Orbital ac spin-Hall effect in the hopping regime

The Rashba and Dresselhaus spin-orbit interactions are both shown to yield the low temperature spin-Hall effect for strongly localized electrons coupled to phonons. A frequency-dependent electric field ${\bf E}(\omega)$ generates a spin-polarization current, normal to ${\bf E}$, due to interference of hopping paths. At zero temperature the corresponding spin-Hall conductivity is real and is proportional to $\omega^{2}$. At non-zero temperatures the coupling to the phonons yields an imaginary term proportional to $\omega$. The interference also yields persistent spin currents at thermal equilibrium, at ${\bf E}=0$. The contributions from the Dresselhaus and Rashba interactions to the interference oppose each other.Comment: 4 pages, no figure

Mesoscopic Spin-Hall Effect in 2D electron systems with smooth boundaries

Spin-Hall effect in ballistic 2D electron gas with Rashba-type spin-orbit coupling and smooth edge confinement is studied. We predict that the interplay of semiclassical electron motion and quantum dynamics of spins leads to several distinct features in spin density along the edge that originate from accumulation of turning points from many classical trajectories. Strong peak is found near a point of the vanishing of electron Fermi velocity in the lower spin-split subband. It is followed by a strip of negative spin density that extends until the crossing of the local Fermi energy with the degeneracy point where the two spin subbands intersect. Beyond this crossing there is a wide region of a smooth positive spin density. The total amount of spin accumulated in each of these features exceeds greatly the net spin across the entire edge. The features become more pronounced for shallower boundary potentials, controlled by gating in typical experimental setups.Comment: 4 pages, 4 figures, published versio

Optical Conductivity of a Two-Dimensional Electron Liquid with Spin-Orbit Interaction

The interplay of electron-electron interactions and spin-orbit coupling leads to a new contribution to the homogeneous optical conductivity of the electron liquid. The latter is known to be insensitive to many-body effects for a conventional electron system with parabolic dispersion. The parabolic spectrum has its origin in the Galilean invariance which is broken by spin-orbit coupling. This opens up a possibility for the optical conductivity to probe electron-electron interactions. We analyze the interplay of interactions and spin-orbit coupling and obtain optical conductivity beyond RPA.Comment: 5 pages, 3 figures; final version, fig. 3 added, minor change

Small-angle impurity scattering and the spin Hall conductivity in 2D systems

An arbitrarily small concentration of impurities can affect the spin Hall conductivity in a two-dimensional semiconductor system. We develop a Boltzmann-like equation that can be used for impurity scattering with arbitrary angular dependence, and for arbitrary angular dependence of the spin-orbit field b(k) around the Fermi surface. For a model applicable to a 2D hole system in GaAs, if the impurity scattering is not isotropic, we find that the spin Hall conductivity depends on the derivative of b with respect to the energy and on deviations from a parabolic band structure, as well as on the angular dependence of the scattering. In principle, the resulting spin Hall conductivity can be larger or smaller than the ``intrinsic value'', and can have opposite sign. In the limit of small angle scattering, in a model appropriate for small hole concentrations, where the band is parabolic and b ~ k^3, the spin Hall conductivity has opposite sign from the intrinsic value, and has larger magnitude. Our analysis assumes that the spin-orbit splitting $b$ and the transport scattering rate tau^{-1} are both small compared to the Fermi energy, but the method is valid for for arbitrary value of b*tau.Comment: Errors corrected, references adde

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.Comment: 5 pages and 1 figure in main text, 5 pages in appendi

Spin orientation of a two-dimensional electron gas by a high-frequency electric field

Coupling of spin states and space motion of conduction electrons due to spin-orbit interaction opens up possibilities for manipulation of the electron spins by electrical means. It is shown here that spin orientation of a two-dimensional electron gas can be achieved by excitation of the carriers with a linearly polarized high-frequency electric field. In (001)-grown quantum well structures excitation with in-plane ac electric field induces orientation of the electron spins along the quantum well normal, with the spin sign and the magnitude depending on the field polarization.Comment: 5 pages, 1 figur

Self Injection length in La0.7 Ca0.3 Mno3-YBa 2Cu3O7-d ferromagnet- superconductor multi layer thin films

We have carried out extensive studies on the self-injection problem in barrierless heterojunctions between La0.7Ca0.3MnO3 (LCMO) and YBa2Cu3O7-d (YBCO). The heterojunctions were grown in situ by sequentially growing LCMO and YBCO films on LaAlO3 (LAO) substrate using a pulsed laser deposition (PLD) system. YBCO micro-bridges with 64 microns width were patterned both on the LAO (control) and LCMO side of the substrate. Critical current, Ic, was measured at 77K on both the control side as well as the LCMO side for different YBCO film thickness. It was observed that while the control side showed a Jc of ~2 x 10E6 A/ cm2 the LCMO side showed about half the value for the same thickness (1800 A). The difference in Jc indicates that a certain thickness of YBCO has become 'effectively' normal due to self-injection. From the measurement of Jc at two different thickness' (1800 A and 1500 A) of YBCO both on the LAO as well as the LCMO side, the value of self-injection length (at 77K) was estimated to be ~900 A self-injection length has been quantified. A control experiment carried out with LaNiO3 deposited by PLD on YBCO did not show any evidence of self-injection.Comment: 6 pages, one figure in .ps forma

Proximity effect, quasiparticle transport, and local magnetic moment in ferromagnet-d-wave superconductor junctions

The proximity effect, quasiparticle transport, and local magnetic moment in ferromagnet--d-wave superconductor junctions with {110}-oriented interface are studied by solving self-consistently the Bogoliubov-de Gennes equations within an extended Hubbard model. It is found that the proximity induced order parameter oscillates in the ferromagnetic region. The modulation period is shortened with the increased exchange field while the oscillation amplitude is depressed by the interfacial scattering. With the determined superconducting energy gap, a transfer matrix method is proposed to compute the subgap conductance within a scattering approach. Many novel features including the zero-bias conductance dip and splitting are exhibited with appropriate values of the exchange field and interfacial scattering strength. The conductance spectrum can be influenced seriously by the spin-flip interfacial scattering. In addition, a sizable local magnetic moment near the {110}-oriented surface of the d-wave superconductor is discussed.Comment: 10 pages, 16 ps-figures, to appear in Phys. Rev.

Ferromagnetic/superconducting proximity effect in La0.7Ca0.3MnO3 / YBa2Cu3O7 superlattices

We study the interplay between magnetism and superconductivity in high quality YBa2Cu3O7 (YBCO) / La0.7Ca0.3MnO3(LCMO)superlattices. We find evidence for the YBCO superconductivity depression in presence of the LCMO layers. We show that due to its short coherence length superconductivity survives in the YBCO down to much smaller thickness in presence of the magnetic layer than in low Tc superconductors. We also find that for a fixed thickness of the superconducting layer, superconductivity is depressed over a thickness interval of the magnetic layer in the 100 nm range. This is a much longer length scale than that predicted by the theory of ferromagnetic/superconducting proximity effect.Comment: 10 pages + 5 figures, submitted to Phys. Rev.

First-principles envelope-function theory for lattice-matched semiconductor heterostructures

In this paper a multi-band envelope-function Hamiltonian for lattice-matched semiconductor heterostructures is derived from first-principles norm-conserving pseudopotentials. The theory is applicable to isovalent or heterovalent heterostructures with macroscopically neutral interfaces and no spontaneous bulk polarization. The key assumption -- proved in earlier numerical studies -- is that the heterostructure can be treated as a weak perturbation with respect to some periodic reference crystal, with the nonlinear response small in comparison to the linear response. Quadratic response theory is then used in conjunction with k.p perturbation theory to develop a multi-band effective-mass Hamiltonian (for slowly varying envelope functions) in which all interface band-mixing effects are determined by the linear response. To within terms of the same order as the position dependence of the effective mass, the quadratic response contributes only a bulk band offset term and an interface dipole term, both of which are diagonal in the effective-mass Hamiltonian. Long-range multipole Coulomb fields arise in quantum wires or dots, but have no qualitative effect in two-dimensional systems beyond a dipole contribution to the band offsets.Comment: 25 pages, no figures, RevTeX4; v3: final published versio