445 research outputs found
Dyakonov-Perel spin relaxation near metal-insulator transition and in hopping transport
In a heavily doped semiconductor with weak spin-orbital interaction the
Dyakonov-Perel spin relaxation rate is known to be proportional to the Drude
conductivity. We argue that in the case of weak spin-orbital interaction this
proportionality goes beyond the Drude mechanism: it stays valid through the
metal-insulator transition and in the range of the exponentially small hopping
conductivity.Comment: 3 page
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
Non-equilibrium spin polarization effects in spin-orbit coupling system and contacting metallic leads
We study theoretically the current-induced spin polarization effect in a
two-terminal mesoscopic structure which is composed of a semiconductor
two-dimensional electron gas (2DEG) bar with Rashba spin-orbit (SO) interaction
and two attached ideal leads. The nonequilibrium spin density is calculated by
solving the scattering wave functions explicitly within the ballistic transport
regime. We found that for a Rashba SO system the electrical current can induce
spin polarization in the SO system as well as in the ideal leads. The induced
polarization in the 2DEG shows some qualitative features of the intrinsic spin
Hall effect. On the other hand, the nonequilibrium spin density in the ideal
leads, after being averaged in the transversal direction, is independent of the
distance measured from the lead/SO system interface, except in the vicinity of
the interface. Such a lead polarization effect can even be enhanced by the
presence of weak impurity scattering in the SO system and may be detectable in
real experiments.Comment: 6 pages,5 figure
Electron-electron scattering effect on spin relaxation in multi-valley nanostructures
We develop a theory of effects of electron-electron collisions on the
Dyakonov-Perel' spin relaxation in multi-valley quantum wells. It is shown that
the electron-electron scattering rate which governs the spin relaxation is
different from that in a single-valley system. The theory is applied to Si/SiGe
(001)-grown quantum wells where two valleys are simultaneously populated by
free carriers. The dependences of the spin relaxation rate on temperature,
electron concentration and valley-orbit splitting are calculated and discussed.
We demonstrate that in a wide range of temperatures the electron-electron
collisions can govern spin relaxation in high-quality Si/SiGe quantum wells.Comment: 6 pages, 4 figures, EPL style, revised versio
Non-equilibrium spin dynamics in a trapped Fermi gas with effective spin-orbit interaction
We consider a trapped atomic system in the presence of spatially varying
laser fields. The laser-atom interaction generates a pseudospin degree of
freedom (referred to simply as spin) and leads to an effective spin-orbit
coupling for the fermions in the trap. Reflections of the fermions from the
trap boundaries provide a physical mechanism for effective momentum relaxation
and non-trivial spin dynamics due to the emergent spin-orbit coupling. We
explicitly consider evolution of an initially spin-polarized Fermi gas in a
two-dimensional harmonic trap and derive non-equilibrium behavior of the spin
polarization. It shows periodic echoes with a frequency equal to the harmonic
trapping frequency. Perturbations, such as an asymmetry of the trap, lead to
the suppression of the spin echo amplitudes. We discuss a possible experimental
setup to observe spin dynamics and provide numerical estimates of relevant
parameters.Comment: 5 pages, 4 figures; published versio
Long-term Dynamics of the Electron-nuclear Spin System of a Semiconductor Quantum Dot
A quasi-classical theoretical description of polarization and relaxation of
nuclear spins in a quantum dot with one resident electron is developed for
arbitrary mechanisms of electron spin polarization. The dependence of the
electron-nuclear spin dynamics on the correlation time of electron
spin precession, with frequency , in the nuclear hyperfine field is
analyzed. It is demonstrated that the highest nuclear polarization is achieved
for a correlation time close to the period of electron spin precession in the
nuclear field. For these and larger correlation times, the indirect hyperfine
field, which acts on nuclear spins, also reaches a maximum. This maximum is of
the order of the dipole-dipole magnetic field that nuclei create on each other.
This value is non-zero even if the average electron polarization vanishes. It
is shown that the transition from short correlation time to
does not affect the general structure of the equation for nuclear spin
temperature and nuclear polarization in the Knight field, but changes the
values of parameters, which now become functions of . For
correlation times larger than the precession time of nuclei in the electron
hyperfine field, it is found that three thermodynamic potentials (,
, ) characterize the polarized electron-nuclear spin
system. The values of these potentials are calculated assuming a sharp
transition from short to long correlation times, and the relaxation mechanisms
of these potentials are discussed. The relaxation of the nuclear spin potential
is simulated numerically showing that high nuclear polarization decreases
relaxation rate.Comment: RevTeX 4, 12 pages, 9 figure
Theory of Spin-Charge Coupled Transport in a Two-Dimensional Electron Gas with Rashba Spin-Orbit Interactions
We use microscopic linear response theory to derive a set of equations that
provide a complete description of coupled spin and charge diffusive transport
in a two-dimensional electron gas (2DEG) with the Rashba spin-orbit (SO)
interaction. These equations capture a number of interrelated effects including
spin accumulation and diffusion, Dyakonov-Perel spin relaxation,
magnetoelectric, and spin-galvanic effects. They can be used under very general
circumstances to model transport experiments in 2DEG systems that involve
either electrical or optical spin injection. We comment on the relationship
between these equations and the exact spin and charge density operator
equations of motion. As an example of the application of our equations, we
consider a simple electrical spin injection experiment and show that a voltage
will develop between two ferromagnetic contacts if a spin-polarized current is
injected into a 2DEG, that depends on the relative magnetization orientation of
the contacts. This voltage is present even when the separation between the
contacts is larger than the spin diffusion length.Comment: 8 pages, 1 eps figure. Corrected an error in the calculation of the
spin-charge coupling coefficient, pointed out in cond-mat/0406730, added
several reference
Spin relaxation in the impurity band of a semiconductor in the external magnetic field
Spin relaxation in the impurity band of a 2D semiconductor with spin-split
spectrum in the external magnetic field is considered. Several mechanisms of
spin relaxation are shown to be relevant. The first one is attributed to
phonon-assisted transitions between Zeeman sublevels of the ground state of an
isolated impurity, while other mechanisms can be described in terms of spin
precession in a random magnetic field during the electron motion over the
impurity band. In the later case there are two contributions to the spin
relaxation: the one given by optimal impurity configurations with the
hop-waiting time inversely proportional to the external magnetic field and
another one related to the electron motion on a large scale. The average spin
relaxation rate is calculated
Phonon-induced decoherence for a quantum dot spin qubit operated by Raman passage
We study single-qubit gates performed via stimulated Raman adiabatic passage
(STIRAP) on a spin qubit implemented in a quantum dot system in the presence of
phonons. We analyze the interplay of various kinds of errors resulting from the
carrier-phonon interaction as well as from quantum jumps related to
nonadiabaticity and calculate the fidelity as a function of the pulse
parameters. We give quantitative estimates for an InAs/GaAs system and identify
the parameter values for which the error is considerably minimized, even to
values below per operation.Comment: Final version; considerable extensions; 18 pages, 7 figure
Bremsstrahlung Spectrum in alpha Decay
Using our previous approach to electromagnetic emission during tunneling, an
explicit, essentially classical, formula describing the bremsstrahlung spectrum
in alpha decay is derived. The role of tunneling motion in photon emission is
discussed. The shape of the spectrum is a universal function of the ratio Eg/Eo
, where Eg is the photon energy and Eo is a characteristic energy depending
only on the nuclear charge and the energy of the alpha particle.Comment: 8 pages, 3 figure
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