Spin-orbit Magnetic Field Research Articles

Tunneling spectroscopy of a spiral Luttinger liquid in contact with superconductors

One-dimensional wires with Rashba spin-orbit coupling, magnetic field, and strong electron-electron interactions are described by a spiral Luttinger liquid model. We develop a theory to investigate the tunneling density of states into a spiral Luttinger liquid under the proximity effect with superconductors. This approach provides a way to disentangle the delicate interplay between superconducting correlations and strong electron interactions. If the wire-superconductor boundary is dominated by Andreev reflection, we find that in the vicinity of the interface the zero-bias tunneling anomaly reveals a power law enhancement with the unusual exponent. Far away from the interface strong correlations inherent to the Luttinger liquid prevail and restore conventional suppression of the tunneling density of states at the Fermi level, which acquire, however, a Friedel-like oscillatory envelope with the period renormalized by the strength of the interaction.

RKKY interaction in carbon nanotubes and graphene nanoribbons

We study Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction in carbon nanotubes (CNTs) and graphene nanoribbons in the presence of spin orbit interactions and magnetic fields. For this we evaluate the static spin susceptibility tensor in real space in various regimes at zero temperature. In metallic CNTs the RKKY interaction depends strongly on the sublattice and, at the Dirac point, is purely ferromagnetic (antiferromagnetic) for the localized spins on the same (different) sublattice, whereas in semiconducting CNTs the spin susceptibility depends only weakly on the sublattice and is dominantly ferromagnetic. The spin orbit interactions break the SU(2) spin symmetry of the system, leading to an anisotropic RKKY interaction of Ising and Moryia-Dzyaloshinsky form, besides the usual isotropic Heisenberg interaction. All these RKKY terms can be made of comparable magnitude by tuning the Fermi level close to the gap induced by the spin orbit interaction. We further calculate the spin susceptibility also at finite frequencies and thereby obtain the spin noise in real space via the fluctuation-dissipation theorem.

Spin-orbit coupling and spin current in mesoscopic devices

Recently, the spin-orbit coupling and spin current in nanodevice have been investigated extensively. In this paper, we review the recent progresses in this field. We introduce the real space Hamiltonian and the second quantization Hamiltonian of a typical quantum transport mesoscopic device, metal-QD-metal configuration, containing the spin-orbit interaction, e-e interactions, and magnetic field. Some noteworthy effects (e.g., the spin-polarized current, spin accumulation, persistent spin current) originated from the spin-orbit interaction are reviewed, and the electric field induced by spin-current is mentioned. Lastly, we introduce some unsolved problems and prospects in this field.

Two-Terminal Spin Filter Using Quantum Dot with Spin-Orbit Interaction in Magnetic Field

We propose a two-terminal spin filter utilizing a quantum dot with spin-orbit interaction and magnetic field. First we examine a quantum dot with two energy-levels, as a minimal model, and obtain an analytical expression for the spin-dependent conductance. When the spacing between the two levels is smaller than the level broadening due to the tunnel coupling to the leads, a largely spin-polarized current is generated around the current peaks of Coulomb oscillation. Next, we perform a numerical simulation using a realistic model for a quantum dot and tunnel barriers and evaluate the efficiency of our spin filter.

Transport spectroscopy ofNSnanowire junctions with Majorana fermions

We investigate transport though normal-superconductor nanowire junctions in the presence of spin-orbit coupling and magnetic field. As the Zeeman field crosses the critical bulk value B_c of the topological transition, a Majorana bound state (MBS) is formed, giving rise to a sharp zero-bias anomaly (ZBA) in the tunneling differential conductance. We identify novel features beyond this picture in wires with inhomogeneous depletion, like the appearance of two MBSs inside a long depleted region for B<B_c. The resulting ZBA is in most cases weakly split and may coexist with Andreev bound states near zero energy. The ZBA may appear without evidence of a topological gap closing. This latter aspect is more evident in the multiband case and stems from a smooth pinch-off barrier. Most of these features are in qualitative agreement with recent experiments [Mourik et al, Science 336, 1003 (2012)]. We also discuss the rich phenomenology of the problem in other regimes which remain experimentally unexplored.

Electrically tunable electron spin lifetimes in GaAs(111)B quantum wells

We investigate the electric tunability of the electron spin lifetime in GaAs(111) quantum wells (QWs) inserted in biased p-i-n and n-i-p diode-like structures. Due to the specific symmetry of these QWs, the Rashba contribution to the spin-orbit magnetic field induced by the applied bias is parallel to the intrinsic Dresselhaus contribution for all directions of the electron wavevector. In particular, the voltage applied to the diode, which controls the amplitude of the Rashba contribution, can be adjusted to attenuate the resulting spin-orbit field, thus strongly suppressing spin dephasing due to the Dyakonov-Perel relaxation mechanism for all spin orientations. Spin lifetimes from below 100 ps at an electric field of −20 kV/cm to values exceeding 4 ns at +8 kV/cm have been measured in 25 nm thick multiple QWs at a temperature of 20 K.

A spin triplet supercurrent in half metal ferromagnet/superconductor junctions with the interfacial Rashba spin-orbit coupling

dc Josephson currents in s-wave superconductor/half metal ferromagnet (HMF)/s-wave superconductor junctions are investigated. We propose the interfacial Rashba spin-orbit coupling as a possible mechanism giving rise to the spin flip Andreev reflection, which results in the singlet-triplet conversion at HMF/superconductor interfaces. When the magnetization direction of the HMF and that of the effective spin-orbit magnetic field are noncolinear, a long-ranged spin triplet supercurrent appears. The supercurrent strongly depends on the magnetization direction of the HMF and can be enhanced by the Rashba spin-orbit coupling strength.

Tunable energy bands and spin filtering in two-dimensional superlattices with spin-orbit interaction

We theoretically investigate the electronic energy bands and spin filtering tuned by Rashba spin-orbit coupling (SOC) and magnetic field in two-dimensional superlattices (2DSLs), where the square rods of quantum barriers, matrix, and wells are imposed periodically. It is shown that electronic energy spectra form a band structure and the energy levels are split up by the Rashba SOC. Correspondingly, the electrical conductance presents a “band-gap” structure against the electron energy. With manipulating the strength of SOC, the conductance in the “band” is enhanced for the spin-up electrons, while it is reduced for the spin-down electrons. Interestingly, by introducing a magnetic modulation, conductance curves for spin-up and spin-down electrons are translated in the different directions. As a result, high spin polarization is observed, and fully spin-polarized conductance is achieved in this 2DSL. Furthermore, the electronic wavefunctions have been obtained, which presents a clear picture of spin filtering. Our investigations achieve potential applications in spin quantum devices and spin filters.

Magnetic-field-induced spin texture in a quantum wire with linear Dresselhaus spin-orbit coupling

A quantum wire with strong Rashba spin-orbit interaction is known to exhibit a so-called spin texturing effect in presence of a moderate external magnetic field acting transverse to the wire axis and parallel to the symmetry-breaking ``gate'' electric field causing Rashba interaction. This effect typically disappears when the external magnetic field is made stronger than the effective spin-orbit magnetic field due to the Rashba interaction. In this work, we present a detailed study of the spin texturing phenomenon in a quantum wire with strong linear Dresselhaus spin-orbit interaction and an external transverse magnetic field. Unlike the pure Rashba case, we observe a persistent spin texture even in the limit when the external magnetic field is large and much stronger than the effective spin-orbit magnetic field due to Dresselhaus interaction. Magnetic field dependence of spin texture is therefore an experimentally viable method for identifying the dominant spin-orbit interaction in a zinc-blende quantum wire. We show that the local distribution of the spin density can be modulated by spin-orbit coupling strength, magnetic field, and the Fermi level of the electron gas.

Majorana Fermions in Equilibrium and in Driven Cold-Atom Quantum Wires

We introduce a new approach to create and detect Majorana fermions using optically trapped 1D fermionic atoms. In our proposed setup, two internal states of the atoms couple via an optical Raman transition-simultaneously inducing an effective spin-orbit interaction and magnetic field-while a background molecular BEC cloud generates s-wave pairing for the atoms. The resulting cold-atom quantum wire supports Majorana fermions at phase boundaries between topologically trivial and nontrivial regions, as well as "Floquet Majorana fermions" when the system is periodically driven. We analyze experimental parameters, detection schemes, and various imperfections.

Properties of Silicon Ballistic Spin Fin-Based Field-Effect Transistors

We investigate the properties of ballistic fin-structured silicon spin field-effect transistors. The spin transistor suggested first by Datta and Das employs spin-orbit coupling to introduce the current modulation. The major contribution to the spin-orbit interaction in silicon films is of the Dresselhaus type due to the interface-induced inversion symmetry breaking. The subband structure in silicon confined systems is obtained with help of a two-band k·p model and is in good agreement with recent density functional calculations. It is demonstrated that fins with [100] orientation display a stronger modulation of the conductance as function of spin-orbit interaction and magnetic field and are thus preferred for practical realizations of silicon SpinFETs.

Scattering effects in resonant magnetotunneling in InAs-based heterostructures

Electron transport through an InGaAs resonant tunneling structure with Rashba spin-orbit interaction and magnetic field parallel to the growth direction was studied theoretically. A nonequilibrium Green's function model was used, wherein interface roughness and longitudinal optical phonon scattering are treated in the self-consistent first Born approximation. The model predicts the main features of the two-dimensional magnetopolaron density of states and the secondary peaks in the I-V curve due to both resonant elastic and inelastic scattering. The I-V curves were studied at magnetic fields around the magnetophonon resonance and the elastic and inelastic contributions identified. At these fields (5 to 7 T), the current spin polarization was found to be dominated by the Zeeman effect and significant even in the presence of scattering events.

Tunability of magnetization in lateral few electron double quantum dots

We demonstrate theoretically a pathway for electrical control of magnetic properties of tunable lateral double InAs quantum dots containing two-electrons, subjected to spin-orbit interaction (SOI), vertical electrical and magnetic fields. In the regime of low temperature and weak magnetic field, interplay of interdot tunnel coupling and hybridized magnetoelectric effect induces “s”-like behavior of magnetization and a peak of susceptibility which are very sensitive to interdot barrier voltage (VB) and interdot distance (d). Sweeping VB and adjusting d not only impact magnetic phase transition, but also switch electronic and magnetic properties from atomic to molecular in nature. Furthermore, SOI broadens the magnetic field interval of paramagnetic phase and changes magnitude of magnetization and susceptibility.

Time-dependent configuration-interaction simulations of spin swap in spin-orbit-coupled double quantum dots

We perform time-dependent simulations of spin exchange for an electron pair in laterally coupled quantum dots. The calculation is based on configuration interaction scheme accounting for spin-orbit (SO) coupling and electron-electron interaction in a numerically exact way. Noninteracting electrons exchange orientations of their spins in a manner that can be understood by interdot tunneling associated with spin precession in an effective SO magnetic field that results in anisotropy of the spin swap. The Coulomb interaction blocks the electron transfer between the dots but the spin transfer and spin precession due to SO coupling is still observed. The electron-electron interaction additionally induces an appearance of spin components in the direction of the effective SO magnetic field which are opposite in both dots. Simulations indicate that the isotropy of the spin swap is restored for equal Dresselhaus and Rashba constants and properly oriented dots.

Strong crystalline field at the Fe site and spin rotation in olivine LiNi0.99 F57e0.01 PO4 material by Mössbauer spectroscopy

The crystal structure of LiNi0.99 F57e0.01 PO4 compound has been determined to be orthorhombic by Rietveld refinement method. Temperature dependence of magnetization M shows an anomalous antiferromagnetic behavior. A sudden change in both the magnitude of magnetic hyperfine field and its slope below 11 K suggests that magnetic phase transition related to the spin ordering takes place abruptly. From the result of Mössbauer measurement, it is shown that a strong electric crystalline field of octahedral symmetry including the contribution of spin-orbit coupling and magnetic hyperfine field by space-modulated spin structure is acted to the sites of Fe2+ ions simultaneously at low temperature.

Bends in nanotubes allow electric spin control and coupling

We investigate combined effects of spin-orbit coupling and magnetic field in carbon nanotubes containing one or more bends along their length. We show how bends can be used to provide electrical control of confined spins, while spins confined in straight segments remain insensitive to electric fields. Device geometries that allow general rotation of single spins are presented and analyzed. In addition, capacitive coupling along bends provides coherent spin-spin interaction, including between otherwise disconnected nanotubes, completing a universal set of one- and two-qubit gates.

Optical out-of-plane spin polarization and charge conductivities in spin-orbit-coupled systems in the presence of an in-plane magnetic field

Out-of-plane spin and charge responses to the terahertz field for a clean two-dimensional electron gas with a Rashba spin-orbit interaction in the presence of an in-plane magnetic field are studied. We show that the characteristic optical spectral behavior is remarkably different from that of the system in the absence of in-plane magnetic fields. It is found that the optical spin polarization normal to the plane is nonzero even for this clean system, in sharp contrast to the static case. Due to the combined effect of spin-orbit coupling and in-plane magnetic field, both diagonal and off-diagonal components of optical charge conductivity tensor are nonvanishing. It is indicated that one can control the spin polarization and the optical current by adjusting the optical frequency. In addition, the out-of-plane spin polarization and conductivities strongly rely on the direction of the external magnetic field. Nevertheless, they meet different angle-dependent relations. This dynamical out-of-plane spin polarization could be measured by the time-resolved Kerr rotation technique.

Current-driven spin torque induced by the Rashba effect in a ferromagnetic metal layer

Methods to manipulate the magnetization of ferromagnets by means of local electric fields or current-induced spin transfer torque allow the design of integrated spintronic devices with reduced dimensions and energy consumption compared with conventional magnetic field actuation. An alternative way to induce a spin torque using an electric current has been proposed based on intrinsic spin-orbit magnetic fields and recently realized in a strained low-temperature ferromagnetic semiconductor. Here we demonstrate that strong magnetic fields can be induced in ferromagnetic metal films lacking structure inversion symmetry through the Rashba effect. Owing to the combination of spin-orbit and exchange interactions, we show that an electric current flowing in the plane of a Co layer with asymmetric Pt and AlO(x) interfaces produces an effective transverse magnetic field of 1 T per 10(8) A cm(-2). Besides its fundamental significance, the high efficiency of this process makes it a realistic candidate for room-temperature spintronic applications.

Evidence for reversible control of magnetization in a ferromagnetic material by means of spin–orbit magnetic field

Conventional computer electronics creates a dichotomy between how information is processed and how it is stored. Silicon chips process information by controlling the flow of charge through a network of logic gates. This information is then stored, most commonly, by encoding it in the orientation of magnetic domains of a computer hard disk. The key obstacle to a more intimate integration of magnetic materials into devices and circuit processing information is a lack of efficient means to control their magnetization. This is usually achieved with an external magnetic field or by the injection of spin-polarized currents. The latter can be significantly enhanced in materials whose ferromagnetic properties are mediated by charge carriers. Among these materials, conductors lacking spatial inversion symmetry couple charge currents to spin by intrinsic spin-orbit (SO) interactions, inducing nonequilibrium spin polarization tunable by local electric fields. Here we show that magnetization of a ferromagnet can be reversibly manipulated by the SO-induced polarization of carrier spins generated by unpolarized currents. Specifically, we demonstrate domain rotation and hysteretic switching of magnetization between two orthogonal easy axes in a model ferromagnetic semiconductor.

Ab initiocalculation of transverse spin current in graphene nanostructures

We report a theoretical analysis of transverse spin current in a four-probe graphene nanostructure in the absence of spin-orbit interaction and magnetic field. The nanostructure consists of a finite-size graphene sheet connected to the outside world by two zigzag graphene nanoribbons GNR and two armchair graphene nanoribbons, forming a cross-shaped two-dimensional device. Due to edge state induced magnetism at zigzag GNR boundaries, our result shows that a pure transverse spin current without an accompanying charge current is induced. We have calculated the transverse spin conductance by an atomic first-principles method where density-functional theory is carried out within the Keldysh nonequilibrium Green’s function framework.