path changes within the boundary, in agreement with the recent work by Tserkovnyak et al (Preprint cond-mat/0610190).

Fano-Rashba effect in a double quantum dot Aharonov-Bohm interferometer

The interplay between the Fano and Rashba spin-orbit (RSO) effects in an Aharonov-Bohm (AB) interferometer containing two noninteracting quantum dots (QDs) in each of its arms is studied by employing the Green’s function equation of motion. When electrons flow through the upper and lower arms of the interferometer, the RSO interaction in the QDs induces a spin-dependent phase factor in the linewidth matrix elements. By the co-action of this phase factor and the magnetic flux penetrating the AB ring, an interesting spin-dependent Fano effect will arise. It is found that the Fano line shapes of the two spin components in the conductance spectra can be tuned either synchronously or individually with the help of the structure parameters relevant to the RSO coupling strength, magnetic flux, and the structure configuration. We focus our attention on the manipulation of the swap operation of the resonances centered at the bonding and antibonding energies, spin-flip processes, and the broadening of the resonances, which might have practical applications in spintronics devices. We also find that both the broadening of the resonances and the spin polarization efficiency can be controlled in terms of the dot-lead coupling strength.

Stark and Rashba effects in GaN nanowires

The effects of an external electric field on the electronic structure of GaN nanowires, as well as GaAs nanowires for comparison, are investigated theoretically. It is found that there is an anti-crossing effect in GaN nanowires caused by a small electric field, the hole energy levels, hole wave functions, and optical oscillator strengths change dramatically when the radius (R) is around a critical radius (Rc), while this effect is absent in GaAs nanowires. When R is slightly smaller than Rc, the highest hole states are optically dark in the absence of the electric field, and a small electric field can change them to be optically bright, due to the coupling of hole states brought by the field. The Rashba spin-orbit effect is also studied. The electron Rashba coefficient α increases linearly with the electric field. While the hole Rashba coefficients β do not increase linearly, but have complicated relationships with the electric field.

Rashba spin-orbit effect on electronic transport in ferromagnetic/semiconductor/ferromagnetic nanostructures under an applied electric field

Using a mode match transfer matrix method, the coherent quantum transport in a ferromagnetic/semiconductor/ferromagnetic nanowire was investigated in the presence of the Rashba spin-orbit interaction, where the angle $\ensuremath{\theta}$ between the two magnetic moments of the left and right ferromagnets varies from 0 to $2\ensuremath{\pi}$. The transmission for spin-up and spin-down electrons shows periodic variation as the angle changes. When the electric field is applied, spin-up and spin-down electrons show considerably different transmission properties both in ferromagnetic alignment $(\ensuremath{\theta}=0)$ and in antiferromagnetic alignment $(\ensuremath{\theta}=\ensuremath{\pi})$. The numerical results also indicate that the effects of the electric field and of Rashba spin-orbit interaction on the tunnel magnetoresistance are significant.