721 research outputs found
Triplet supercurrent in ferromagnetic Josephson junctions by spin injection
We show that injecting nonequilibrium spins into the superconducting leads
strongly enhances the stationary Josephson current through a
superconductor-ferromagnet-superconductor junction. The resulting long-range
super-current through a ferromagnet is carried by triplet Cooper pairs that are
formed in s-wave superconductors by the combined effects of spin injection and
exchange interaction. We quantify the exchange interaction in terms of Landau
Fermi-liquid factors. The magnitude and direction of the long-range Josephson
current can be manipulated by varying the angles of the injected polarizations
with respect to the magnetization in the ferromagnet
Drift-diffusion model for spin-polarized transport in a non-degenerate 2DEG controlled by a spin-orbit interaction
We apply the Wigner function formalism to derive drift-diffusion transport
equations for spin-polarized electrons in a III-V semiconductor single quantum
well. Electron spin dynamics is controlled by the linear in momentum spin-orbit
interaction. In a studied transport regime an electron momentum scattering rate
is appreciably faster than spin dynamics. A set of transport equations is
defined in terms of a particle density, spin density, and respective fluxes.
The developed model allows studying of coherent dynamics of a non-equilibrium
spin polarization. As an example, we consider a stationary transport regime for
a heterostructure grown along the (0, 0, 1) crystallographic direction. Due to
the interplay of the Rashba and Dresselhaus spin-orbit terms spin dynamics
strongly depends on a transport direction. The model is consistent with results
of pulse-probe measurement of spin coherence in strained semiconductor layers.
It can be useful for studying properties of spin-polarized transport and
modeling of spintronic devices operating in the diffusive transport regime.Comment: 16 pages, 3 figure
Multi-subband effect in spin dephasing in semiconductor quantum wells
Multi-subband effect on spin precession and spin dephasing in -type GaAs
quantum wells is studied with electron-electron and electron-phonon scattering
explicitly included. The effects of temperature, well width and applied
electric field (in hot-electron regime) on the spin kinetics are thoroughly
investigated. It is shown that due to the strong inter-subband scattering, the
spin procession and the spin dephasing rate of electrons in different subbands
are almost identical despite the large difference in the D'yakonov-Perel' (DP)
terms of different subbands. It is also shown that for quantum wells with small
well width at temperatures where only the lowest subband is occupied, the spin
dephasing time increases with the temperature as well as the applied in-plane
electric field until the contribution from the second subband is no longer
negligible. For wide quantum wells the spin dephasing time tends to decrease
with the temperature and the electric field.Comment: 6 pages, 4 figures in eps forma
Unconventional sign-reversing superconductivity in LaFeAsO_(1-x)F_x
We argue that the newly discovered superconductivity in a nearly magnetic,
Fe-based layered compound is unconventional and mediated by antiferromagnetic
spin fluctuations, though different from the usual superexchange and specific
to this compound. This resulting state is an example of extended s-wave pairing
with a sign reversal of the order parameter between different Fermi surface
sheets. The main role of doping in this scenario is to lower the density of
states and suppress the pair-breaking ferromagnetic fluctuations
Spin diffusion/transport in -type GaAs quantum wells
The spin diffusion/transport in -type (001) GaAs quantum well at high
temperatures ( K) is studied by setting up and numerically solving the
kinetic spin Bloch equations together with the Poisson equation
self-consistently. All the scattering, especially the electron-electron Coulomb
scattering, is explicitly included and solved in the theory. This enables us to
study the system far away from the equilibrium, such as the hot-electron effect
induced by the external electric field parallel to the quantum well. We find
that the spin polarization/coherence oscillates along the transport direction
even when there is no external magnetic field. We show that when the scattering
is strong enough, electron spins with different momentums oscillate in the same
phase which leads to equal transversal spin injection length and ensemble
transversal injection length. It is also shown that the intrinsic scattering is
already strong enough for such a phenomena. The oscillation period is almost
independent on the external electric field which is in agreement with the
latest experiment in bulk system at very low temperature [Europhys. Lett. {\bf
75}, 597 (2006)]. The spin relaxation/dephasing along the diffusion/transport
can be well understood by the inhomogeneous broadening, which is caused by the
momentum-dependent diffusion and the spin-orbit coupling, and the scattering.
The scattering, temperature, quantum well width and external magnetic/electric
field dependence of the spin diffusion is studied in detail.Comment: 12 pages, 6 figures, to be published in J Appl. Phy
Current-Induced Polarization and the Spin Hall Effect at Room Temperature
Electrically-induced electron spin polarization is imaged in n-type ZnSe
epilayers using Kerr rotation spectroscopy. Despite no evidence for an
electrically-induced internal magnetic field, current-induced in-plane spin
polarization is observed with characteristic spin lifetimes that decrease with
doping density. The spin Hall effect is also observed, indicated by an
electrically-induced out-of-plane spin polarization with opposite sign for
spins accumulating on opposite edges of the sample. The spin Hall conductivity
is estimated as 3 +/- 1.5 Ohms**-1 m**-1/|e| at 20 K, which is consistent with
the extrinsic mechanism. Both the current-induced spin polarization and the
spin Hall effect are observed at temperatures from 10 K to 295 K.Comment: 5 pages, 4 figure
Comment on "Antilocalization in a 2D Electron Gas in a Random Magnetic Field"
In a recent Letter, Taras-Semchuk and Efetov reconsider the problem of
electron localization in a random magnetic field in two dimensions. They claim
that due to the long-range nature of the vector potential correlations an
additional term appears in the effective field theory (-model) of the
problem, leading to delocalization at the one-loop level. This calls into
question the results of earlier analytical studies, where the random magnetic
field problem was mapped onto the conventional unitary-class -model,
implying that the leading quantum correction is of two-loop order and of a
localizing nature. We show in this Comment, however, that the new term in fact
does not exist and was erroneously obtained by Taras-Semchuk and Efetov because
of an inconsistent treatment violating gauge invariance.Comment: 1 page, 2 figure
Spin coherence of a two-dimensional electron gas induced by resonant excitation of trions and excitons in CdTe/(Cd,Mg)Te quantum wells
The mechanisms for generation of long-lived spin coherence in a
two-dimensional electron gas (2DEG) have been studied experimentally by means
of a picosecond pump-probe Kerr rotation technique. CdTe/(Cd,Mg)Te quantum
wells with a diluted 2DEG were investigated. The strong Coulomb interaction
between electrons and holes, which results in large binding energies of neutral
excitons and negatively charged excitons (trions), allows one to address
selectively the exciton or trion states by resonant optical excitation.
Different scenarios of spin coherence generation were analyzed theoretically,
among them the direct trion photocreation, the formation of trions from
photogenerated excitons and the electron-exciton exchange scattering. Good
agreement between experiment and theory is found.Comment: 18 pages, 20 figure
Restrictions on modeling spin injection by resistor networks
Because of the technical difficulties of solving spin transport equations in
inhomogeneous systems, different resistor networks are widely applied for
modeling spin transport. By comparing an analytical solution for spin injection
across a ferromagnet - paramagnet junction with a resistor model approach, its
essential limitations stemming from inhomogeneous spin populations are
clarified.Comment: To be published in a special issue of Semicond. Sci. Technol., Guest
editor Prof. G. Landweh
Shape Analysis of the Level Spacing Distribution around the Metal Insulator Transition in the Three Dimensional Anderson Model
We present a new method for the numerical treatment of second order phase
transitions using the level spacing distribution function . We show that
the quantities introduced originally for the shape analysis of eigenvectors can
be properly applied for the description of the eigenvalues as well. The
position of the metal--insulator transition (MIT) of the three dimensional
Anderson model and the critical exponent are evaluated. The shape analysis of
obtained numerically shows that near the MIT is clearly different
from both the Brody distribution and from Izrailev's formula, and the best
description is of the form , with
. This is in good agreement with recent analytical results.Comment: 14 pages in plain TeX, 6 figures upon reques
- …