1,298 research outputs found
Sound modes in composite incommensurate crystals
We propose a simple phenomenological model describing composite crystals,
constructed from two parallel sets of periodic inter-penetrating chains. In the
harmonic approximation and neglecting thermal fluctuations we find the
eigenmodes of the system. It is shown that at high frequencies there are two
longitudinal sound modes with standard attenuation, while in the low frequency
region there is one propagating sound mode and an over-damped phase mode. The
crossover between these two regions is analyzed numerically and the dynamical
structure factor is calculated. It is shown that the qualitative features of
the experimentally observed spectra can be consistently described by our model.Comment: 12 pages, 2 eps figures, Revtex, accepted to European Physics Journal
B, (2002
Efficiency and spectrum of internal gamma-ray burst shocks
We present an analysis of the Internal Shock Model of GRBs, where gamma-rays
are produced by internal shocks within a relativistic wind. We show that
observed GRB characteristics impose stringent constraints on wind and source
parameters. We find that a significant fraction, of order 20 %, of the wind
kinetic energy can be converted to radiation, provided the distribution of
Lorentz factors within the wind has a large variance and provided the minimum
Lorentz factor is higher than 10^(2.5)L_(52)^(2/9), where L=10^(52)L_(52)erg/s
is the wind luminosity. For a high, >10 %, efficiency wind, spectral energy
breaks in the 0.1 to 1 MeV range are obtained for sources with dynamical time
R/c < 1 ms, suggesting a possible explanation for the observed clustering of
spectral break energies in this range. The lower limit to wind Lorenz factor
and the upper limit, around (R/10^7 cm)^(-5/6) MeV to observed break energies
are set by Thomson optical depth due to electron positron pairs produced by
synchrotron photons. Natural consequences of the model are absence of bursts
with peak emission energy significantly exceeding 1 MeV, and existence of low
luminosity bursts with low, 1 keV to 10 keV, break energies.Comment: 10 pages, 5 ps-figures. Expanded discussion of magnetic field and
electron energy fraction. Accepted for publication in Astrophysical Journa
Resonance effects due to the excitation of surface Josephson plasma waves in layered superconductors
We analytically examine the excitation of surface Josephson plasma waves
(SJPWs) in periodically-modulated layered superconductors. We show that the
absorption of the incident electromagnetic wave can be substantially increased,
for certain incident angles, due to the resonance excitation of SJPWs. The
absorption increase is accompanied by the decrease of the specular reflection.
Moreover, we find the physical conditions guaranteeing the total absorption
(and total suppression of the specular reflection). These conditions can be
realized for Bi2212 superconductor films.Comment: 17 pages, 3 figure
Paramagnetic anisotropic magnetoresistance in thin films of SrRuO3
SrRuO3 is an itinerant ferromagnet and in its thin film form when grown on
miscut SrTiO3 it has Tc of ~ 150 K and strong uniaxial anisotropy. We measured
both the Hall effect and the magnetoresistance (MR) of the films as a function
of the angle between the applied field and the normal to the films at
temperatures above Tc. We extracted the extraordinary Hall effect that is
proportional to the perpendicular component of the magnetization and thus the
MR for each angle of the applied field could be correlated with the magnitude
and orientation of the induced magnetization. We successfully fit the MR data
with a second order magnetization expansion, which indicates large anisotropic
MR in the paramagnetic state. The extremum values of resistivity are not
obtained for currents parallel or perpendicular to the magnetization, probably
due to the crystal symmetry.Comment: 3 pages, 3 figure
Dynamic Fluctuation Phenomena in Double Membrane Films
Dynamics of double membrane films is investigated in the long-wavelength
limit including the overdamped squeezing mode. We demonstrate that thermal
fluctuations essentially modify the character of the mode due to its nonlinear
coupling to the transversal shear hydrodynamic mode. The corresponding Green
function acquires as a function of the frequency a cut along the imaginary
semi-axis. Fluctuations lead to increasing the attenuation of the squeezing
mode it becomes larger than the `bare' value.Comment: 7 pages, Revte
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