3,173 research outputs found
Bulk Spin-Hall Effect
We show that a two-dimensional spin-orbit-coupled system in the presence of a
charge/spin-density wave with a wave-vector perpendicular to an applied
electric field supports bulk manifestations of the direct/inverse spin-Hall
effect. We develop a theory of this phenomenon in the framework of the spin
diffusion equation formalism and show that, due to the inhomogeneity created by
a spin-grating, an anomalous bulk charge-density wave is induced away from
sample boundaries. The optimal conditions for the observation of the effect are
determined. The main experimental manifestation of the bulk spin-Hall effect,
the induced charge/spin-density-wave, is characterized by a pi/2-phase shift
relative to the initial non-homogeneous spin/charge-polarization profile and
has a non-monotonic time-varying amplitude.Comment: 4 pages, 4 figure
Surface states, Friedel oscillations, and spin accumulation in p-doped semiconductors
We consider a hole-doped semiconductor with a sharp boundary and study the
boundary spin accumulation in response to a charge current. First, we solve
exactly a single-particle quantum mechanics problem described by the isotropic
Luttinger model in half-space and construct an orthonormal basis for the
corresponding Hamiltonian. It is shown that the complete basis includes two
types of eigenstates. The first class of states contains conventional incident
and reflected waves, while the other class includes localized surface states.
Second, we consider a many-body system in the presence of a charge current
flowing parallel to the boundary. It is shown that the localized states
contribute to spin accumulation near the surface. We also show that the spin
density exhibits current-induced Friedel oscillations with three different
periods determined by the Fermi momenta of the light and heavy holes. We find
an exact asymptotic expression for the Friedel oscillations far from the
boundary. We also calculate numerically the spin density profile and compute
the total spin accumulation, which is defined as the integral of the spin
density in the direction perpendicular to the boundary. The total spin
accumulation is shown to fit very well the simple formula S ~(1 - m_L/m_H)^2,
where m_L and m_H are the light- and heavy-hole masses. The effects of disorder
are discussed. We estimate the spin relaxation time in the Luttinger model and
argue that spin physics cannot be described within the diffusion approximation.Comment: 22 pages, 8 color figure
Spin relaxation in a generic two-dimensional spin-orbit coupled system
We study the relaxation of a spin density injected into a two-dimensional
electron system with generic spin-orbit interactions. Our model includes the
Rashba as well as linear and cubic Dresselhaus terms. We explicitly derive a
general spin-charge coupled diffusion equation. Spin diffusion is characterized
by just two independent dimensionless parameters which control the interplay
between different spin-orbit couplings. The real-time representation of the
diffuson matrix (Green's function of the diffusion equation) is evaluated
analytically. The diffuson describes space-time dynamics of the injected spin
distribution. We explicitly study two regimes: The first regime corresponds to
negligible spin-charge coupling and is characterized by standard charge
diffusion decoupled from the spin dynamics. It is shown that there exist
several qualitatively different dynamic behaviors of the spin density, which
correspond to various domains in the spin-orbit coupling parameter space. We
discuss in detail a few interesting phenomena such as an enhancement of the
spin relaxation times, real space oscillatory dynamics, and anisotropic
transport. In the second regime, we include the effects of spin-charge
coupling. It is shown that the spin-charge coupling leads to an enhancement of
the effective charge diffusion coefficient. We also find that in the case of
strong spin-charge coupling, the relaxation rates formally become complex and
the spin/charge dynamics is characterized by real time oscillations. These
effects are qualitatively similar to those observed in spin-grating experiments
[Weber et al., Nature 437, 1330 (2005)].Comment: 18 pages, 7 figure
Two-dimensional surface charge transport in topological insulators
We construct a theory of charge transport by the surface states of topological insulators in three dimensions. The focus is on the experimentally relevant case when the electron doping is such that the Fermi energy and transport scattering time satisfy , but sufficiently low that lies below the bottom of the conduction band. Our theory is based on the spin density matrix and takes the quantum Liouville equation as its starting point. The scattering term is determined accurately to linear order in the impurity density. We consider scattering by charged impurities and short-range scatterers such as surface roughness. We calculate also the polarization function in topological insulators, emphasizing the differences from graphene. We find that the main contribution to the conductivity is , where is the impurity density, and will have different carrier density dependencies for different forms of scattering. Two different contributions to this conductivity are traced to the scalar and spin-dependent terms in the Hamiltonian and their relative weight depends on the doping density. Our results contain all contributions to the conductivity to orders zero and one in the impurity density. We discuss also a way to determine the dominant scattering angles by studying the ratio of the transport relaxation time to the Bloch lifetime as a function of the Wigner-Seitz radius . We also discuss the effect on the surface states of adding metallic contacts. Comment: 16 pages, 3 figure
Role of disorder in half-filled high Landau levels
We study the effects of disorder on the quantum Hall stripe phases in
half-filled high Landau levels using exact numerical diagonalization. We show
that, in the presence of weak disorder, a compressible, striped charge density
wave, becomes the true ground state. The projected electron density profile
resembles that of a smectic liquid. With increasing disorder strength W, we
find that there exists a critical value, W_c \sim 0.12 e^2/\epsilon l, where a
transition/crossover to an isotropic phase with strong local electron density
fluctuations takes place. The many-body density of states are qualitatively
distinguishable in these two phases and help elucidate the nature of the
transition.Comment: 4 pages, 4 figure
Superconductivity in a two dimensional extended Hubbard model
The Roth's two-pole approximation has been used by the present authors to
investigate the role of hybridization in the superconducting properties
of an extended Hubbard model. Superconductivity with singlet
-wave pairing is treated by following Beenen and Edwards
formalism. In this work, the Coulomb interaction, the temperature and the
superconductivity have been considered in the calculation of some relevant
correlation functions present in the Roth's band shift. The behavior of the
order parameter associated with temperature, hybridization, Coulomb interaction
and the Roth's band shift effects on superconductivity are studied.Comment: 14 pages, 8 figures, accepted for publication in European Physical
Journal
Structural and magnetic properties of CoPt mixed clusters
In this present work, we report a structural and magnetic study of mixed
Co58Pt42 clusters. MgO, Nb and Si matrix can be used to embed clusters,
avoiding any magnetic interactions between particles. Transmission Electron
Microscopy (TEM) observations show that Co58Pt42 supported isolated clusters
are about 2nm in diameter and crystallized in the A1 fcc chemically disordered
phase. Grazing Incidence Small Angle X-ray Scattering (GISAXS) and Grazing
Incidence Wide Angle X-ray Scattering (GIWAXS) reveal that buried clusters
conserve these properties, interaction with matrix atoms being limited to their
first atomic layers. Considering that 60% of particle atoms are located at
surface, this interactions leads to a drastic change in magnetic properties
which were investigated with conventional magnetometry and X-Ray Magnetic
Circular Dichro\"{i}sm (XMCD). Magnetization and blocking temperature are
weaker for clusters embedded in Nb than in MgO, and totally vanish in silicon
as silicides are formed. Magnetic volume of clusters embedded in MgO is close
to the crystallized volume determined by GIWAXS experiments. Cluster can be
seen as a pure ferromagnetic CoPt crystallized core surrounded by a
cluster-matrix mixed shell. The outer shell plays a predominant role in
magnetic properties, especially for clusters embedded in niobium which have a
blocking temperature 3 times smaller than clusters embedded in MgO
Finite Temperature Density Instability at High Landau Level Occupancy
We study here the onset of charge density wave instabilities in quantum Hall
systems at finite temperature for Landau level filling . Specific
emphasis is placed on the role of disorder as well as an in-plane magnetic
field. Beyond some critical value, disorder is observed to suppress the charge
density wave melting temperature to zero. In addition, we find that a
transition from perpendicular to parallel stripes (relative to the in-plane
magnetic field) exists when the electron gas thickness exceeds \AA.
The perpendicular alignment of the stripes is in agreement with the
experimental finding that the easy conduction direction is perpendicular to the
in-plane field.Comment: 4 pages, 2 eps figures. We show explicitly that a transition from
perpendicular to parallel stripes (relative to the in-plane magnetic field)
exists when the electron gas thickness exceeds \AA. The
perpendicular alignment of the stripes is in agreement with the experimental
finding that the easy conduction direction is perpendicular to the in-plane
fiel
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