541 research outputs found
Langevin dynamics with a tilted periodic potential
We study a Langevin equation for a particle moving in a periodic potential in
the presence of viscosity and subject to a further external field
. For a suitable choice of the parameters and the
related deterministic dynamics yields heteroclinic orbits. In such a regime, in
absence of stochastic noise both confined and unbounded orbits coexist. We
prove that, with the inclusion of an arbitrarly small noise only the confined
orbits survive in a sub-exponential time scale.Comment: 38 pages, 6 figure
The Hydrodynamics of M-Theory
We consider the low energy limit of a stack of N M-branes at finite
temperature. In this limit, the M-branes are well described, via the AdS/CFT
correspondence, in terms of classical solutions to the eleven dimensional
supergravity equations of motion. We calculate Minkowski space two-point
functions on these M-branes in the long-distance, low-frequency limit, i.e. the
hydrodynamic limit, using the prescription of Son and Starinets
[hep-th/0205051]. From these Green's functions for the R-currents and for
components of the stress-energy tensor, we extract two kinds of diffusion
constant and a viscosity. The N dependence of these physical quantities may
help lead to a better understanding of M-branes.Comment: 1+19 pages, references added, section 5 clarified, eq. (72) correcte
Hydrodynamic obstruction to bubble expansion
We discuss a hydrodynamic obstruction to bubble wall acceleration during a
cosmological first-order phase transition. The obstruction results from the
heating of the plasma in the compression wave in front of the phase transition
boundary. We provide a simple criterion for the occurrence of the obstruction
at subsonic bubble wall velocity in terms of the critical temperature, the
phase transition temperature, and the latent heat of the model under
consideration. The criterion serves as a sufficient condition of subsonic
bubble wall velocities as required by electroweak baryogenesis.Comment: 18 pages, 4 figures; comments and reference added, published versio
Energy Budget of Cosmological First-order Phase Transitions
The study of the hydrodynamics of bubble growth in first-order phase
transitions is very relevant for electroweak baryogenesis, as the baryon
asymmetry depends sensitively on the bubble wall velocity, and also for
predicting the size of the gravity wave signal resulting from bubble
collisions, which depends on both the bubble wall velocity and the plasma fluid
velocity. We perform such study in different bubble expansion regimes, namely
deflagrations, detonations, hybrids (steady states) and runaway solutions
(accelerating wall), without relying on a specific particle physics model. We
compute the efficiency of the transfer of vacuum energy to the bubble wall and
the plasma in all regimes. We clarify the condition determining the runaway
regime and stress that in most models of strong first-order phase transitions
this will modify expectations for the gravity wave signal. Indeed, in this
case, most of the kinetic energy is concentrated in the wall and almost no
turbulent fluid motions are expected since the surrounding fluid is kept mostly
at rest.Comment: 36 pages, 14 figure
Coordinate Representation of the One-Spinon One-Holon Wavefunction and Spinon-Holon Interaction
By deriving and studying the coordinate representation for the one-spinon
one-holon wavefunction we show that spinons and holons in the supersymmetric model with interaction attract each other. The interaction causes
a probability enhancement in the one-spinon one-holon wavefunction at short
separation between the particles. We express the hole spectral function for a
finite lattice in terms of the probability enhancement, given by the one-spinon
one-holon wavefunction at zero separation. In the thermodynamic limit, the
spinon-holon attraction turns into the square-root divergence in the hole
spectral function.Comment: 20 pages, 3 .eps figure
Spontaneous symmetry breaking and the limit
We point out a basic ambiguity in the limit of the connected
propagator in a spontaneously broken phase. This may represent an indication
that the conventional singlet Higgs boson, rather than being a purely massive
field, might have a gap-less branch. This would dominate the energy spectrum
for and give rise to a very weak, long-range force. The
natural interpretation is in terms of density fluctuations of the `Higgs
condensate': in the region of very long wavelengths, infinitely larger than the
Fermi scale, it cannot be treated as a purely classical c-number field.Comment: 17 pages, LaTex, small changes and some comments adde
Proximity effect in ultrathin Pb/Ag multilayers within the Cooper limit
We report on transport and tunneling measurements performed on ultra-thin
Pb/Ag (strong coupled superconductor/normal metal) multilayers evaporated by
quench condensation. The critical temperature and energy gap of the
heterostructures oscillate with addition of each layer, demonstrating the
validity of the Cooper limit model in the case of multilayers. We observe
excellent agreement with a simple theory for samples with layer thickness
larger than 30\AA . Samples with single layers thinner than 30\AA deviate from
the Cooper limit theory. We suggest that this is due to the "inverse proximity
effect" where the normal metal electrons improve screening in the
superconducting ultrathin layer and thus enhance the critical temperature.Comment: 4 pages, 4 figure
Strong Phase Separation in a Model of Sedimenting Lattices
We study the steady state resulting from instabilities in crystals driven
through a dissipative medium, for instance, a colloidal crystal which is
steadily sedimenting through a viscous fluid. The problem involves two coupled
fields, the density and the tilt; the latter describes the orientation of the
mass tensor with respect to the driving field. We map the problem to a 1-d
lattice model with two coupled species of spins evolving through conserved
dynamics. In the steady state of this model each of the two species shows
macroscopic phase separation. This phase separation is robust and survives at
all temperatures or noise levels--- hence the term Strong Phase Separation.
This sort of phase separation can be understood in terms of barriers to
remixing which grow with system size and result in a logarithmically slow
approach to the steady state. In a particular symmetric limit, it is shown that
the condition of detailed balance holds with a Hamiltonian which has
infinite-ranged interactions, even though the initial model has only local
dynamics. The long-ranged character of the interactions is responsible for
phase separation, and for the fact that it persists at all temperatures.
Possible experimental tests of the phenomenon are discussed.Comment: To appear in Phys Rev E (1 January 2000), 16 pages, RevTex, uses
epsf, three ps figure
Nonequilibrium thermodynamics versus model grain growth: derivation and some physical implications
Nonequilibrium thermodynamics formalism is proposed to derive the flux of
grainy (bubbles-containing) matter, emerging in a nucleation growth process.
Some power and non-power limits, due to the applied potential as well as owing
to basic correlations in such systems, have been discussed. Some encouragement
for such a discussion comes from the fact that the nucleation and growth
processes studied, and their kinetics, are frequently reported in literature as
self-similar (characteristic of algebraic correlations and laws) both in basic
entity (grain; bubble) size as well as time scales.Comment: 8 pages, 1 figur
Phenomenological approach to non-linear Langevin equations
In this paper we address the problem of consistently construct Langevin
equations to describe fluctuations in non-linear systems. Detailed balance
severely restricts the choice of the random force, but we prove that this
property together with the macroscopic knowledge of the system is not enough to
determine all the properties of the random force. If the cause of the
fluctuations is weakly coupled to the fluctuating variable, then the
statistical properties of the random force can be completely specified. For
variables odd under time-reversal, microscopic reversibility and weak coupling
impose symmetry relations on the variable-dependent Onsager coefficients. We
then analyze the fluctuations in two cases: Brownian motion in position space
and an asymmetric diode, for which the analysis based in the master equation
approach is known. We find that, to the order of validity of the Langevin
equation proposed here, the phenomenological theory is in agreement with the
results predicted by more microscopic models.Comment: LaTex file, 2 figures available upon request, to appear in Phys.Rev.
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