1,050 research outputs found
Modeling spin transport in electrostatically-gated lateral-channel silicon devices: role of interfacial spin relaxation
Using a two-dimensional finite-differences scheme to model spin transport in
silicon devices with lateral geometry, we simulate the effects of spin
relaxation at interfacial boundaries, i.e. the exposed top surface and at an
electrostatically-controlled backgate with SiO_2 dielectric. These
gate-voltage-dependent simulations are compared to previous experimental
results and show that strong spin relaxation due to extrinsic effects yield an
Si/SiO_2 interfacial spin lifetime of ~ 1ns, orders of magnitude lower than
lifetimes in the bulk Si, whereas relaxation at the top surface plays no
substantial role. Hall effect measurements on ballistically injected electrons
gated in the transport channel yield the carrier mobility directly and suggest
that this reduction in spin lifetime is only partially due to enhanced
interfacial momentum scattering which induces random spin flips as in the
Elliott effect. Therefore, other extrinsic mechanisms such as those caused by
paramagnetic defects should also be considered in order to explain the dramatic
enhancement in spin relaxation at the gate interface over bulk values
The Larmor clock and anomalous spin dephasing in silicon
Drift-diffusion theory - which fully describes charge transport in
semiconductors - is also universally used to model transport of spin-polarized
electrons in the presence of longitudinal electric fields. By transforming spin
transit time into spin orientation with precession (a technique called the
"Larmor clock") in current-sensing vertical-transport intrinsic Si devices, we
show that spin diffusion (and concomitant spin dephasing) can be greatly
enhanced with respect to charge diffusion, in direct contrast to predictions of
spin Coulomb-drag diffusion suppression.Comment: minor edits and updated ref
Field-induced negative differential spin lifetime in silicon
We show that the electric field-induced thermal asymmetry between the
electron and lattice systems in pure silicon substantially impacts the identity
of the dominant spin relaxation mechanism. Comparison of empirical results from
long-distance spin transport devices with detailed Monte-Carlo simulations
confirms a strong spin depolarization beyond what is expected from the standard
Elliott-Yafet theory already at low temperatures. The enhanced spin-flip
mechanism is attributed to phonon emission processes during which electrons are
scattered between conduction band valleys that reside on different crystal
axes. This leads to anomalous behavior, where (beyond a critical field)
reduction of the transit time between spin-injector and spin-detector is
accompanied by a counterintuitive reduction in spin polarization and an
apparent negative spin lifetime
Tunneling ``zero-bias'' anomaly in the quasi-ballistic regime
For the first time, we study the tunneling density of states (DOS) of the
interacting electron gas beyond the diffusive limit. A strong correction to the
DOS persists even at electron energies exceeding the inverse transport
relaxation time, which could not be expected from the well-known
Altshuler-Aronov-Lee (AAL) theory. This correction originates from the
interference between the electron waves scattered by an impurity and by the
Friedel oscillation this impurity creates. Account for such processes also
revises the AAL formula for the DOS in the diffusive limit.Comment: 4 pages, 2 .eps figures, submitted to Phys. Rev. Let
Nonequilibrium Transport through a Kondo Dot in a Magnetic Field: Perturbation Theory
Using nonequilibrium perturbation theory, we investigate the nonlinear
transport through a quantum dot in the Kondo regime in the presence of a
magnetic field. We calculate the leading logarithmic corrections to the local
magnetization and the differential conductance, which are characteristic of the
Kondo effect out of equilibrium. By solving a quantum Boltzmann equation, we
determine the nonequilibrium magnetization on the dot and show that the
application of both a finite bias voltage and a magnetic field induces a novel
structure of logarithmic corrections not present in equilibrium. These
corrections lead to more pronounced features in the conductance, and their form
calls for a modification of the perturbative renormalization group.Comment: 16 pages, 7 figure
Coherent spin transport through a 350-micron-thick Silicon wafer
We use all-electrical methods to inject, transport, and detect spin-polarized
electrons vertically through a 350-micron-thick undoped single-crystal silicon
wafer. Spin precession measurements in a perpendicular magnetic field at
different accelerating electric fields reveal high spin coherence with at least
13pi precession angles. The magnetic-field spacing of precession extrema are
used to determine the injector-to-detector electron transit time. These transit
time values are associated with output magnetocurrent changes (from in-plane
spin-valve measurements), which are proportional to final spin polarization.
Fitting the results to a simple exponential spin-decay model yields a
conduction electron spin lifetime (T1) lower bound in silicon of over 500ns at
60K.Comment: Accepted in PR
Suppression of Kondo effect in a quantum dot by external irradiation
We demonstrate that the external irradiation brings decoherence in the spin
states of the quantum dot. This effect cuts off the Kondo anomaly in
conductance even at zero temperature. We evaluate the dependence of the DC
conductance in the Kondo regime on the power of irradiation, this dependence
being determined by the decoherence.Comment: 4 pages, 1 figur
The Kondo Effect in Non-Equilibrium Quantum Dots: Perturbative Renormalization Group
While the properties of the Kondo model in equilibrium are very well
understood, much less is known for Kondo systems out of equilibrium. We study
the properties of a quantum dot in the Kondo regime, when a large bias voltage
V and/or a large magnetic field B is applied. Using the perturbative
renormalization group generalized to stationary nonequilibrium situations, we
calculate renormalized couplings, keeping their important energy dependence. We
show that in a magnetic field the spin occupation of the quantum dot is
non-thermal, being controlled by V and B in a complex way to be calculated by
solving a quantum Boltzmann equation. We find that the well-known suppression
of the Kondo effect at finite V>>T_K (Kondo temperature) is caused by inelastic
dephasing processes induced by the current through the dot. We calculate the
corresponding decoherence rate, which serves to cut off the RG flow usually
well inside the perturbative regime (with possible exceptions). As a
consequence, the differential conductance, the local magnetization, the spin
relaxation rates and the local spectral function may be calculated for large
V,B >> T_K in a controlled way.Comment: 9 pages, invited paper for a special edition of JPSJ "Kondo Effect --
40 Years after the Discovery", some typos correcte
Circadian clocks, rhythmic synaptic plasticity and the sleep-wake cycle in zebrafish
The circadian clock and homeostatic processes are fundamental mechanisms that regulate sleep. Surprisingly, despite decades of research, we still do not know why we sleep. Intriguing hypotheses suggest that sleep regulates synaptic plasticity and consequently has a beneficial role in learning and memory. However, direct evidence is still limited and the molecular regulatory mechanisms remain unclear. The zebrafish provides a powerful vertebrate model system that enables simple genetic manipulation, imaging of neuronal circuits and synapses in living animals, and the monitoring of behavioral performance during day and night. Thus, the zebrafish has become an attractive model to study circadian and homeostatic processes that regulate sleep. Zebrafish clock- and sleep-related genes have been cloned, neuronal circuits that exhibit circadian rhythms of activity and synaptic plasticity have been studied, and rhythmic behavioral outputs have been characterized. Integration of this data could lead to a better understanding of sleep regulation. Here, we review the progress of circadian clock and sleep studies in zebrafish with special emphasis on the genetic and neuroendocrine mechanisms that regulate rhythms of melatonin secretion, structural synaptic plasticity, locomotor activity and sleep
Nonlinear Response of a Kondo system: Direct and Alternating Tunneling Currents
Non - equilibrium tunneling current of an Anderson impurity system subject to
both constant and alternating electric fields is studied. A time - dependent
Schrieffer - Wolff transformation maps the time - dependent Anderson
Hamiltonian onto a Kondo one. Perturbation expansion in powers of the Kondo
coupling strength is carried out up to third order, yielding a remarkably
simple analytical expression for the tunneling current. It is found that the
zero - bias anomaly is suppressed by an ac - field. Both dc and the first
harmonic are equally enhanced by the Kondo effect, while the higher harmonics
are relatively small. These results are shown to be valid also below the Kondo
temperature.Comment: 7 pages, RevTeX, 3 PS figures attached, the article has been
significantly developed: time - dependent Schrieffer - Wolff transformation
is presented in the full form, the results are applied to the change in the
direct current induced by an alternating field (2 figures are new
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