arXiv subjects

Emmanuel I. Rashba

Publications and source records attributed to Emmanuel I. Rashba.

At least 37 records · Page 2Linked to original sources

arXiv2007

Theory of Spin Hall Effects in Semiconductors

Spin Hall effects are a collection of phenomena, resulting from spin-orbit coupling, in which an electrical current flowing through a sample can lead to spin transport in a perpendicular direction and spin accumulation at lateral boundaries. These effects, which do not require an applied magnetic field, can originate in a variety of intrinsic and extrinsic spin-orbit coupling mechanisms and depend on geometry, dimension, impurity scattering, and carrier density of the system--making the analysis of these effects a diverse field of research. In this article, we give an overview of the theoretical background of the spin Hall effects and summarize some of the most important results. First, we explain effective spin-orbit Hamiltonians, how they arise from band structure, and how they can be understood from symmetry considerations; including intrinsic coupling due to bulk inversion or structure asymmetry or due to strain, and extrinsic coupling due to impurities. This leads to different mechanisms of spin transport: spin precession, skew scattering, and side jump. Then we present the kinetic (Boltzmann) equations, which describe the spin-dependent distribution function of charge carriers, and the diffusion equation for spin polarization density. Next, we define the notion of spin currents and discuss their relation to spin polarization. Finally, we explain the electrically induced spin effects; namely, spin polarization and currents in bulk and near boundaries (the focus of most current theoretical research efforts), and spin injection, as well as effects in mesoscopic systems and in edge states.

cond-mat.mes-hall↗

arXiv2007

Spin generation away from boundaries by nonlinear transport

In several situations of interest, spin polarization may be generated far from the boundaries of a sample by nonlinear effects of an electric current, even when such a generation is forbidden by symmetry in the linear regime. We present an analytically solvable model where spin accumulation results from a combination of current gradients, nonlinearity, and cubic anisotropy. Further, we show that even with isotropic conductivity, nonlinear effects in a low symmetry geometry can generate spin polarization far away from boundaries. Finally, we find that drift from the boundaries results in spin polarization patterns that dominate in recent experiments on GaAs by Sih et al. [Phys. Rev. Lett. 97, 096605 (2006)].

cond-mat.mes-hall↗

arXiv2006

Semiconductor Spintronics: Progress and Challenges

Brief review of the recent progress in semiconductor spintronics (theory and experiment) and the current theoretical problems in it is presented. Invited paper at the 2006 Advanced Research Workshop "Future Trends in Microelectronics: Up to Nano Creek" (Aghia Pelaghia, Crete, June 26-30, 2006). To be published in Workshop Proceedings (Wiley)

cond-mat.mes-hall↗

arXiv2006

Out-of-plane spin polarization from in-plane electric and magnetic fields

We show that the joint effect of spin-orbit and magnetic fields leads to a spin polarization perpendicular to the plane of a two-dimensional electron system with Rashba spin-orbit coupling and in-plane parallel dc magnetic and electric fields, for angle-dependent impurity scattering or nonparabolic energy spectrum, while only in-plane polarization persists for simplified models. We derive Bloch equations, describing the main features of recent experiments, including the magnetic field dependence of static and dynamic responses.

cond-mat.mes-hall↗

arXiv2005

Spin-orbit coupling and spin transport

Recent achievements in semiconductor spintronics are discussed. Special attention is paid to spin-orbit interaction, coupling of electron spins to external electric fields, and spin transport in media with spin-orbit coupling, including the mechanisms of spin-Hall effect. Importance of spin-transport parameters at spin-precession wave vector $k_{\rm so}$ is emphasized, and existence of an universal relation between spin currents and spin accumulation at the spatial scale of $\ell_{\rm so}\approx k_{\rm so}^{-1}$ is conjectured.

cond-mat.mes-hall↗

arXiv2005

Theory of Spin Hall conductivity in n-doped GaAs

We develop a theory of extrinsic spin currents in semiconductors, resulting from spin-orbit coupling at charged scatterers, which leads to skew scattering and side jump contributions to the spin Hall conductance. Applying the theory to bulk n-GaAs, without any free parameters, we find spin currents that are in reasonable agreement with recent experiments by Kato et al. [Science 306, 1910 (2004)].

cond-mat.mes-hall↗

arXiv2004

Sum rules for spin-Hall conductivity cancelation

It has been shown recently that the universal dc spin conductivity of two-dimensional electrons with a Rashba spin-orbit interaction is canceled by vertex corrections in a weak scattering regime. We prove that the zero bulk spin conductivity is an intrinsic property of the free-electron Hamiltonian and scattering is merely a tool to reveal this property in terms of the diagrammatic technique. When Zeeman energy is neglected, the zero dc conductivity persists in a magnetic field. Spin conductivity increases resonantly at the cyclotron frequency and then decays towards the universal value.

cond-mat.mes-hall↗

arXiv2004

Spin Dynamics and Spin Transport

Spin-orbit (SO) interaction critically influences electron spin dynamics and spin transport in bulk semiconductors and semiconductor microstructures. This interaction couples electron spin to dc and ac electric fields. Spin coupling to ac electric fields allows efficient spin manipulating by the electric component of electromagnetic field through the electric dipole spin resonance (EDSR) mechanism. Usually, it is much more efficient than the magnetic manipulation due to a larger coupling constant and the easier access to spins at a nanometer scale. The dependence of the EDSR intensity on the magnetic field direction allows measuring the relative strengths of the competing SO coupling mechanisms in quantum wells. Spin coupling to an in-plane electric field is much stronger than to a perpendicular field. Because electron bands in microstructures are spin split by SO interaction, electron spin is not conserved and spin transport in them is controlled by a number of competing parameters, hence, it is rather nontrivial. The relation between spin transport, spin currents, and spin populations is critically discussed. Importance of transients and sharp gradients for generating spin magnetization by electric fields and for ballistic spin transport is clarified.

cond-mat.mes-hall↗

arXiv2004

Spin currents, spin populations, and dielectric function

In Maxwellian electrodynamics, specific properties of the responses to external fields are included in constitutive equations. For noncentrosymmetric semiconductors, spin conductivity can be expressed in terms of the contribution of electric-dipole transitions between spin-split spectrum branches to the dielectric function. In a dissipationless regime, a spin current driven by an external electric field is tantamount to a background current in an equilibrium system with a reduced symmetry. The importance of transients and gradients for efficient spin-current injection is emphasized.

cond-mat.mes-hall↗

arXiv2003

Spin currents in thermodynamic equilibrium?

The standard definition of a spin current, applied to the conductors lacking inversion symmetry, results in nonzero spin currents. I demonstrate that the spin currents do not vanish even in the thermodynamic equilibrium, in the absence of external fields. These currents are dissipationless and are not associated with a real spin transport. The result should be taken as a warning indicating problems inherent in the theory of transport spin currents driven by external fields.

cond-mat.mes-hall↗

arXiv2003

Inelastic scattering approach to the theory of a tunnel magnetic transistor source

The high efficiency of a tunnel magnetic transistor as a source of spin-polarized electrons has been proven recently [X. Jiang {\it et al.}, Phys. Rev. Lett. {\bf 90}, 256603 (2003)]. A concept of this device based on an active group of hot electrons controlling the collector current and experiencing predominantly inelastic scattering in the base is developed. It takes into account the interconnection between the injection and filtering of spin-polarized electrons in different elements of the device. Explicit expressions for the parameters of the device in terms of the basic parameters of the system are presented.

cond-mat.mes-hall↗

arXiv2003

Electron spin operation by electric fields: spin dynamics and spin injection

Spin-orbit interaction couples electron spins to electric fields and allows electrical monitoring of electron spins and electrical detection of spin dynamics. Competing mechanisms of spin-orbit interaction are compared, and optimal conditions for the electric operation of electrons spins in a quantum well by a gate voltage are established. Electric spin injection into semiconductors is discussed with a special emphasis on the injection into ballistic microstructures. Dramatic effect of a long range Coulomb interaction on transport phenomena in space-quantized low-dimensional conductors is discussed in conclusion.

cond-mat.mes-hall↗

arXiv2002

Spin injection into a ballistic semiconductor microstructure

A theory of spin injection across a ballistic ferromagnet-semiconductor-ferromagnet junction is developed for the Boltzmann regime. Spin injection coefficient $γ$ is suppressed by the Sharvin resistance of the semiconductor $r_N^*=(h/e^2)(π^2/S_N)$, where $S_N$ is the Fermi-surface cross-section. It competes with the diffusion resistances of the ferromagnets $r_F$, and $γ\sim r_F/r_N^*\ll 1$ in the absence of contact barriers. Efficient spin injection can be ensured by contact barriers. Explicit formulae for the junction resistance and the spin-valve effect are presented.

cond-mat.mes-hall↗

arXiv2002

Diffusion theory of spin injection through resistive contacts

Insertion of a resistive contact between a ferromagnetic metal and a semiconductor microstructure is of critical importance for achieving efficient spin injection into a semiconductor. However, the equations of the diffusion theory are rather cumbersome for the junctions including such contacts. A technique based on deriving a system of self-consistent equations for the coefficients of spin injection, "gamma", through different contacts are developed. These equations are concise when written in the proper notations. Moreover, the resistance of a two-contact junction can be expressed in terms of "gamma"'s of both contacts. This equation makes calculating the spin valve effect straightforward, allows to find an explicit expression for the junction resistance and to prove that its nonequilibrium part is positive. Relation of these parameters to different phenomena like spin-e.m.f. and the junction transients is established. Comparative effect of the Coulomb screening on different parameters is clarified. It is also shown that the spin non-conservation in a contact can have a dramatic effect on the non-equilibrium resistance of the junction.

cond-mat.mtrl-sci↗

arXiv2002

Classical properties of low-dimensional conductors: Giant capacitance and non-Ohmic potential drop

Electrical field arising around an inhomogeneous conductor when an electrical current passes through it is not screened, as distinct from 3D conductors, in low-dimensional conductors. As a result, the electrical field depends on the global distribution of the conductivity sigma(x) rather than on the local value of it, inhomogeneities of sigma(x) produce giant capacitances C(omega) that show frequency dependence at relatively low omega, and electrical fields develop in vast regions around the inhomogeneities of sigma(x). A theory of these phenomena is presented for 2D conductors.

cond-mat.mes-hall↗

arXiv2002

Theory of spin injection

Diffusive theory of spin injection is reviewed and a number of new results is presented for the dc and ac regimes. They were derived by means of the "gamma-technique" allowing to simplify the calculations by choosing the spin injection coefficients through different interfaces as the basic variables. The prospects for increasing spin injection by using resistive spin-selective contacts are emphasized and spin non-conserving contacts are introduced. Finding the basic parameters of a junction from the ac data is discussed.

cond-mat.mes-hall↗

arXiv2002

Complex impedance of a spin injecting junction

Theory of the ac spin injection from a ferromagnetic electrode into a normal conductor through a tunnel or Schottky contact is developed. Diffusion and relaxation of non-equilibrium spins results in a frequency dependent complex impedance controlled by the spin relaxation rates and the resistances involved. Explicit expression for the impedance is presented. Experimental investigation of the frequency dependence of the impedance should allow measuring spin relaxation times in both conductors, their effective resistances, and also the parameters of the contact controlling the spin injection.

cond-mat.mes-hall↗

arXiv2001

Superconducting 2D system with lifted spin degeneracy: Mixed singlet-triplet state

Motivated by recent experimental findings, we have developed a theory of the superconducting state for 2D metals without inversion symmetry modeling the geometry of a surface superconducting layer in a field-effect-transistor or near the boundary doped by adsorbed ions. In such systems the two-fold spin degeneracy is lifted by spin-orbit interaction, and singlet and triplet pairings are mixed in the wave function of the Cooper pairs. As a result, spin magnetic susceptibility becomes anisotropic and Knight shift retains finite and rather high value at T=0.

cond-mat.supr-con↗