936 research outputs found
Interaction of a surface acoustic wave with a two-dimensional electron gas
When a surface acoustic wave propagates on the surface of a GaAs
semiconductor, coupling between electrons in the two-dimensional electron gas
beneath the interface and the elastic host crystal through piezoelectric
interaction will attenuate the SAW. The coupling coefficient is calculated for
the SAW propagating along an arbitrary direction. It is found that the coupling
strength is largely dependent on the propagating direction. When the SAW
propagates along the [011] direction, the coupling becomes quite weak.Comment: 3 figure
Electron density stratification in two-dimensional structures tuned by electric field
A new kinetic instability which results in formation of charge density waves
is proposed. The instability is of a purely classical nature. A spatial period
of arising space-charge and field configuration is inversely proportional to
electric field and can be tuned by applied voltage. The instability has no
interpretation in the framework of traditional hydrodynamic approach, since it
arises from modulation of an electron distribution function both in coordinate
and energy spaces. The phenomenon can be observed in thin 2D nanostructures at
relatively low electron density.Comment: 4 pages, 2 figure
Filtered Cathodic Vacuum Arc Deposition of Porous and Nanostructured Carbon and Hybrid C-Mo Thin Films for Fuel Cell Membranes
In our proton exchange membrane (PEM) fuel cells, the electrolyte is a catalyst platinum (Pt) coated
nafion membrane. In order to increase the electrochemically active catalyst area to enhance the cell
performance but without sacrificing the electrical conductivity of the membrane, carbon thin films were
deposited onto the membrane by using the filtered cathodic vacuum arc deposition (FCVAD) technique.
With varying the deposition conditions, it was possible to form porous carbon films and nanoisland and
nanorod structure surface which increased the catalyst area when a higher He working gas pressure and a
low number of pulses were used. Based on this result, hybrid C-Mo thin films were deposited for further
enhancing the Pt catalytic effect. Under the varied deposition conditions, the surface morphology and C
and Mo grain sizes of the hybrid thin films were measured and their relations with the catalytic
performance were studied.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3486
The current of fermions scattered off a bubble wall
Proceeding from WKB quantization conditions, we derive a semiclassical
expression for the current of fermions scattered off a propagating bubble wall
in the presence of longitudinal gauge field. It agrees with the expression used
by Nasser and Turok in semiclassical analysis of instability of electroweak
bubble walls with respect to longitudinal condensation. We discuss the
resulting dispersion relation for longitudinal field and show that light
species are important for the analysis of stability, because of their large
contribution to plasma frequency.Comment: 7 pages, latex, no figures; misprint in eq.(12) correcte
Effect on Genetic Mutation Induction from Nano-Ranged Low-Energy Plasma Ion Bombardment of DNA and Gene Fragment
Symplectic structures associated to Lie-Poisson groups
The Lie-Poisson analogues of the cotangent bundle and coadjoint orbits of a
Lie group are considered. For the natural Poisson brackets the symplectic
leaves in these manifolds are classified and the corresponding symplectic forms
are described. Thus the construction of the Kirillov symplectic form is
generalized for Lie-Poisson groups.Comment: 30 page
Self-consistent calculation of the coupling constant in the Gross-Pitaevskii equation
A method is proposed for a self-consistent evaluation of the coupling
constant in the Gross-Pitaevskii equation without involving a pseudopotential
replacement. A renormalization of the coupling constant occurs due to medium
effects and the trapping potential, e.g. in quasi-1D or quasi-2D systems. It is
shown that a simplified version of the Hartree-Fock-Bogoliubov approximation
leads to a variational problem for both the condensate and a two-body wave
function describing the behaviour of a pair of bosons in the Bose-Einstein
condensate. The resulting coupled equations are free of unphysical divergences.
Particular cases of this scheme that admit analytical estimations are
considered and compared to the literature. In addition to the well-known cases
of low-dimensional trapping, cross-over regimes can be studied. The values of
the kinetic, interaction, external, and release energies in low dimensions are
also evaluated and contributions due to short-range correlations are found to
be substantial.Comment: 15 pages, ReVTEX, no figure
On modulational instability and energy localization in anharmonic lattices at finite energy density
The localization of vibrational energy, induced by the modulational
instability of the Brillouin-zone-boundary mode in a chain of classical
anharmonic oscillators with finite initial energy density, is studied within a
continuum theory. We describe the initial localization stage as a gas of
envelope solitons and explain their merging, eventually leading to a single
localized object containing a macroscopic fraction of the total energy of the
lattice. The initial-energy-density dependences of all characteristic time
scales of the soliton formation and merging are described analytically. Spatial
power spectra are computed and used for the quantitative explanation of the
numerical results.Comment: 12 pages, 7 figure
Optical metrics and birefringence of anisotropic media
The material tensor of linear response in electrodynamics is constructed out
of products of two symmetric second rank tensor fields which in the
approximation of geometrical optics and for uniaxial symmetry reduce to
"optical" metrics, describing the phenomenon of birefringence. This
representation is interpreted in the context of an underlying internal
geometrical structure according to which the symmetric tensor fields are
vectorial elements of an associated two-dimensional space.Comment: 24 pages, accepted for publication in GR
Uniformly Accelerated Charge in a Quantum Field: From Radiation Reaction to Unruh Effect
We present a stochastic theory for the nonequilibrium dynamics of charges
moving in a quantum scalar field based on the worldline influence functional
and the close-time-path (CTP or in-in) coarse-grained effective action method.
We summarize (1) the steps leading to a derivation of a modified
Abraham-Lorentz-Dirac equation whose solutions describe a causal semiclassical
theory free of runaway solutions and without pre-acceleration patholigies, and
(2) the transformation to a stochastic effective action which generates
Abraham-Lorentz-Dirac-Langevin equations depicting the fluctuations of a
particle's worldline around its semiclassical trajectory. We point out the
misconceptions in trying to directly relate radiation reaction to vacuum
fluctuations, and discuss how, in the framework that we have developed, an
array of phenomena, from classical radiation and radiation reaction to the
Unruh effect, are interrelated to each other as manifestations at the
classical, stochastic and quantum levels. Using this method we give a
derivation of the Unruh effect for the spacetime worldline coordinates of an
accelerating charge. Our stochastic particle-field model, which was inspired by
earlier work in cosmological backreaction, can be used as an analog to the
black hole backreaction problem describing the stochastic dynamics of a black
hole event horizon.Comment: Invited talk given by BLH at the International Assembly on
Relativistic Dynamics (IARD), June 2004, Saas Fee, Switzerland. 19 pages, 1
figur
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