24,193 research outputs found

### Comment on "Off-diagonal Long-range Order in Bose Liquids: Irrotational Flow and Quantization of Circulation"

In the context of an application to superfluidity, it is elaborated how to do
quantum mechanics of a system with a rotational velocity. Especially, in both
the laboratory frame and the non-inertial co-rotating frame, the canonical
momentum, which corresponds to the quantum mechanical momentum operator,
contains a part due to the rotational velocity.Comment: 2 page, comment on cond-mat/010435

### A heuristic approach to the weakly interacting Bose gas

Some thermodynamic properties of weakly interacting Bose systems are derived
from dimensional and heuristic arguments and thermodynamic relations, without
resorting to statistical mechanics

### Nonlinear Landau-Zener Tunnelling in Coupled Waveguide Arrays

The possibility of direct observation of Nonlinear Landau-Zener tunnelling
effect with a device consisting of two waveguide arrays connected with a tilted
reduced refractive index barrier is discussed. Numerical simulations on this
realistic setup are interpreted via simplified double well system and different
asymmetric tunnelling scenarios were predicted just varying injected beam
intensity.Comment: 5 pages, 6 figure

### Dynamic Structure Factor of Normal Fermi Gas from Collisionless to Hydrodynamic Regime

The dynamic structure factor of a normal Fermi gas is investigated by using
the moment method for the Boltzmann equation. We determine the spectral
function at finite temperatures over the full range of crossover from the
collisionless regime to the hydrodynamic regime. We find that the Brillouin
peak in the dynamic structure factor exhibits a smooth crossover from zero to
first sound as functions of temperature and interaction strength. The dynamic
structure factor obtained using the moment method also exhibits a definite
Rayleigh peak ($/omega /sim 0$), which is a characteristic of the hydrodynamic
regime. We compare the dynamic structure factor obtained by the moment method
with that obtained from the hydrodynamic equations.Comment: 19 pages, 9 figure

### Generalized Emission Functions for Photon Emission from Quark-Gluon Plasma

The Landau-Pomeranchuk-Migdal effects on photon emission from the quark gluon
plasma have been studied as a function of photon mass, at a fixed temperature
of the plasma. The integral equations for the transverse vector function (${\bf
\tilde{f}(\tilde{p}_\perp)}$) and the longitudinal function ($\tilde{g}({\bf
\tilde{p}_\perp})$) consisting of multiple scattering effects are solved by the
self consistent iterations method and also by the variational method for the
variable set \{$p_0,q_0,Q^2$\}, considering the bremsstrahlung and the $\bf
aws$ processes. We define four new dynamical scaling variables,
$x^b_T$,$x^a_T$,$x^b_L$,$x^a_L$ for bremsstrahlung and {\bf aws} processes and
analyse the transverse and longitudinal components as a function of
\{$p_0,q_0,Q^2$\}. We generalize the concept of photon emission function and we
define four new emission functions for massive photon emission represented by
$g^b_T$, $g^a_T$, $g^b_L$, $g^a_L$. These have been constructed using the exact
numerical solutions of the integral equations. These four emission functions
have been parameterized by suitable simple empirical fits. In terms of these
empirical emission functions, the virtual photon emission from quark gluon
plasma reduces to one dimensional integrals that involve folding over the
empirical $g^{b,a}_{T,L}$ functions with appropriate quark distribution
functions and the kinematic factors. Using this empirical emission functions,
we calculated the imaginary part of the photon polarization tensor as a
function of photon mass and energy.Comment: In nuclear physics journals and arxiv listings, my name used to
appear as S.V.S. Sastry. Hereafter, my name will appear as, S.V.
Suryanarayan

### The bound on viscosity and the generalized second law of thermodynamics

We describe a new paradox for ideal fluids. It arises in the accretion of an
\textit{ideal} fluid onto a black hole, where, under suitable boundary
conditions, the flow can violate the generalized second law of thermodynamics.
The paradox indicates that there is in fact a lower bound to the correlation
length of any \textit{real} fluid, the value of which is determined by the
thermodynamic properties of that fluid. We observe that the universal bound on
entropy, itself suggested by the generalized second law, puts a lower bound on
the correlation length of any fluid in terms of its specific entropy. With the
help of a new, efficient estimate for the viscosity of liquids, we argue that
this also means that viscosity is bounded from below in a way reminiscent of
the conjectured Kovtun-Son-Starinets lower bound on the ratio of viscosity to
entropy density. We conclude that much light may be shed on the
Kovtun-Son-Starinets bound by suitable arguments based on the generalized
second law.Comment: 11 pages, 1 figure, published versio

### Equilibrium topology of the intermediate state in type-I superconductors of different shapes

High-resolution magneto-optical technique was used to analyze flux patterns
in the intermediate state of bulk Pb samples of various shapes - cones,
hemispheres and discs. Combined with the measurements of macroscopic
magnetization these results allowed studying the effect of bulk pinning and
geometric barrier on the equilibrium structure of the intermediate state.
Zero-bulk pinning discs and slabs show hysteretic behavior due to geometric
barrier that results in a topological hysteresis -- flux tubes on penetration
and lamellae on flux exit. (Hemi)spheres and cones do not have geometric
barrier and show no hysteresis with flux tubes dominating the intermediate
field region. It is concluded that flux tubes represent the equilibrium
topology of the intermediate state in reversible samples, whereas laminar
structure appears in samples with magnetic hysteresis (either bulk or
geometric). Real-time video is available in
http://www.cmpgroup.ameslab.gov/supermaglab/video/Pb.html
NOTE: the submitted images were severely downsampled due to Arxiv's
limitations of 1 Mb total size

### Measurement of the complex Faraday angle in thin-film metals and high temperature superconductors

A sensitive polarization modulation technique uses photoelastic modulation
and hetrodyne detection to simultaneously measure the Faraday rotation and
induced ellipticity in light transmitted by semiconducting and metallic
samples. The frequencies measured are in the mid-infrared and correspond to the
spectral lines of a CO2 laser. The measured temperature range is continuous and
extends from 35 to 330K. Measured samples include GaAs and Si substrates, gold
and copper films, and YBCO and BSCCO high temperature superconductors.Comment: 12 pages of text, 6 figures, fixed typos in formulas, added figur

### Relativistic viscoelastic fluid mechanics

A detailed study is carried out for the relativistic theory of
viscoelasticity which was recently constructed on the basis of Onsager's linear
nonequilibrium thermodynamics. After rederiving the theory using a local
argument with the entropy current, we show that this theory universally reduces
to the standard relativistic Navier-Stokes fluid mechanics in the long time
limit. Since effects of elasticity are taken into account, the dynamics at
short time scales is modified from that given by the Navier-Stokes equations,
so that acausal problems intrinsic to relativistic Navier-Stokes fluids are
significantly remedied. We in particular show that the wave equations for the
propagation of disturbance around a hydrostatic equilibrium in Minkowski
spacetime become symmetric hyperbolic for some range of parameters, so that the
model is free of acausality problems. This observation suggests that the
relativistic viscoelastic model with such parameters can be regarded as a
causal completion of relativistic Navier-Stokes fluid mechanics. By adjusting
parameters to various values, this theory can treat a wide variety of materials
including elastic materials, Maxwell materials, Kelvin-Voigt materials, and (a
nonlinearly generalized version of) simplified Israel-Stewart fluids, and thus
we expect the theory to be the most universal description of single-component
relativistic continuum materials. We also show that the presence of strains and
the corresponding change in temperature are naturally unified through the
Tolman law in a generally covariant description of continuum mechanics.Comment: 52pages, 11figures; v2: minor corrections; v3: minor corrections, to
appear in Physical Review E; v4: minor change

### Unified approach to structure factors and neutrino processes in nucleon matter

We present a unified approach to neutrino processes in nucleon matter based
on Landau's theory of Fermi liquids that includes one- and
two-quasiparticle-quasihole pair states as well as mean-field effects. We show
how rates of neutrino processes involving two nucleons may be calculated in
terms of the collision integral in the Landau transport equation for
quasiparticles. Using a relaxation time approximation, we solve the transport
equation for density and spin-density fluctuations and derive a general form
for the response functions. We apply our approach to neutral-current processes
in neutron matter, where the spin response function is crucial for calculations
of neutrino elastic and inelastic scattering, neutrino-pair bremsstrahlung and
absorption from strongly-interacting nucleons. We calculate the relaxation
rates using modern nuclear interactions and including many-body contributions,
and find that rates of neutrino processes are reduced compared with estimates
based on the one-pion exchange interaction, which is used in current
simulations of core-collapse supernovae.Comment: 16 pages, 4 figures; NORDITA-2008-30; published versio

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