Very Large Rashba Spin–Orbit Splitting in HgTe Quantum Wells

A large Rashba spin splitting has been observed in the first conduction subband of n-type modulation doped HgTe quantum wells with an inverted band structure via an investigation of Shubnikov–de Haas oscillations as a function of gate voltage. Self-consistent Hartree calculations of the band structure based on an 8 × 8 k p model quantitatively describe the experimental results. It has been shown that the heavy-hole nature of the H1 conduction subband greatly influences the spatial distribution of electrons in the quantum well and also enhances the Rashba spin splitting at large electron densities. These are unique features of type III heterostructures in the inverted band regime. The k ³ dispersion predicted by an analytical model is a good approximation of the self-consistent Hartree calculations for small values of the in-plane wave vector k . This is in contrast to the commonly used k dispersion for the conduction subband in type I heterojunctions.     

Enhancement of Rashba Spin–Orbit Interaction Due to Wave Function Engineering

We have demonstrated an enhancement of Rashba spin–orbit interaction (SOI) in In_0.53Ga_0.47As/In_0.7Ga_0.3As/In_0.53Ga_0.47As double-step structure in comparison with In_0.53Ga_0.47As normal quantum well. In the double-step structure, high electron probability density is located on the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface to enhance the interface contribution of Rashba SOI. The double-step structure is designed based on k⋅p formalism considering field and interface contributions separately. The Rashba parameter α calculated by the k⋅p formalism shows good agreement with the experimental value by analyzing weak antilocalization. The large carrier density dependence of α is due to the In_0.53Ga_0.47As/In_0.7Ga_0.3As heterointerface contribution as well as the energy-band bending in the In_0.7Ga_0.3As quantum well. The results of this study suggest that the precise control of interface and field contributions in Rashba SOI will make its application to semiconductor spintronics.     

Quantum Correlations in Three-Qubit Ising Model with Added Dzyaloshinskii–Moriya Interaction

The thermal behavior of the quantum correlations in a three-qubit Ising model with added Dzyaloshinskii–Moriya (DM) interaction is studied. Quantum correlations are calculated in the three-qubit system through symmetric (SY) and nonsymmetric (NYS) ways. In particular, the study is focused on the concurrence (C) and the quantum discord (QD). Intriguing results are found by comparing the finite-temperature results with the ground-state results. Especially, an opposite trend of the ground state QD and C through both SY and NSY ways is found. The thermal behavior of the C (TC) through the SY way is shown to be a robust trend against thermal fluctuations. Two kinds of zero and nonzero plateau behavior in the thermal QD curve are also found. The width of the QD plateau is calculated, and it is found that it scales as D ^ε , where the critical exponent varies from ε=0.5 in the antiferromagnetic case to ε=?0.6 in the ferromagnetic case.     

Vortex–Antivortex-Pair Lattices in Spin–Orbit Coupled Bose–Einstein Condensates

We investigate theoretically the ground states of Bose–Einstein condensates with Rashba spin–orbit coupling in optical lattices within mean-field framework. We obtain numerically the Bloch states and energy spectrum for the single particle Hamiltonian, meanwhile the analytical solution of Bloch states is also presented. For a spin–orbit coupling Bose–Einstein condensates with a weak interaction, we show the existence of the vortex–antivortex-pair lattices state by simulating the Gross–Pitaevskii equation.     

On the Theory of Kinetic Phenomena in 2D Doped Antiferromagnet with Strong Spin–Hole Interaction

The problem of constructing the kinetic equation for a hole motion in systems with strong spin-hole interaction (such as HTSC) is treated in terms of the spin polaron concept for a regular 2d antiferromagnetic (AFM) s–d model. It is shown that kinetics is determined by the properties of the spin polaron bands (rather than “bar hole”) for which the hole residues Z _k can be far from 1. In both cases of low and optimal doping, the obtained electrical resistivity ρ(T) and Hall coefficient R(T) T-dependence demonstrates qualitative agreement with experimental data if we take into account the rearrangement of the lower polaron band spectrum and Z _k residues on doping, as well as a strong anisotropy of hole–spin scattering.     

Electric-Field Induced Spin Excitation in Quantum Wells with Rashba–Dresselhaus Spin-Orbit Coupling

The spin-rotation symmetry in spin-orbit coupled two-dimensional electron systems gives rise to a long-lived spin excitation that is robust against short-range impurity scattering. The influence of a constant in-plane electric field on this persistent spin helix is studied. To probe field-mediated spin modes, a surface acoustic wave is exploited that provides the wave-vector for resonant excitation. The approach takes advantage of methods worked out in the field of space-charge waves. A sharp resonance in the electric field dependence of the magnetization is identified.     

Current-Induced Spin–Orbit Torques for Spintronic Applications

Control of magnetization in magnetic nanostructures is essential for development of spintronic devices because it governs fundamental device characteristics such as energy consumption, areal density, and operation speed. In this respect, spin–orbit torque (SOT), which originates from the spin–orbit interaction, has been widely investigated due to its efficient manipulation of the magnetization using in-plane current. SOT spearheads novel spintronic applications including high-speed magnetic memories, reconfigurable logics, and neuromorphic computing. Herein, recent advances in SOT research, highlighting the considerable benefits and challenges of SOT-based spintronic devices, are reviewed. First, the materials and structural engineering that enhances SOT efficiency are discussed. Then major experimental results for field-free SOT switching of perpendicular magnetization are summarized, which includes the introduction of an internal effective magnetic field and the generation of a distinct spin current with out-of-plane spin polarization. Finally, advanced SOT functionalities are presented, focusing on the demonstration of reconfigurable and complementary operation in spin logic devices.     

Pseudo-gap and Vertex Correction of Electron–Phonon Interaction in High Transition-Temperature Superconductors

The strong-coupling Eliashberg theory plus vertex correction is used to calculate the maps of transition temperature (T _c) in parameter-space characterizing superconductivity. Based on these T _c maps, complex crossover behaviors are found when electron?Cphonon interaction increases from weak-coupling region to strong-coupling region. The doping-dependent T _c of cuprate superconductors and most importantly the emergence of pseudo-gap region can be explained as the effects of vertex correction.     

Conductance and Thermopower of a Quantum Dot with Fano–Rashba Effect

We investigate the conductance and thermopower of a Rashba quantum dot coupled to ferromagnetic leads. We show that the interference of localized electron states with resonant electron states leads to the appearance of the Fano–Rashba effect. This effect occurs due to the interference of bound levels of spin-polarized electrons with the continuum of electronic states with an opposite spin polarization. We obtain an important enhancement of the thermopower due to the Fano–Rashba effect.     

Strongly Surface State Carrier-Dependent Spin–Orbit Torque in Magnetic Topological Insulators

The topological surface states (TSS) in topological insulators (TIs) can exert strong spin–orbit torque (SOT) on adjacent magnetization, offering great potential in implementing energy-efficient magnetic memory devices. However, there are large discrepancies among the reported spin Hall angle values in TIs, and its temperature dependence still remains elusive. Here, the spin Hall angle in a modulation-doped Cr-Bi_xSb_2−xTe₃ (Cr-BST) film is quantitatively determined via both transport and optic approaches, where consistent results are obtained. A large spin Hall angle of ≈90 in the modulation-doped Cr-BST film is demonstrated at 2.5 K, and the spin Hall angle drastically decreases to 0.3–0.5 as the temperature increases. Moreover, by tuning the top TSS carrier concentration, a competition between the top and bottom TSS in contributing to SOT is observed. The above phenomena can account for the large discrepancies among the previously reported spin Hall angle values and reveal the unique role of TSS in generating SOT.     

Deterministic Magnetization Switching Using Lateral Spin–Orbit Torque

Current-induced magnetization switching by spin–orbit torque (SOT) holds considerable promise for next generation ultralow-power memory and logic applications. In most cases, generation of spin–orbit torques has relied on an external injection of out-of-plane spin currents into the magnetic layer, while an external magnetic field along the electric current direction is generally required for realizing deterministic switching by SOT. Here, deterministic current-induced SOT full magnetization switching by lateral spin–orbit torque in zero external magnetic field is reported. The Pt/Co/Pt magnetic structure is locally annealed by a laser track along the in-plane current direction, resulting in a lateral Pt gradient within the ferromagnetic layer, as confirmed by microstructure and chemical composition analysis. In zero magnetic field, the direction of the deterministic current-induced magnetization switching depends on the location of the laser track, but shows no dependence on the net polarization of external out-of-plane spin currents. From the behavior under external magnetic fields, two independent mechanisms giving rise to SOT are identified, i.e., the lateral Pt–Co asymmetry as well as out-of-plane injected spin currents, where the polarization and the magnitude of the SOT in the former case depends on the relative location and the laser power of the annealing track.     

The Role of the Spin–Orbit Couplings and Optical Phonons on the Anisotropic Resistivity

We have investigated the influence of the electron–phonon interaction on magnetoelectric properties and spin-related transport effects of a two dimensional electron gas in the presence of the spin–orbit couplings. We have employed a semiclassical method that has been extended to include the anisotropic effects of band structure in the presence of the spin–orbit couplings. We found that resistivity and anisotropic resistance (AR) can be controlled by Rashba spin–orbit coupling.     

Site-Controlled Uniform Ge/Si Hut Wires with Electrically Tunable Spin–Orbit Coupling

Semiconductor nanowires have been playing a crucial role in the development of nanoscale devices for the realization of spin qubits, Majorana fermions, single photon emitters, nanoprocessors, etc. The monolithic growth of site-controlled nanowires is a prerequisite toward the next generation of devices that will require addressability and scalability. Here, combining top-down nanofabrication and bottom-up self-assembly, the growth of Ge wires on prepatterned Si (001) substrates with controllable position, distance, length, and structure is reported. This is achieved by a novel growth process that uses a SiGe strain-relaxation template and can be potentially generalized to other material combinations. Transport measurements show an electrically tunable spin–orbit coupling, with a spin–orbit length similar to that of III–V materials. Also, charge sensing between quantum dots in closely spaced wires is observed, which underlines their potential for the realization of advanced quantum devices. The reported results open a path toward scalable qubit devices using nanowires on silicon.     

Infinite Spin Diffusion Length of Any Spin Polarization Along Direction Perpendicular to Effective Magnetic Field from Dresselhaus and Rashba Spin–Orbit Couplings with Identical Strengths in (001) GaAs Quantum Wells

In this note, we show that the latest spin grating measurement of spin helix by Koralek et al. (Nature 458:610, 2009) provides strong evidence of the infinite spin diffusion length of any spin polarization along the direction perpendicular to the effective magnetic field from the Dresselhaus and Rashba spin–orbit couplings with identical strengths in (001) GaAs quantum wells, predicted by Cheng et al. (Phys. Rev. B 75:205328, 2007).     

Magnon BEC in Antiferromagnets with Suhl–Nakamura Interaction

From atomic physics one knows the phenomenon of Bose–Einstein condensation (BEC), where a macroscopic ensemble of particles occupy coherently a single state. Similar phenomena were observed for different types of quasiparticles in condensed matter. Here we present the results of investigations on the BEC of elementary magnetic excitations—magnons—in antiferromagnets with a dynamical frequency shift.     

On the Electron–Electron Interaction in Superconductors

By using an attractive potential that varies in reverse proportion with respect to the third power of the distance between the two particles, we resolved the Schrodinger equation for an electron pair. We applied these statements to superconductors and obtained a good agreement with experimental data and BCS theory.