269 research outputs found
Transverse Spin-Orbit Force in the Spin Hall Effect in Ballistic Semiconductor Wires
We introduce the spin and momentum dependent {\em force operator} which is
defined by the Hamiltonian of a {\em clean} semiconductor quantum wire with
homogeneous Rashba spin-orbit (SO) coupling attached to two ideal (i.e., free
of spin and charge interactions) leads. Its expectation value in the
spin-polarized electronic wave packet injected through the leads explains why
the center of the packet gets deflected in the transverse direction. Moreover,
the corresponding {\em spin density} will be dragged along the transverse
direction to generate an out-of-plane spin accumulation of opposite signs on
the lateral edges of the wire, as expected in the phenomenology of the spin
Hall effect, when spin- and spin- polarized packets
(mimicking the injection of conventional unpolarized charge current) propagate
simultaneously through the wire. We also demonstrate that spin coherence of the
injected spin-polarized wave packet will gradually diminish (thereby
diminishing the ``force'') along the SO coupled wire due to the entanglement of
spin and orbital degrees of freedom of a single electron, even in the absence
of any impurity scattering.Comment: 5 pages, 4 color EPS figures; 2 new figures and expanded discussion
on the sign of spin Hall quantities. To appear in Phys. Rev. B 72 (2005
Band structure and magnetotransport of a two-dimensional electron gas in the presence of spin-orbit interaction
The band structure and magnetotransport of a two-dimensional electron gas
(2DEG), in the presence of the Rashba (RSOI) and Dresselhaus (DSOI) terms of
the spin-orbit interaction and of a perpendicular magnetic field, is
investigated. Exact and approximate analytical expressions for the band
structure are obtained and used to calculate the density of states (DOS) and
the longitudinal magnetoresitivity assuming a Gaussian type of level
broadening. The interplay between the Zeeman coupling and the two terms of the
SOI is discussed. If the strengths and , of the RSOI and DSOI,
respectively, are equal and the factor vanishes, the two spin states are
degenerate and a shifted Landau-level structure appears. With the increase of
the difference , a novel beating pattern of the DOS and of the
Shubnikov-de Haas (SdH) oscillations appears distinctly different from that
occurring when one of these strengths vanishes
Spin relaxation in -type (111) GaAs quantum wells
We investigate the spin relaxation limited by the D'yakonov-Perel' mechanism
in -type (111) GaAs quantum wells, by means of the kinetic spin Bloch
equation approach. In (111) GaAs quantum wells, the in-plane effective magnetic
field from the D'yakonov-Perel' term can be suppressed to zero on a special
momentum circle under the proper gate voltage, by the cancellation between the
Dresselhaus and Rashba spin-orbit coupling terms. When the spin-polarized
electrons mainly distribute around this special circle, the in-plane
inhomogeneous broadening is small and the spin relaxation can be suppressed,
especially for that along the growth direction of quantum well. This
cancellation effect may cause a peak (the cancellation peak) in the density or
temperature dependence of the spin relaxation time. In the density
(temperature) dependence, the interplay between the cancellation peak and the
ordinary density (Coulomb) peak leads to rich features of the density
(temperature) dependence of the spin relaxation time. The effect of impurities,
with its different weights on the cancellation peak and the Coulomb peak in the
temperature dependence of the spin relaxation, is revealed. We also show the
anisotropy of the spin relaxation with respect to the spin-polarization
direction.Comment: 8 pages, 6 figure
Spin polarization decay in spin-1/2 and spin-3/2 systems
We present a general unifying theory for spin polarization decay due to the
interplay of spin precession and momentum scattering that is applicable to both
spin-1/2 electrons and spin-3/2 holes. Our theory allows us to identify and
characterize a wide range of qualitatively different regimes. For strong
momentum scattering or slow spin precession we recover the D'yakonov-Perel
result, according to which the spin relaxation time is inversely proportional
to the momentum relaxation time. On the other hand, we find that, in the
ballistic regime the carrier spin polarization shows a very different
qualitative behavior. In systems with isotropic spin splitting the spin
polarization can oscillate indefinitely, while in systems with anisotropic spin
splitting the spin polarization is reduced by spin dephasing, which is
non-exponential and may result in an incomplete decay of the spin polarization.
For weak momentum scattering or fast spin precession, the oscillations or
non-exponential spin dephasing are modulated by an exponential envelope
proportional to the momentum relaxation time. Nevertheless, even in this case
in certain systems a fraction of the spin polarization may survive at long
times. Finally it is shown that, despite the qualitatively different nature of
spin precession in the valence band, spin polarization decay in spin-3/2 hole
systems has many similarities to its counterpart in spin-1/2 electron systems.Comment: 4 pages, 1 figure, to appear in Phys. Rev.
Decoherence of coupled electron spins via nuclear spin dynamics in quantum dots
In double quantum dots, the exchange interaction between two electron spins
renormalizes the excitation energy of pair-flips in the nuclear spin bath,
which in turn modifies the non-Markovian bath dynamics. As the energy
renormalization varies with the Overhauser field mismatch between the quantum
dots, the electron singlet-triplet decoherence resulting from the bath dynamics
depends on sampling of nuclear spin states from an ensemble, leading to the
transition from exponential decoherence in single-sample dynamics to power-law
decay under ensemble averaging. In contrast, the decoherence of a single
electron spin in one dot is essentially the same for different choices of the
nuclear spin configuration.Comment: 4 pages 3 figure
Relaxation mechanism for electron spin in the impurity band of n-doped semiconductors
We propose a mechanism to describe spin relaxation in n-doped III-V
semiconductors close to the Mott metal-insulator transition. Taking into
account the spin-orbit interaction induced spin admixture in the hydrogenic
donor states, we build a tight-binding model for the spin-dependent impurity
band. Since the hopping amplitudes with spin flip are considerably smaller than
the spin conserving counterparts, the resulting spin lifetime is very large. We
estimate the spin lifetime from the diffusive accumulation of spin rotations
associated with the electron hopping. Our result is larger but of the same
order of magnitude than the experimental value. Therefore the proposed
mechanism has to be included when describing spin relaxation in the impurity
band.Comment: 4 page
Pauli blockade of the electron spin flip in bulk GaAs
By means of time-resolved optical orientation under strong optical pumping,
the k-dependence of the electron spin-flip time (t_sf) in undoped GaAs is
experimentally determined. t_sf monotonically decreases by more than one order
of magnitude when the electron kinetic energy varies from 2 to 30 meV. At the
high excitation densities and low temperatures of the reported experiments the
main spin-flip mechanism of the conduction band electrons is the
Bir-Aronov-Pikus. By means of Monte-Carlo simulations we evidence that
phase-space filling effects result in the blocking of the spin flip, yielding
an increase of t_sf with excitation density. These effects obtain values of
t_sf up to 30 ns at k=0, the longest reported spin-relaxation time in undoped
GaAs in the absence of a magnetic field.Comment: new author added, major changes in section IV (phenomenological
model), minor changes throughout the entire manuscrip
Dynamical symmetry breaking as the origin of the zero--resistance state in an -driven system
Under a strong drive the zero-frequency linear response dissipative
resistivity of a homogeneous state is allowed to become
negative. We show that such a state is absolutely unstable. The only
time-independent state of a system with a is characterized by
a current which almost everywhere has a magnitude fixed by the
condition that the nonlinear dissipative resistivity .
As a result, the dissipative component of the electric field vanishes. The
total current may be varied by rearranging the current pattern appropriately
with the dissipative component of the -electric field remaining zero. This
result, together with the calculation of Durst \emph{et. al.}, indicating the
existence of regimes of applied microwave field and magnetic field
where , explains the zero-resistance state observed by Mani
\emph{et. al.} and Zudov \emph{et. al.}.Comment: Published versio
Triplet-singlet relaxation in semiconductor single and double quantum dots
We study the triplet-singlet relaxation in two-electron semiconductor quantum
dots. Both single dots and vertically coupled double dots are discussed. In our
work, the electron-electron Coulomb interaction, which plays an important role
in the electronic structure, is included. The spin mixing is caused by
spin-orbit coupling which is the key to the triplet-singlet relaxation. We show
that the selection rule widely used in the literature is incorrect unless near
the crossing/anticrossing point in single quantum dots. The triplet/singlet
relaxation in double quantum dots can be markedly changed by varying barrier
height, inter-dot distance, external magnetic field and dot size.Comment: 7 pages, 4 figures, PRB in pres
Fulde-Ferrel-Larkin-Ovchinnikov Inhomogeneous Superconducting State and Phase Transitions Induced by Spin Accumulation in a Ferromagnet--Wave Superconductor-Ferromagnet Tunnel Junction
Fulde-Ferrel-Larkin-Ovchinnikov (FFLO) inhomogeneous superconducting (SC)
state, first- and second-order phase transitions, and quantum criticality
induced by spin accumulation in a ferromagnet--wave
superconductor-ferromagnet tunnel junction are theoretically predicted. A
complex phase diagram in the temperature-bias voltage plane is determined. It
is found that the phase transitions from the homogeneous BCS state to the
inhomogeneous FFLO state, and from the FFLO state with the momentum 's azimuthal angle to that with , are of the first-order; while the transitions from all SC
states to the normal state at critical voltages are of the second-order. A
Lifshitz point, a bicritical point and a quantum critical point are identified.Comment: 5 pages, 5 figure
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