2,057 research outputs found
On the temperature dependence of ballistic Coulomb drag in nanowires
We have investigated within the theory of Fermi liquid dependence of Coulomb
drag current in a passive quantum wire on the applied voltage across an
active wire and on the temperature for any values of . We assume
that the bottoms of the 1D minibands in both wires almost coincide with the
Fermi level. We come to conclusions that 1) within a certain temperature
interval the drag current can be a descending function of the temperature ;
2) the experimentally observed temperature dependence of the drag
current can be interpreted within the framework of Fermi liquid theory; 3) at
relatively high applied voltages the drag current as a function of the applied
voltage saturates; 4) the screening of the electron potential by metallic gate
electrodes can be of importance.Comment: 7 pages, 1 figur
Absorption suppression in photonic crystals
We study electromagnetic properties of periodic composite structures, such as
photonic crystals, involving lossy components. We show that in many cases a
properly designed periodic structure can dramatically suppress the losses
associated with the absorptive component, while preserving or even enhancing
its useful functionality. As an example, we consider magnetic photonic
crystals, in which the lossy magnetic component provides nonreciprocal Faraday
rotation. We show that the electromagnetic losses in the composite structure
can be reduced by up to two orders of magnitude, compared to those of the
uniform magnetic sample made of the same lossy magnetic material. Importantly,
the dramatic absorption reduction is not a resonance effect and occurs over a
broad frequency range covering a significant portion of photonic frequency
band
Multiple hydrodynamical shocks induced by Raman effect in photonic crystal fibres
We theoretically predict the occurrence of multiple hydrodynamical-like shock
phenomena in the propagation of ultrashort intense pulses in a suitably
engineered photonic crystal fiber. The shocks are due to the Raman effect,
which acts as a nonlocal term favoring their generation in the focusing regime.
It is shown that the problem is mapped to shock formation in the presence of a
slope and a gravity-like potential. The signature of multiple shocks in XFROG
signals is unveiled
Mass Transfer Mechanism in Real Crystals by Pulsed Laser Irradiation
The dynamic processes in the surface layers of metals subjected activity of a
pulsing laser irradiation, which destroyed not the crystalline structure in
details surveyed. The procedure of calculation of a dislocation density
generated in bulk of metal during the relaxation processes and at repeated
pulse laser action is presented. The results of evaluations coincide with high
accuracy with transmission electron microscopy dates. The
dislocation-interstitial mechanism of laser-stimulated mass-transfer in real
crystals is presented on the basis of the ideas of the interaction of structure
defects in dynamically deforming medium. The good compliance of theoretical and
experimental results approves a defining role of the presented mechanism of
mass transfer at pulse laser action on metals. The possible implementation this
dislocation-interstitial mechanism of mass transfer in metals to other cases of
pulsing influences is justifiedComment: 10 pages, 2 figures, Late
Giant Oscillations of Acoustoelectric Current in a Quantum Channel
A theory of d.c. electric current induced in a quantum channel by a
propagating surface acoustic wave (acoustoelectric current) is worked out. The
first observation of the acoustoelectric current in such a situation was
reported by J. M. Shilton et al., Journ. Phys. C (to be published). The authors
observed a very specific behavior of the acoustoelectric current in a
quasi-one-dimensional channel defined in a GaAs-AlGaAs heterostructure by a
split-gate depletion -- giant oscillations as a function of the gate voltage.
Such a behavior was qualitatively explained by an interplay between the
energy-momentum conservation law for the electrons in the upper transverse mode
with a finite temperature splitting of the Fermi level. In the present paper, a
more detailed theory is developed, and important limiting cases are considered.Comment: 7 pages, 2 Postscript figures, RevTeX 3.
Quantum line bundles on noncommutative sphere
Noncommutative (NC) sphere is introduced as a quotient of the enveloping
algebra of the Lie algebra su(2). Using the Cayley-Hamilton identities we
introduce projective modules which are analogues of line bundles on the usual
sphere (we call them quantum line bundles) and define a multiplicative
structure in their family. Also, we compute a pairing between certain quantum
line bundles and finite dimensional representations of the NC sphere in the
spirit of the NC index theorem. A new approach to constructing the differential
calculus on a NC sphere is suggested. The approach makes use of the projective
modules in question and gives rise to a NC de Rham complex being a deformation
of the classical one.Comment: LaTeX file, 15 pp, no figures. Some clarifying remarks are added at
the beginning of section 2 and into section
Electron and Phonon Temperature Relaxation in Semiconductors Excited by Thermal Pulse
Electron and phonon transient temperatures are analyzed in the case of
nondegenerate semiconductors. An analytical solution is obtained for
rectangular laser pulse absorption. It is shown that thermal diffusion is the
main energy relaxation mechanism in the phonon subsystem. The mechanism depends
on the correlation between the sample length and the electron cooling length in
an electron subsystem. Energy relaxation occurs by means of the electron
thermal diffusion in thin samples (), and by means of the electron-phonon
energy interaction in thick samples (). Characteristic relaxation times are
obtained for all the cases, and analysis of these times is made. Electron and
phonon temperature distributions in short and long samples are qualitatively
and quantitatively analyzed for different correlations between the laser pulse
duration and characteristic times.Comment: 33 pages, 16 figure
Formation of shock waves in a Bose-Einstein condensate
We consider propagation of density wave packets in a Bose-Einstein
condensate. We show that the shape of initially broad, laser-induced, density
perturbation changes in the course of free time evolution so that a shock wave
front finally forms. Our results are well beyond predictions of commonly used
zero-amplitude approach, so they can be useful in extraction of a speed of
sound from experimental data. We discuss a simple experimental setup for shock
propagation and point out possible limitations of the mean-field approach for
description of shock phenomena in a BEC.Comment: 8 pages & 6 figures, minor changes, more references, to appear in
Phys. Rev.
Multi-band superconductivity in LaFeAsO_{0.9}F_{0.1} single crystals probed by high-field vortex torque magnetometry
To probe manifestations of multiband superconductivity in oxypnictides, we
measured the angular dependence of the magnetic torque in the
mixed state of LaOFFeAs single crystals as a function of
temperature and magnetic fields up to 18 T. The paramagnetic
contribution of the Fe ions is properly treated in order to extract the
effective mass anisotropy parameter from
. We show that depends strongly on both and ,
reaching a maximum value of 10 followed by a decrease towards values
close to 1 as is lowered. The observed field dependencies of the London
penetration depth and suggest the onset of suppression
of a superconducing gap at .Comment: 7 pages, 7 figure
Unsteady undular bores in fully nonlinear shallow-water theory
We consider unsteady undular bores for a pair of coupled equations of
Boussinesq-type which contain the familiar fully nonlinear dissipationless
shallow-water dynamics and the leading-order fully nonlinear dispersive terms.
This system contains one horizontal space dimension and time and can be
systematically derived from the full Euler equations for irrotational flows
with a free surface using a standard long-wave asymptotic expansion.
In this context the system was first derived by Su and Gardner. It coincides
with the one-dimensional flat-bottom reduction of the Green-Naghdi system and,
additionally, has recently found a number of fluid dynamics applications other
than the present context of shallow-water gravity waves. We then use the
Whitham modulation theory for a one-phase periodic travelling wave to obtain an
asymptotic analytical description of an undular bore in the Su-Gardner system
for a full range of "depth" ratios across the bore. The positions of the
leading and trailing edges of the undular bore and the amplitude of the leading
solitary wave of the bore are found as functions of this "depth ratio". The
formation of a partial undular bore with a rapidly-varying finite-amplitude
trailing wave front is predicted for ``depth ratios'' across the bore exceeding
1.43. The analytical results from the modulation theory are shown to be in
excellent agreement with full numerical solutions for the development of an
undular bore in the Su-Gardner system.Comment: Revised version accepted for publication in Phys. Fluids, 51 pages, 9
figure
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