5,613 research outputs found
A Digital Switch and Femto-Tesla Magnetic Field Sensor Based on Fano Resonance in a Spin Field Effect Transistor
We show that a Spin Field Effect Transistor, realized with a semiconductor
quantum wire channel sandwiched between half-metallic ferromagnetic contacts,
can have Fano resonances in the transmission spectrum. These resonances appear
because the ferromagnets are half-metallic, so that the Fermi level can be
placed above the majority but below the minority spin band. In that case, the
majority spins will be propagating, but the minority spins will be evanescent.
At low temperatures, the Fano resonances can be exploited to implement a
digital binary switch that can be turned on or off with a very small gate
voltage swing of few tens of microvolts, leading to extremely small dynamic
power dissipation during switching. An array of 500,000 x 500,000 such
transistors can detect ultrasmall changes in a magnetic field with a
sensitivity of 1 femto-Tesla/sqrt{Hz}, if each transistor is biased near a Fano
resonance
Pion chemical equilibration in heavy ion collisions : relativistic quantum molecular dynamic analysis
In the framework of relativistic quantum molecular dynamics the authors find that the pion system produced in central heavy-ion collisions at Elab/A approximately 1 GeV/nucl. is out of chemical equilibrium. Pion chemical potential is large and decreases during the expansion stage
COHERENT SPIN TRANSPORT IN NANOWIRE SPIN VALVES AND NOVEL SPINTRONIC DEVICE POSSIBILITIES
We have proposed a spintronic infrared photodetector backed by experimental evidence and matched with theoretical prediction obtained in our labs. Unlike conventional photodetectors, it can work at room temperature with ideally infinite light-to-dark contrast ratio, infinite detectivity and zero dark current. The proposed idea is based on smart implementation of spin polarized transport. Electrons while travelling through one-dimensional channel show long spin relaxation length if they can be confined to a single conduction subband because of the elimination of major spin relaxation mechanism, namely the D’yakonov-Perel’ mechanism. With infrared light, electrons can be excited to higher subbands, resulting in the revival of DP mechanism which shortens the spin relaxation length. A noticeable change in current in a nanowire spin-valve (a semiconductor nanowire with two ferromagnetic contacts) can be observed due to this shortening and this phenomenon can be manipulated to implement infrared photo-detection. An array of tri-layer nanowires have been fabricated using electrodeposition where a narrow band semiconductor InSb has been sandwiched between two ferromagnetic contacts, Cobalt and Nickel. The two magnetic contacts act as spin injector and detector, where in the InSb layer, spin polarization is modulated using infrared light. The spin-valve effect and the Hanle effect have been demonstrated in these structures, which gives the confidence that the proposed device is indeed capable of injecting, coherently transporting and detecting spin of the electrons at room temperature even in the presence of thermal drift, background magnetoresistance, low spin injection and detection efficiency. When the same experiment was done under the infrared light, spin-valve effect was still there but muted, which means, infrared light is responsible weakening the spin polarization of carriers in the InSb layer. With choice of other materials, which show better spin injection and detection efficiency, the detectivity and sensitivity can be made more prominent.https://scholarscompass.vcu.edu/gradposters/1008/thumbnail.jp
Zero-field spin splitting in a two-dimensional electron gas with the spin-orbit interaction revisited
We consider a two-dimensional electron gas (2DEG) with the Rashba spin-orbit
interaction (SOI) in presence of a perpendicular magnetic field. We derive
analytical expressions of the density of states (DOS) of a 2DEG with the Rashba
SOI in presence of magnetic field by using the Green's function technique. The
DOS allows us to obtain the analytical expressions of the magnetoconductivities
for spin-up and spin-down electrons. The conductivities for spin-up and
spin-down electrons oscillate with different frequencies and gives rise to the
beating patterns in the amplitude of the Shubnikov de Hass (SdH) oscillations.
We find a simple equation which determines the zero-field spin splitting energy
if the magnetic field corresponding to any beat node is known from the
experiment. Our analytical results reproduce well the experimentally observed
non-periodic beating patterns, number of oscillations between two successive
nodes and the measured zero-field spin splitting energy.Comment: 5 pages, 2 figure
Density of defects and the scaling law of the entanglement entropy in quantum phase transition of one dimensional spin systems induced by a quench
We have studied quantum phase transition induced by a quench in different one
dimensional spin systems. Our analysis is based on the dynamical mechanism
which envisages nonadiabaticity in the vicinity of the critical point. This
causes spin fluctuation which leads to the random fluctuation of the Berry
phase factor acquired by a spin state when the ground state of the system
evolves in a closed path. The two-point correlation of this phase factor is
associated with the probability of the formation of defects. In this framework,
we have estimated the density of defects produced in several one dimensional
spin chains. At the critical region, the entanglement entropy of a block of
spins with the rest of the system is also estimated which is found to increase
logarithmically with . The dependence on the quench time puts a constraint
on the block size . It is also pointed out that the Lipkin-Meshkov-Glick
model in point-splitting regularized form appears as a combination of the XXX
model and Ising model with magnetic field in the negative z-axis. This unveils
the underlying conformal symmetry at criticality which is lost in the sharp
point limit. Our analysis shows that the density of defects as well as the
scaling behavior of the entanglement entropy follows a universal behavior in
all these systems.Comment: 4 figures, Accepted in Phys. Rev.
Spin relaxation of "upstream" electrons in quantum wires: Failure of the drift diffusion model
The classical drift diffusion (DD) model of spin transport treats spin
relaxation via an empirical parameter known as the ``spin diffusion length''.
According to this model, the ensemble averaged spin of electrons drifting and
diffusing in a solid decays exponentially with distance due to spin dephasing
interactions. The characteristic length scale associated with this decay is the
spin diffusion length. The DD model also predicts that this length is different
for ``upstream'' electrons traveling in a decelerating electric field than for
``downstream'' electrons traveling in an accelerating field. However this
picture ignores energy quantization in confined systems (e.g. quantum wires)
and therefore fails to capture the non-trivial influence of subband structure
on spin relaxation. Here we highlight this influence by simulating upstream
spin transport in a multi-subband quantum wire, in the presence of
D'yakonov-Perel' spin relaxation, using a semi-classical model that accounts
for the subband structure rigorously.
We find that upstream spin transport has a complex dynamics that defies the
simplistic definition of a ``spin diffusion length''.
In fact, spin does not decay exponentially or even monotonically with
distance, and the drift diffusion picture fails to explain the qualitative
behavior, let alone predict quantitative features accurately. Unrelated to spin
transport, we also find that upstream electrons undergo a ``population
inversion'' as a consequence of the energy dependence of the density of states
in a quasi one-dimensional structure.Comment: 13 figures. To appear in Phys. Rev.
Infrared Spectroscopy of GX 1+4/V2116 Oph: Evidence for a Fast Red Giant Wind?
We present infrared spectroscopy of the low-mass X-ray binary GX 1+4/V2116
Oph. This symbiotic binary consists of a 2-min accretion-powered pulsar and an
M5 III red giant. A strong He I 1.083 micron emission line with a pronounced P
Cygni profile was observed. From the blue edge of this feature, we infer an
outflow velocity of 250(50) km/s. This is an order of magnitude faster than a
typical red giant wind, and we suggest that radiation from the accretion disk
or the neutron star may contribute to the acceleration of the outflow. We infer
a wind mass loss rate of around 10^-6 Msun/yr. Accretion from such a strong
stellar wind provides a plausible alternative to Roche lobe overflow for
supplying the accretion disk which powers the X-ray source. The H I Paschen
beta and He I 1.083 micron lines showed no evidence for the dramatic changes
previously reported in some optical lines, and no evidence for pulsations at
the 2-min pulsar period.Comment: 11 pages including 2 PS figures. To appear in ApJ Letter
The role of electron-electron scattering in spin transport
We investigate spin transport in quasi 2DEG formed by III-V semiconductor
heterojunctions using the Monte Carlo method. The results obtained with and
without electron-electron scattering are compared and appreciable difference
between the two is found. The electron-electron scattering leads to suppression
of Dyakonov-Perel mechanism (DP) and enhancement of Elliott-Yafet mechanism
(EY). Finally, spin transport in InSb and GaAs heterostructures is investigated
considering both DP and EY mechanisms. While DP mechanism dominates spin
decoherence in GaAs, EY mechanism is found to dominate in high mobility InSb.
Our simulations predict a lower spin relaxation/decoherence rate in wide gap
semiconductors which is desirable for spin transport.Comment: to appear in Journal of Applied Physic
Memory in nanomagnetic systems: Superparamagnetism versus Spinglass behavior
The slow dynamics and concomitant memory (aging) effects seen in nanomagnetic
systems are analyzed on the basis of two separate paradigms : superparamagnets
and spinglasses. It is argued that in a large class of aging phenomena it
suffices to invoke superparamagnetic relaxation of individual single domain
particles but with a distribution of their sizes. Cases in which interactions
and randomness are important in view of distinctive experimental signatures,
are also discussed.Comment: 11 pages and 19 figure
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