9,769 research outputs found
On giant piezoresistance effects in silicon nanowires and microwires
The giant piezoresistance (PZR) previously reported in silicon nanowires is
experimentally investigated in a large number of surface depleted silicon nano-
and micro-structures. The resistance is shown to vary strongly with time due to
electron and hole trapping at the sample surfaces. Importantly, this time
varying resistance manifests itself as an apparent giant PZR identical to that
reported elsewhere. By modulating the applied stress in time, the true PZR of
the structures is found to be comparable with that of bulk silicon
An exactly solvable model of a superconducting to rotational phase transition
We consider a many-fermion model which exhibits a transition from a
superconducting to a rotational phase with variation of a parameter in its
Hamiltonian. The model has analytical solutions in its two limits due to the
presence of dynamical symmetries. However, the symmetries are basically
incompatible with one another; no simple solution exists in intermediate
situations. Exact (numerical) solutions are possible and enable one to study
the behavior of competing but incompatible symmetries and the phase transitions
that result in a semirealistic situation. The results are remarkably simple and
shed light on the nature of phase transitions.Comment: 11 pages including 1 figur
Effect of the Pauli principle on photoelectron spin transport in GaAs
In p+ GaAs thin films, the effect of photoelectron degeneracy on spin
transport is investigated theoretically and experimentally by imaging the spin
polarization profile as a function of distance from a tightly-focussed light
excitation spot. Under degeneracy of the electron gas (high concentration, low
temperature), a dip at the center of the polarization profile appears with a
polarization maximum at a distance of about from the center. This
counterintuitive result reveals that photoelectron diffusion depends on spin,
as a direct consequence of the Pauli principle. This causes a concentration
dependence of the spin stiffness while the spin dependence of the mobility is
found to be weak in doped material. The various effects which can modify spin
transport in a degenerate electron gas under local laser excitation are
considered. A comparison of the data with a numerical solution of the coupled
diffusion equations reveals that ambipolar coupling with holes increases the
steady-state photo-electron density at the excitation spot and therefore the
amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents
are predicted to depend on spin under degeneracy (spin Soret currents), but
these currents are negligible except at very high excitation power where they
play a relatively small role. Coulomb spin drag and bandgap renormalization are
negligible due to electrostatic screening by the hole gas
Geometrical dependence of decoherence by electronic interactions in a GaAs/GaAlAs square network
We investigate weak localization in metallic networks etched in a two
dimensional electron gas between mK and mK when electron-electron
(e-e) interaction is the dominant phase breaking mechanism. We show that, at
the highest temperatures, the contributions arising from trajectories that wind
around the rings and trajectories that do not are governed by two different
length scales. This is achieved by analyzing separately the envelope and the
oscillating part of the magnetoconductance. For K we find
\Lphi^\mathrm{env}\propto{T}^{-1/3} for the envelope, and
\Lphi^\mathrm{osc}\propto{T}^{-1/2} for the oscillations, in agreement with
the prediction for a single ring \cite{LudMir04,TexMon05}. This is the first
experimental confirmation of the geometry dependence of decoherence due to e-e
interaction.Comment: LaTeX, 5 pages, 4 eps figure
The use of preferred social stimuli as rewards for rhesus macaques in behavioural neuroscience
Macaques are often motivated to perform in neuroscientific experiments by implementing fluid restriction protocols. Daily access to water is controlled and the monkeys are rewarded with droplets of fluid for performing correct trials in the laboratory. Although these protocols are widely used and highly effective, it is important from a 3Rs perspective to investigate refinements that may help to lessen the severity of the fluid restriction applied. We assessed the use of social stimuli (images of conspecifics) as rewards for four rhesus macaques performing simple cognitive tasks. We found that individual preferences for images of male faces, female perinea and control stimuli could be identified in each monkey. However, using preferred images did not translate into effective motivators on a trial-by-trial basis: animals preferred fluid rewards, even when fluid restriction was relaxed. There was no difference in the monkeysâ performance of a task when using greyscale versus colour images. Based on our findings, we cannot recommend the use of social stimuli, in this form, as a refinement to current fluid restriction protocols. We discuss the potential alternatives and possibilities for future research
Spin and recombination dynamics of excitons and free electrons in p-type GaAs : effect of carrier density
Carrier and spin recombination are investigated in p-type GaAs of acceptor
concentration NA = 1.5 x 10^(17) cm^(-3) using time-resolved photoluminescence
spectroscopy at 15 K. At low pho- tocarrier concentration, acceptors are mostly
neutral and photoelectrons can either recombine with holes bound to acceptors
(e-A0 line) or form excitons which are mostly trapped on neutral acceptors
forming the (A0X) complex. It is found that the spin lifetime is shorter for
electrons that recombine through the e-A0 transition due to spin relaxation
generated by the exchange scattering of free electrons with either trapped or
free holes, whereas spin flip processes are less likely to occur once the
electron forms with a free hole an exciton bound to a neutral acceptor. An
increase of exci- tation power induces a cross-over to a regime where the
bimolecular band-to-band (b-b) emission becomes more favorable due to screening
of the electron-hole Coulomb interaction and ionization of excitonic complexes
and free excitons. Then, the formation of excitons is no longer possible, the
carrier recombination lifetime increases and the spin lifetime is found to
decrease dramatically with concentration due to fast spin relaxation with free
photoholes. In this high density regime, both the electrons that recombine
through the e-A0 transition and through the b-b transition have the same spin
relaxation time.Comment: 4 pages, 5 figure
Spin dependent photoelectron tunnelling from GaAs into magnetic Cobalt
The spin dependence of the photoelectron tunnel current from free standing
GaAs films into out-of- plane magnetized Cobalt films is demonstrated. The
measured spin asymmetry (A) resulting from a change in light helicity, reaches
+/- 6% around zero applied tunnel bias and drops to +/- 2% at a bias of -1.6 V
applied to the GaAs. This decrease is a result of the drop in the photoelectron
spin polarization that results from a reduction in the GaAs surface
recombination velocity. The sign of A changes with that of the Cobalt
magnetization direction. In contrast, on a (nonmagnetic) Gold film A ~ 0%
Vector coherent state representations, induced representations, and geometric quantization: II. Vector coherent state representations
It is shown here and in the preceeding paper (quant-ph/0201129) that vector
coherent state theory, the theory of induced representations, and geometric
quantization provide alternative but equivalent quantizations of an algebraic
model. The relationships are useful because some constructions are simpler and
more natural from one perspective than another. More importantly, each approach
suggests ways of generalizing its counterparts. In this paper, we focus on the
construction of quantum models for algebraic systems with intrinsic degrees of
freedom. Semi-classical partial quantizations, for which only the intrinsic
degrees of freedom are quantized, arise naturally out of this construction. The
quantization of the SU(3) and rigid rotor models are considered as examples.Comment: 31 pages, part 2 of two papers, published versio
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