1,748 research outputs found
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.
Spin Accumulation in Quantum Wires with Strong Rashba Spin-Orbit Coupling
We present analytical and numerical results for the effect of Rashba
spin-orbit coupling on band structure, transport, and interaction effects in
quantum wires when the spin precession length is comparable to the wire width.
In contrast to the weak-coupling case, no common spin-quantization axis can be
defined for eigenstates within a single-electron band. The situation with only
the lowest spin-split subbands occupied is particularly interesting because
electrons close to Fermi points of the same chirality can have approximately
parallel spins. We discuss consequences for spin-dependent transport and
effective Tomonaga-Luttinger descriptions of interactions in the quantum wire.Comment: 4 pages, 4 figures, expanded discussion of spin accumulatio
Magnetization of a two-dimensional electron gas with a second filled subband
We have measured the magnetization of a dual-subband two-dimensional electron
gas, confined in a GaAs/AlGaAs heterojunction. In contrast to two-dimensional
electron gases with a single subband, we observe non-1/B-periodic, triangularly
shaped oscillations of the magnetization with an amplitude significantly less
than per electron. All three effects are explained by a
field dependent self-consistent model, demonstrating the shape of the
magnetization is dominated by oscillations in the confining potential.
Additionally, at 1 K, we observe small oscillations at magnetic fields where
Landau-levels of the two different subbands cross.Comment: 4 pages, 4 figure
Filtering spin with tunnel-coupled electron wave guides
We show how momentum-resolved tunneling between parallel electron wave guides
can be used to observe and exploit lifting of spin degeneracy due to Rashba
spin-orbit coupling. A device is proposed that achieves spin filtering without
using ferromagnets or the Zeeman effect.Comment: 4 pages, 4 figures, RevTex
Rashba precession in quantum wires with interaction
Rashba precession of spins moving along a one-dimensional quantum channel is
calculated, accounting for Coulomb interactions. The Tomonaga--Luttinger model
is formulated in the presence of spin-orbit scattering and solved by
Bosonization. Increasing interaction strength at decreasing carrier density is
found to {\sl enhance} spin precession and the nominal Rashba parameter due to
the decreasing spin velocity compared with the Fermi velocity. This result can
elucidate the observed pronounced changes of the spin splitting on applied gate
voltages which are estimated to influence the interface electric field in
heterostructures only little.Comment: now replaced by published versio
Spin Orientation and Spin Precession in Inversion-Asymmetric Quasi Two-Dimensional Electron Systems
Inversion asymmetry induced spin splitting of the electron states in quasi
two-dimensional (2D) systems can be attributed to an effective magnetic field B
which varies in magnitude and orientation as a function of the in-plane wave
vector k||. Using a realistic 8x8 Kane model that fully takes into account spin
splitting because of both bulk inversion asymmetry and structure inversion
asymmetry we investigate the spin orientation and the effective field B for
different configurations of a quasi 2D electron system. It is shown that these
quantities depend sensitively on the crystallographic direction in which the
quasi 2D system was grown as well as on the magnitude and orientation of the
in-plane wave vector k||. These results are used to discuss how spin-polarized
electrons can precess in the field B(k||). As a specific example we consider
GaInAs-InP quantum wells.Comment: 10 pages, 6 figure
Coherent spin valve phenomena and electrical spin injection in ferromagnetic/semiconductor/ferromagnetic junctions
Coherent quantum transport in ferromagnetic/ semiconductor/ ferromagnetic
junctions is studied theoretically within the Landauer framework of ballistic
transport. We show that quantum coherence can have unexpected implications for
spin injection and that some intuitive spintronic concepts which are founded in
semi-classical physics no longer apply: A quantum spin-valve (QSV) effect
occurs even in the absence of a net spin polarized current flowing through the
device, unlike in the classical regime. The converse effect also arises, i.e. a
zero spin-valve signal for a non-vanishing spin-current. We introduce new
criteria useful for analyzing quantum and classical spin transport phenomena
and the relationships between them. The effects on QSV behavior of
spin-dependent electron transmission at the interfaces, interface Schottky
barriers, Rashba spin-orbit coupling and temperature, are systematically
investigated. While the signature of the QSV is found to be sensitive to
temperature, interestingly, that of its converse is not. We argue that the QSV
phenomenon can have important implications for the interpretation of
spin-injection in quantum spintronic experiments with spin-valve geometries.Comment: 15 pages including 11 figures. To appear in PR
Search for the Higgs boson in events with missing transverse energy and b quark jets produced in proton-antiproton collisions at s**(1/2)=1.96 TeV
We search for the standard model Higgs boson produced in association with an
electroweak vector boson in events with no identified charged leptons, large
imbalance in transverse momentum, and two jets where at least one contains a
secondary vertex consistent with the decay of b hadrons. We use ~1 fb-1
integrated luminosity of proton-antiproton collisions at s**(1/2)=1.96 TeV
recorded by the CDF II experiment at the Tevatron. We find 268 (16) single
(double) b-tagged candidate events, where 248 +/- 43 (14.4 +/- 2.7) are
expected from standard model background processes. We place 95% confidence
level upper limits on the Higgs boson production cross section for several
Higgs boson masses ranging from 110 GeV/c2 to 140 GeV/c2. For a mass of 115
GeV/c2 the observed (expected) limit is 20.4 (14.2) times the standard model
prediction.Comment: 8 pages, 2 figures, submitted to Phys. Rev. Let
Search for Third Generation Vector Leptoquarks in p anti-p Collisions at sqrt(s) = 1.96 TeV
We describe a search for a third generation vector leptoquark (VLQ3) that
decays to a b quark and tau lepton using the CDF II detector and 322 pb^(-1) of
integrated luminosity from the Fermilab Tevatron. Vector leptoquarks have been
proposed in many extensions of the standard model (SM). Observing a number of
events in agreement with SM expectations, assuming Yang-Mills (minimal)
couplings, we obtain the most stringent upper limit on the VLQ3 pair production
cross section of 344 fb (493 fb) and lower limit on the VLQ3 mass of 317
GeV/c^2 (251 GeV/c^2) at 95% C.L.Comment: 7 pages, 2 figures, submitted to PR
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