253 research outputs found
Spin relaxation in a GaAs quantum dot embedded inside a suspended phonon cavity
The phonon-induced spin relaxation in a two-dimensional quantum dot embedded
inside a semiconductor slab is investigated theoretically. An enhanced
relaxation rate is found due to the phonon van Hove singularities. Oppositely,
a vanishing deformation potential may also result in a suppression of the spin
relaxation rate. For larger quantum dots, the interplay between the spin orbit
interaction and Zeeman levels causes the suppression of the relaxation at
several points. Furthermore, a crossover from confined to bulk-like systems is
obtained by varying the width of the slab.Comment: 5 pages, 4 figures, to apper in Phys. Rev. B (2006
Suppression of the D'yakonov-Perel' spin relaxation mechanism for all spin components in [111] zincblende quantum wells
We apply the D'yakonov-Perel' (DP) formalism to [111]-grown zincblende
quantum wells (QWs) to compute the spin lifetimes of electrons in the
two-dimensional electron gas. We account for both bulk and structural inversion
asymmetry (Rashba) effects. We see that, under certain conditions, the spin
splitting vanishes to first order in k, which effectively suppresses the DP
spin relaxation mechanism for all spin components. We predict extended spin
lifetimes as a result, giving rise to the possibility of enhanced spin storage.
We also study [110]-grown QWs, where the effect of structural inversion
asymmetry is to augment the spin relaxation rate of the component perpendicular
to the well. We derive analytical expressions for the spin lifetime tensor and
its proper axes, and see that they are dependent on the relative magnitude of
the BIA- and SIA-induced splittings.Comment: v1: 5 pages, 2 figures, submitted to PRL v2: added 1 figure and
supporting content, PRB forma
Spin-orbit Hanle effect in high-mobility quantum wells
We study the depolarization of optically oriented electrons in quantum wells
subjected to an in-plane magnetic field and show that the Hanle curve
drastically depends on the carrier mobility. In low-mobility structures, the
Hanle curve is described by a Lorentzian with the width determined by the
effective g-factor and the spin lifetime. In contrast, the magnetic field
dependence of spin polarization in high-mobility quantum wells is nonmonotonic:
The spin polarization rises with the magnetic field induction at small fields,
reaches maximum and then decreases. We show that the position of the Hanle
curve maximum can be used to directly measure the spin-orbit Rashba/Dresselhaus
magnetic field.Comment: 4 pages, 3 figure
Asymptotically self-similar propagation of the spherical ionization waves
It is shown that a new type of the self-similar spherical ionization waves
may exist in gases. All spatial scales and the propagation velocity of such
waves increase exponentially in time. Conditions for existence of these waves
are established, their structure is described and approximate analytical
relationships between the principal parameters are obtained. It is notable that
spherical ionization waves can serve as the simplest, but structurally complete
and physically transparent model of streamer in homogeneous electric field.Comment: Corrected typos, the more precise formulas are obtaine
Spin dynamics of two-dimensional electrons with Rashba spin-orbit coupling and electron-electron interactions
We study the spin dynamics of two dimensional electron gases (2DEGs) with
Rashba spin-orbit coupling by taking account of electron-electron interactions.
The diffusion equations for charge and spin densities are derived by making use
of the path-integral approach and the quasiclassical Green's function.
Analyzing the effect of the interactions, we show that the spin-relaxation time
can be enhanced by the electron-electron interaction in the ballistic regime.Comment: accepted for publication in Phys. Rev.
Semiclassical theory of weak antilocalization and spin relaxation in ballistic quantum dots
We develop a semiclassical theory for spin-dependent quantum transport in
ballistic quantum dots. The theory is based on the semiclassical Landauer
formula, that we generalize to include spin-orbit and Zeeman interaction.
Within this approach, the orbital degrees of freedom are treated
semiclassically, while the spin dynamics is computed quantum mechanically.
Employing this method, we calculate the quantum correction to the conductance
in quantum dots with Rashba and Dresselhaus spin-orbit interaction. We find a
strong sensitivity of the quantum correction to the underlying classical
dynamics of the system. In particular, a suppression of weak antilocalization
in integrable systems is observed. These results are attributed to the
qualitatively different types of spin relaxation in integrable and chaotic
quantum cavities.Comment: 20 page
Spin relaxation dynamics of quasiclassical electrons in ballistic quantum dots with strong spin-orbit coupling
We performed path integral simulations of spin evolution controlled by the
Rashba spin-orbit interaction in the semiclassical regime for chaotic and
regular quantum dots. The spin polarization dynamics have been found to be
strikingly different from the D'yakonov-Perel' (DP) spin relaxation in bulk
systems. Also an important distinction have been found between long time spin
evolutions in classically chaotic and regular systems. In the former case the
spin polarization relaxes to zero within relaxation time much larger than the
DP relaxation, while in the latter case it evolves to a time independent
residual value. The quantum mechanical analysis of the spin evolution based on
the exact solution of the Schroedinger equation with Rashba SOI has confirmed
the results of the classical simulations for the circular dot, which is
expected to be valid in general regular systems. In contrast, the spin
relaxation down to zero in chaotic dots contradicts to what have to be expected
from quantum mechanics. This signals on importance at long time of the
mesoscopic echo effect missed in the semiclassical simulations.Comment: 14 pages, 9 figure
Spin-polarized electron transport in ferromagnet/semiconductor heterostructures: Unification of ballistic and diffusive transport
A theory of spin-polarized electron transport in ferromagnet/semiconductor
heterostructures, based on a unified semiclassical description of ballistic and
diffusive transport in semiconductor structures, is developed. The aim is to
provide a framework for studying the interplay of spin relaxation and transport
mechanism in spintronic devices. A key element of the unified description of
transport inside a (nondegenerate) semiconductor is the thermoballistic current
consisting of electrons which move ballistically in the electric field arising
from internal and external electrostatic potentials, and which are thermalized
at randomly distributed equilibration points. The ballistic component in the
unified description gives rise to discontinuities in the chemical potential at
the boundaries of the semiconductor, which are related to the Sharvin interface
conductance. By allowing spin relaxation to occur during the ballistic motion
between the equilibration points, a thermoballistic spin-polarized current and
density are constructed in terms of a spin transport function. An integral
equation for this function is derived for arbitrary values of the momentum and
spin relaxation lengths. For field-driven transport in a homogeneous
semiconductor, the integral equation can be converted into a second-order
differential equation that generalizes the standard spin drift-diffusion
equation. The spin polarization in ferromagnet/semiconductor heterostructures
is obtained by invoking continuity of the current spin polarization and
matching the spin-resolved chemical potentials on the ferromagnet sides of the
interfaces. Allowance is made for spin-selective interface resistances.
Examples are considered which illustrate the effects of transport mechanism and
electric field.Comment: 23 pages, 8 figures, REVTEX 4; minor corrections introduced; to
appear in Phys. Rev.
Kinetic investigation on extrinsic spin Hall effect induced by skew scattering
The kinetics of the extrinsic spin Hall conductivity induced by the skew
scattering is performed from the fully microscopic kinetic spin Bloch equation
approach in GaAs symmetric quantum well. In the steady state, the
extrinsic spin Hall current/conductivity vanishes for the linear-
dependent spin-orbit coupling and is very small for the cubic-
dependent spin-orbit coupling. The spin precession induced by the
Dresselhaus/Rashba spin-orbit coupling plays a very important role in the
vanishment of the extrinsic spin Hall conductivity in the steady state. An
in-plane spin polarization is induced by the skew scattering, with the help of
the spin-orbit coupling. This spin polarization is very different from the
current-induced spin polarization.Comment: 5 pages, 2 figures, to be published in JPC
Electronic spin transport in graphene field effect transistors
Spin transport experiments in graphene, a single layer of carbon atoms,
indicate spin relaxation times that are significantly shorter than the
theoretical predictions. We investigate experimentally whether these short spin
relaxation times are due to extrinsic factors, such as spin relaxation caused
by low impedance contacts, enhanced spin flip processes at the device edges or
the presence of an aluminium oxide layer on top of graphene in some samples.
Lateral spin valve devices using a field effect transistor geometry allowed for
the investigation of the spin relaxation as a function of the charge density,
going continuously from metallic hole to electron conduction (charge densities
of cm) via the Dirac charge neutrality point (). The results are quantitatively described by a one dimensional spin
diffusion model where the spin relaxation via the contacts is taken into
account. Spin valve experiments for various injector/detector separations and
spin precession experiments reveal that the longitudinal (T) and the
transversal (T) relaxation times are similar. The anisotropy of the spin
relaxation times and , when the spins are injected
parallel or perpendicular to the graphene plane, indicates that the effective
spin orbit fields do not lie exclusively in the two dimensional graphene plane.
Furthermore, the proportionality between the spin relaxation time and the
momentum relaxation time indicates that the spin relaxation mechanism is of the
Elliott-Yafet type. For carrier mobilities of 2-5 cm2^/Vs and
for graphene flakes of 0.1-2 m in width, we found spin relaxation times of
the order of 50-200 ps, times which appear not to be determined by the
extrinsic factors mentioned above.Comment: 11 pages, 13 figure
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