A virtual intersubband spin–flip spin–orbit coupling induced spin relaxation in GaAs (110) quantum wells

A spin relaxation mechanism is proposed based on a second-order spin–flip intersubband spin–orbit coupling together with the spin-conserving scattering. The corresponding spin relaxation time is calculated via the Fermi golden rule. It is shown that this mechanism is important in symmetric GaAs (110) quantum wells with high impurity density. The dependencies of the spin relaxation time on electron density, temperature and well width are studied with the underlying physics analyzed.     

B与N掺杂对单层石墨纳米带自旋极化输运的影响

通过第一性原理计算研究了具有锯齿状边沿并且具有反铁磁构型的单层石墨纳米带的自旋极化输运.研究发现,在中心散射区同一位置掺入单个B和N原子,尽管对整个体系磁矩的影响完全相同,但对两个自旋分量电流的影响却完全相反.掺B时,自旋向上的电流显著大于自旋向下的电流;而掺N时,自旋向下的电流显著大于自旋向上的电流.这是由于不管掺B还是掺N都将打破自旋简并,使得导带和价带中自旋向上的能级比自旋向下的能级更高.掺B引入空穴,使完全占据的价带变为部分占据,从而自旋向上的能级正好处于费米能级,使得电子透射能力更强、电流更大,而自旋向下的能级则离费米能级较远使电子透射的能力较弱.掺N则引入电子,使得原来全空的导带变为部分占据,从而费米能级穿过导带中自旋向下的能级,使得自旋向下的电子比自旋向上的电子透射能力更强. 关键词： 自旋极化输运 单层石墨纳米带 第一性原理 非平衡格林函数     

Spin Polarization and Andreev Conductance through a Diluted Magnetic Semiconductor Quantum Wire with Spin--Orbit Interaction

Spin-dependent Andreev reflection and spin polarization through a diluted magnetic semiconductor quantum wire coupled to normal metallic and superconductor electrodes are investigated using scattering theory. When the spin-orbit coupling is considered, more Andreev conductance steps appear at the same Fermi energy. Magnetic semiconductor quantum wire separates the spin-up and spin-down electrons. The Fermi energy, at which different- spin-state electrons begin to separate, becomes lower due to the effect of the spin-orbit interaction. The spin filter effect can be measured more easily by investigating the Andreev conductance than by investigating the normal conductance.     

The Fano antiresonance effect in the current-voltage characteristics of a nanostructure with a single magnetic impurity

A theoretical investigation is performed of quantum coherent electron transport through a nanostructure that contains an impurity ion with an uncompensated magnetic moment. It is shown that the transmission coefficient of spin-polarized electrons has the Fano antiresonance. This effect appears as a result of exchange interaction between the spin of transmitted electron and the spin of impurity ion. It is shown that Fano antiresonance leads to a qualitative modification of the current-voltage characteristic of the structure responsible for the large value of magnetoresistance.     

Magnetic properties of X<Subscript>0.5</Subscript>Mn<Subscript>0.5</Subscript>/Cu(001) overlayers,X = Fe,Mn, and Ir: ab-initio calculations

We study the effect of a magnetic impurity with spin-half on a single propagating electron in a one-dimensional model system via the tight-binding approach. Due to the spin-dependent interaction, the scattering channel for the flying qubit is split, and its transmission spectrum is obtained. It is found that, the spin orientation of the impurity plays the role as a spin state filter for a flying qubit.     

Spin-polarized transport in graphene nanoribbon superlattices

By the Green’s function method,we investigate spin transport properties of a zigzag graphene nanoribbon superlattice(ZGNS) under a ferromagnetic insulator and edge effect.The exchange splitting induced by the ferromagnetic insulator eliminates the spin degeneracy,which leads to spin-polarized transport in structure.Spin-dependent minibands and minigaps are exhibited in the conductance profile near the Fermi energy.The location and width of the miniband are associated with the geometry of the ZGNS.In the optimal structure,the spin-up and spin-down minibands can be separated completely near the Fermi energy.Therefore,a wide,perfect spin polarization with clear stepwise pattern is observed,i.e.,the perfect spin-polarized transport can be tuned from spin up to spin down by varying the electron energy.     

Current-induced spin transfer torque in ferromagnet-marginal Fermi liquid double tunnel junctions

Current-induced spin transfer torque through a marginal Fermi liquid (MFL) which is connected to two noncollinearly aligned ferromagnets via tunnel junctions is discussed in terms of the nonequilibrium Green function method. It is found that in the absence of the spin–flip scattering, the magnitude of the torque increases with the polarization and the coupling constant λ of the MFL, whose maximum increases with λ linearly, showing that the interactions between electrons tend to enhance the spin torque. When the spin–flip scattering is included, an additional spin torque is induced. It is found that the spin–flip scattering enhances the spin torque and gives rise to a nonlinear angular shift.     

Hydrogen impurities and μSR in ferromagnetic Ni: A self-consistent calculation

We present the first spin-polarized band calculation for hydrogen impurities in ferromagnetic Ni. A set of impurity states is split off from the bottom of the Ni conduction bands. The impurities are effectively screened, and one electron per impurity is filled in states above the pure Ni Fermi energy. The work function is raised by hydrogenation, and the magnetic moment of the Ni atoms surrounding the impurity is reduced. The contact spin density at the impurity compares favourably with μSR data.     

稀磁合金中“电阻极大”现象的双杂质散射理论

本文基于s-d相互作用,考虑杂质之间存在RKKY相互作用,提出了一种新的双杂质散射模型。按照这个模型,当一个杂质作自旋翻转散射时,由于杂质之间存在着RKKY关联,它的自旋作为内部自由度会受到限制。由于这种关联,抑制了杂质的自旋翻转散射,结合Kondo的logT项,能形成电阻极大。本文计算了所有可能的“DIS”图(双杂质自能图),在Kondo电阻公式中加入了A/(T₀²—T²)这样的项。其中A是一个正常数。T₀是一个临界温度。当T≤T₀时,这个公式不再有意义。这个理论和已有的分子场理论在本质上是不同的.因为它并不依赖于合金中的磁有序.因此当T≥T_c时(T_c是磁有序转变温度),这种机制仍起作用,但分子场理论则不行.这是一种顺磁效应.我们和Cd-Mn(杂质浓度从0.01到0.1at./0)的实验曲线进行了比较,发现符合得很好.最后,我们认为即使在极低浓度下这种机制也是消除Kondo logT发散的主要原因. 关键词：     

Anomalous Spin Textures in a 2D Topological Superconductor Induced by Point Impurities

Topological superconductors are foreseen as good candidates for the search of Majorana zero modes, where they appear as edge states and can be used for quantum computation. In this context, it becomes necessary to study the robustness and behavior of electron states in topological superconductors when a magnetic or non-magnetic impurity is present. The focus is on scattering resonances in the bands and on spin texture to know what the spin behavior of the electrons in the system will be. It is found that the scattering resonances appear outside the superconducting gap, thus providing evidence of topological robustness. Non-trivial and anisotropic spin textures related to the Dzyaloshinskii–Moriya interaction are also found. The spin textures show a Ruderman–Kittel–Kasuya–Yosida interaction governed by Friedel oscillations. It is believed that the results are useful for further studies which consider many-point-impurity scattering or a more structured impurity potential with a finite range.     

Spin polarization of the current in a correlated one-dimensional conductor with an impurity

The spin transport as the current flows through an impurity in a one-dimensional conductor is analyzed. The interacting electrons are described in terms of the Luttinger liquid theory. Both the Coulomb and short-range interactions are considered; the latter appears when the gate screens the long-range part of the Coulomb potential. The cases of magnetic and nonmagnetic impurities are considered. It has been revealed that, for a magnetic impurity, the electric current flow induces the generation of the spin current, which has direct and alternating components. At low temperatures and voltages, the current can be completely spin-polarized. For a nonmagnetic impurity, the spin current generation is absent. The spin current flowing though the wire affects the current-voltage characteristic for both magnetic and nonmagnetic impurities. The results have been obtained for a rather strong electron-electron interaction.     

Spin-polarized transport induced by photoirradiation in zigzag silicene nanosystem

We study the spin-polarized transport induced by photoirradiation in zigzag silicene nanosystem, based on tight-binding approach, Green's function method and Landauer–Büttiker formula. By applying strong circular polarized light, silicene nanosystem can be transformed into a quantum Hall insulator, where the spin-down subband is gapped while the spin-up subband persists gapless edge state. Therefore, the dc conductance is dominated by the spin-up electrons, and the spin polarization can reach almost 100% around the Fermi energy. The spatial-resolved local density of states confirm that the spin-up electrons transport at two edges of the nanosystem in opposite current directions. Furthermore, because of the topological origin of the edge state, the spin-polarized transport is very robust against the size change of the nanosystem.     

Non-universal Fermi polaron in quasi two-dimensional quantum gases

We consider an impurity problem in a quasi-two-dimensional Fermi gas, where a spin-down impurity is immersed in a Fermi sea of N spin-up atoms. Using a variational approach and an effective two-channel model, we obtain the energy for a wide range of interaction strength and for various different mass ratios between the impurity and the background fermion in the context of heteronuclear mixture. We demonstrate that in a quasi-two-dimensional Fermi gas there exists a transition of the ground state from polaron in the weakly interacting region to molecule in the strongly interacting region. The critical interaction strength of the polaron-molecule transition is non-universal and depends on the particle density of the background Fermi sea. We also investigate the excited repulsive polaron state, and find similar non-universal behavior.     

Magnetic Impurity Band and Photoinduced Magnetic Phase Transition in Diluted Magnetic Semiconductors

Applying the dynamical coherent potential approximation (dynamical CPA) to a model of diluted magnetic semiconductors (DMSs), in which both random impurity distribution and thermal fluctuation of localized spins are taken into account, the spin-polarized band and the carrier spin polarization are calculated for various magnetizations. In order to clarify the role of impurity depth on the occurrence of ferromagnetism, three typical cases are investigated: (a) II-VI DMS, (b) deep impurity level, and (c) strong exchange interaction. The present study reveals that the impurity depth of magnetic ions strongly enhances the carrier spin polarization (CSP) and accordingly, leads to a high Curie temperature. This means that photoinduced ferromagnetism with high Curie temperature can be expected in a DMS with a deep impurity depth and strong exchange interaction.     

Effect of single magnetic atom on spin-polarized transport of armchair graphene nanoribbons

Using a perturbative method, the influence of a single magnetic impurity on the spin-polarized current flowing through a metallic armchair graphene nanoribbon is investigated theoretically. It is shown that the nonlinear correction depends strongly on the relative spin orientations in the two spin-polarized reservoirs. Also, the effects of magnetic impurity position and width of nanoribbon on the nonlinear conductance are discussed.     

Scattering of spin-polarized electron in an Aharonov-Bohm potential

The scattering of spin-polarized electrons in an Aharonov-Bohm vector potential is considered. We solve the Pauli equation in 3 + 1 dimensions taking into account explicitly the interaction between the three-dimensional spin magnetic moment of electron and magnetic field. Expressions for the scattering amplitude and the cross section are obtained for spin-polarized electron scattered off a flux tube of small radius. It is also shown that bound electron states cannot occur in this quantum system. The scattering problem for the model of a flux tube of zero radius in the Born approximation is briefly discussed.     

Quantum spin Hall effect in a square-lattice model under a uniform magnetic field

We study a toy square-lattice model under a uniform magnetic field. Using the Landauer-Bttiker formula, we calculate the transport properties of the system on a two-terminal, a four-terminal and a six-terminal device. We find that the quantum spin Hall (QSH) effect appears in energy ranges where the spin-up and spin-down subsystems have different filling factors. We also study the robustness of the resulting QSH effect and find that it is robust when the Fermi levels of both spin subsystems are far away from the energy plateaus but is fragile when the Fermi level of any spin subsystem is near the energy plateaus. These results provide an example of the QSH effect with a physical origin other than time-reversal (TR) preserving spin-orbit coupling (SOC).     

Spin-polarized current through a lateral double quantum dot with spin–orbit interaction

We study the spin-polarized current through a vertical double quantum dot scheme. Both the Rashba spin–orbit (RSO) interaction inside one of the quantum dots and the strong intradot Coulomb interactions on the two dots are taken into account by using the second-quantized form of the Hamiltonian. Due to the existence of the RSO interaction, spin-up and spin-down electrons couple to the external leads with different strengths, and then a spin polarized current can be driven out of the middle lead by controlling a set of structure parameters and the external bias voltage. Moreover, by properly adjusting the dot levels and the external bias voltages, a pure spin current with no accompanying charge current can be generated in the weak coupling regime. We show that the difference between the intradot Coulomb interactions strongly influences the spin-polarized currents flowing through the middle lead and is undesirable in the generation of the net spin current. Based on the RSO interaction, the structure we propose can efficiently polarize the electron spin without the usage of any magnetic field or ferromagnetic material. This device can be used as a spin-battery and is realizable using the present available technologies.     

Circularly polarized light stimulation of spin transport in zinc-blende semiconductors

An experimental and theoretical study on the optically stimulated spin transport in zinc-blende semiconductors is presented. The first part of the paper describes an experiment which investigates the effect of a longitudinal electric field on the spin-polarized carriers induced by a circularly polarized light. Since the photo-generated hole spins relaxation is extremely fast, the experiment observes only the effect resulting from spin-polarized electrons accumulating at the transverse edges of the sample, as a result of left-right asymmetries in scattering for spin-up and spin-down electrons in the presence of spin–orbit (SO) interaction. It is found that the effect depends on the longitudinal electric field and doping density as well as on temperature. The results are discussed. The second part of the paper deals with a theoretical investigation using norm-conserving pseudopotential and Green function formalism to analyse the SO mechanism responsible for the light-induced Hall voltage. The findings resulting from the investigation are discussed and are compared with experimental data.     

Spin-polarized current produced by a double barrier resonant tunneling diode

The spin-polarized tunneling current through a double barrier resonant tunneling diode (RTD) made with a semimagnetic semiconductor is studied theoretically. The calculated spin-polarized current and polarization degree are in agreement with recent experimental results. It is predicted that the polarization degree can be modulated continuously from +1 to −1 by changing the external voltage such that the quasi-confined spin-up and spin-down energy levels shift downwards from the Fermi level to the bottom of the conduction band. The RTD with low potential barrier or the tunneling through the second quasi-confined state produces larger spin-polarized current. Furthermore a higher magnetic field enhances the polarization degree of the tunneling current.