7,121 research outputs found
Electrical spin protection and manipulation via gate-locked spin-orbit fields
The spin-orbit (SO) interaction couples electron spin and momentum via a
relativistic, effective magnetic field. While conveniently facilitating
coherent spin manipulation in semiconductors, the SO interaction also
inherently causes spin relaxation. A unique situation arises when the Rashba
and Dresselhaus SO fields are matched, strongly protecting spins from
relaxation, as recently demonstrated. Quantum computation and spintronics
devices such as the paradigmatic spin transistor could vastly benefit if such
spin protection could be expanded from a single point into a broad range
accessible with in-situ gate-control, making possible tunable SO rotations
under protection from relaxation. Here, we demonstrate broad, independent
control of all relevant SO fields in GaAs quantum wells, allowing us to tune
the Rashba and Dresselhaus SO fields while keeping both locked to each other
using gate voltages. Thus, we can electrically control and simultaneously
protect the spin. Our experiments employ quantum interference corrections to
electrical conductivity as a sensitive probe of SO coupling. Finally, we
combine transport data with numerical SO simulations to precisely quantify all
SO terms.Comment: 5 pages, 4 figures (color), plus supplementary information 18 pages,
8 figures (color) as ancillary arXiv pd
An efficient user-oriented method for calculating compressible flow in an about three-dimensional inlets
A panel method is used to calculate incompressible flow about arbitrary three-dimensional inlets with or without centerbodies for four fundamental flow conditions: unit onset flows parallel to each of the coordinate axes plus static operation. The computing time is scarcely longer than for a single solution. A linear superposition of these solutions quite rigorously gives incompressible flow about the inlet for any angle of attack, angle of yaw, and mass flow rate. Compressibility is accounted for by applying a well-proven correction to the incompressible flow. Since the computing times for the combination and the compressibility correction are small, flows at a large number of inlet operating conditions are obtained rather cheaply. Geometric input is aided by an automatic generating program. A number of graphical output features are provided to aid the user, including surface streamline tracing and automatic generation of curves of curves of constant pressure, Mach number, and flow inclination at selected inlet cross sections. The inlet method and use of the program are described. Illustrative results are presented
Phonon driven spin distribution due to the spin-Seebeck effect
Here we report on measurements of the spin-Seebeck effect of GaMnAs over an
extended temperature range alongside the thermal conductivity, specific heat,
magnetization, and thermoelectric power. The amplitude of the spin-Seebeck
effect in GaMnAs scales with the thermal conductivity of the GaAs substrate and
the phonon-drag contribution to the thermoelectric power of the GaMnAs,
demonstrating that phonons drive the spin redistribution. A phenomenological
model involving phonon-magnon drag explains the spatial and temperature
dependence of the measured spin distribution.Comment: 12 pages, 3 figure
A three-dimensional scalar field theory model of center vortices and its relation to k-string tensions
In d=3 SU(N) gauge theory, we study a scalar field theory model of center
vortices that furnishes an approach to the determination of so-called k-string
tensions. This model is constructed from string-like quantum solitons
introduced previously, and exploits the well-known relation between string
partition functions and scalar field theories in d=3. Center vortices
corresponding to magnetic flux J (in units of 2\pi /N) are composites of J
elementary J=1 constituent vortices that come in N-1 types, with repulsion
between like constituents and attraction between unlike constituents. The
scalar field theory involves N scalar fields \phi_i (one of which is
eliminated) that can merge, dissociate, and recombine while conserving flux mod
N. The properties of these fields are deduced directly from the corresponding
gauge-theory quantum solitons. Every vacuum Feynman graph of the theory
corresponds to a real-space configuration of center vortices. We study
qualitatively the problem of k-string tensions at large N, whose solution is
far from obvious in center-vortex language. We construct a simplified dynamical
picture of constituent-vortex merging, dissociation, and recombination, which
allows in principle for the determination of vortex areal densities and
k-string tensions. This picture involves point-like "molecules" (cross-sections
of center vortices) made of constituent "atoms" that combine and disassociate
dynamically in a d=2 test plane . The vortices evolve in a Euclidean "time"
which is the location of the test plane along an axis perpendicular to the
plane. A simple approximation to the molecular dynamics is compatible with
k-string tensions that are linear in k for k<< N, as naively expected.Comment: 21 pages; RevTeX4; 4 .eps figure
Optimal probabilistic cloning and purification of quantum states
We investigate the probabilistic cloning and purification of quantum states.
The performance of these probabilistic operations is quantified by the average
fidelity between the ideal and actual output states. We provide a simple
formula for the maximal achievable average fidelity and we explictly show how
to construct a probabilistic operation that achieves this fidelity. We
illustrate our method on several examples such as the phase covariant cloning
of qubits, cloning of coherent states, and purification of qubits transmitted
via depolarizing channel and amplitude damping channel. Our examples reveal
that the probabilistic cloner may yield higher fidelity than the best
deterministic cloner even when the states that should be cloned are linearly
dependent and are drawn from a continuous set.Comment: 9 pages, 2 figure
Reduced tillage in corn production
Caption title."Corn Belt Branch, Soil and Water Conservation Research Division, Agricultural Research Service, U.S. Department of Agriculture, Columbia, Missouri and the Agricultural Engineering Department of the Missouri Agricultural Experiment Station Cooperating"--Page [2]
Oscillatory Spin Polarization and Magneto-Optic Kerr Effect in Fe3O4 Thin Films on GaAs(001)
The spin dependent properties of epitaxial Fe3O4 thin films on GaAs(001) are
studied by the ferromagnetic proximity polarization (FPP) effect and
magneto-optic Kerr effect (MOKE). Both FPP and MOKE show oscillations with
respect to Fe3O4 film thickness, and the oscillations are large enough to
induce repeated sign reversals. We attribute the oscillatory behavior to
spin-polarized quantum well states forming in the Fe3O4 film. Quantum
confinement of the t2g states near the Fermi level provides an explanation for
the similar thickness dependences of the FPP and MOKE oscillations.Comment: to appear in Phys. Rev. Let
Detrimental adsorbate fields in experiments with cold Rydberg gases near surfaces
We observe the shift of Rydberg levels of rubidium close to a copper surface
when atomic clouds are repeatedly deposited on it. We measure transition
frequencies of rubidium to S and D Rydberg states with principal quantum
numbers n between 31 and 48 using the technique of electromagnetically induced
transparency. The spectroscopic measurement shows a strong increase of electric
fields towards the surface that evolves with the deposition of atoms. Starting
with a clean surface, we measure the evolution of electrostatic fields in the
range between 30 and 300 \mum from the surface. We find that after the
deposition of a few hundred atomic clouds, each containing ~10^6 atoms, the
field of adsorbates reaches 1 V/cm for a distance of 30 \mum from the surface.
This evolution of the electrostatic field sets serious limitations on cavity
QED experiments proposed for Rydberg atoms on atom chips.Comment: 4 pages, 3 figures Submitted to Phys. Rev.
Stretchable persistent spin helices in GaAs quantum wells
The Rashba and Dresselhaus spin-orbit (SO) interactions in 2D electron gases
act as effective magnetic fields with momentum-dependent directions, which
cause spin decay as the spins undergo arbitrary precessions about these
randomly-oriented SO fields due to momentum scattering. Theoretically and
experimentally, it has been established that by fine-tuning the Rashba
and Dresselhaus couplings to equal strengths
, the total SO field becomes unidirectional thus rendering the
electron spins immune to dephasing due to momentum scattering. A robust
persistent spin helix (PSH) has already been experimentally realized at this
singular point . Here we employ the suppression of weak
antilocalization as a sensitive detector for matched SO fields together with a
technique that allows for independent electrical control over the SO couplings
via top gate voltage and back gate voltage . We demonstrate for the
first time the gate control of and the of
the SO fields at , i.e., we are able to vary both and
controllably and continuously with and , while keeping them
locked at equal strengths. This makes possible a new concept: "stretchable
PSHs", i.e., helical spin patterns with continuously variable pitches over
a wide parameter range. The extracted spin-diffusion lengths and decay times as
a function of show a significant enhancement near
. Since within the continuous-locking regime quantum transport
is diffusive (2D) for charge while ballistic (1D) for spin and thus amenable to
coherent spin control, stretchable PSHs could provide the platform for the much
heralded long-distance communication m between solid-state
spin qubits.Comment: 5 color figures, with supplementary info available on arXiv. arXiv
admin note: substantial text overlap with arXiv:1403.351
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