1,941 research outputs found
Kinetics of Ordering in Fluctuation-Driven First-Order Transitions: Simulations and Dynamical Renormalization
Many systems where interactions compete with each other or with constraints
are well described by a model first introduced by Brazovskii. Such systems
include block copolymers, alloys with modulated phases, Rayleigh-Benard Cells
and type-I superconductors. The hallmark of this model is that the fluctuation
spectrum is isotropic and has a minimum at a nonzero wave vector represented by
the surface of a d-dimensional hyper-sphere. It was shown by Brazovskii that
the fluctuations change the free energy structure from a to a
form with the disordered state metastable for all quench depths.
The transition from the disordered to the periodic, lamellar structure changes
from second order to first order and suggests that the dynamics is governed by
nucleation. Using numerical simulations we have confirmed that the equilibrium
free energy function is indeed of a form. A study of the dynamics,
however, shows that, following a deep quench, the dynamics is described by
unstable growth rather than nucleation. A dynamical calculation, based on a
generalization of the Brazovskii calculations shows that the disordered state
can remain unstable for a long time following the quench.Comment: 18 pages, 15 figures submitted to PR
Efficient method for simulating quantum electron dynamics under the time dependent Kohn-Sham equation
A numerical scheme for solving the time-evolution of wave functions under the
time dependent Kohn-Sham equation has been developed. Since the effective
Hamiltonian depends on the wave functions, the wave functions and the effective
Hamiltonian should evolve consistently with each other. For this purpose, a
self-consistent loop is required at every time-step for solving the
time-evolution numerically, which is computationally expensive. However, in
this paper, we develop a different approach expressing a formal solution of the
TD-KS equation, and prove that it is possible to solve the TD-KS equation
efficiently and accurately by means of a simple numerical scheme without the
use of any self-consistent loops.Comment: 5 pages, 3 figures. Physical Review E, 2002, in pres
Static- and dynamical-phase transition in multidimensional voting models on continua
A voting model (or a generalization of the Glauber model at zero temperature)
on a multidimensional lattice is defined as a system composed of a lattice each
site of which is either empty or occupied by a single particle. The reactions
of the system are such that two adjacent sites, one empty the other occupied,
may evolve to a state where both of these sites are either empty or occupied.
The continuum version of this model in a Ddimensional region with boundary is
studied, and two general behaviors of such systems are investigated. The
stationary behavior of the system, and the dominant way of the relaxation of
the system toward its stationary state. Based on the first behavior, the static
phase transition (discontinuous changes in the stationary profiles of the
system) is studied. Based on the second behavior, the dynamical phase
transition (discontinuous changes in the relaxation-times of the system) is
studied. It is shown that the static phase transition is induced by the bulk
reactions only, while the dynamical phase transition is a result of both bulk
reactions and boundary conditions.Comment: 10 pages, LaTeX2
Universality and Scaling for the Structure Factor in Dynamic Order-Disorder Transitions
The universal form for the average scattering intensity from systems
undergoing order-disorder transitions is found by numerical integration of the
Langevin dynamics. The result is nearly identical for simulations involving two
different forms of the local contribution to the free energy, supporting the
idea that the Model A dynamical universality class includes a wide range of
local free-energy forms. An absolute comparison with no adjustable parameters
is made to the forms predicted by the theories of Ohta-Jasnow-Kawasaki and
Mazenko. The numerical results are well described by the former theory, except
in the cross-over region between scattering dominated by domain geometry and
scattering determined by Porod's law.Comment: 10 pages incl. 3 figures, Revtex. Submitted to PR
Singular Structure and Enhanced Friedel Oscillations in the Two-Dimensional Electron Gas
We calculate the leading order corrections (in ) to the static
polarization , with dynamically screened interactions, for the
two-dimensional electron gas. The corresponding diagrams all exhibit singular
logarithmic behavior in their derivatives at and provide significant
enhancement to the proper polarization particularly at low densities. At a
density of , the contribution from the leading order {\em fluctuational}
diagrams exceeds both the zeroth order (Lindhard) response and the self-energy
and exchange contributions. We comment on the importance of these diagrams in
two-dimensions and make comparisons to an equivalent three-dimensional electron
gas; we also consider the impact these finding have on computed
to all orders in perturbation theory
Dynamical properties of the Landau-Ginzburg model with long-range correlated quenched impurities
We investigate the critical dynamics of the time-dependent Landau-Ginzburg
model with non conserved n-component order parameter (Model A) in the presence
of long-range correlated quenched impurities. We use a special kind of
long-range correlations, previously introduced by Weinrib and Halperin. Using a
double expansion in \epsilon and \delta we calculate the critical exponent z up
to second order on the small parameters. We show that the quenched impurities
of this kind affect the critical dynamics already in first order of \epsilon
and \delta, leading to a relevant correction for the mean field value of the
exponent zComment: 7 pages, REVTEX, to be published in Phys. Rev.
Statics and Dynamics of an Interface in a Temperature Gradient
The response and nonconserved dynamics of a two-phase interface in the
presence of a temperature gradient oriented normally to the interface are
considered. Two types of boundary conditions on the order parameter are
considered, and the structure of the effective free energy and the Langevin
equation for the collective coordinate specifying the interface position are
analyzed.Comment: 15 pages, Revtex 3.0, 5 figures available upon reques
Structure and vibrational spectra of carbon clusters in SiC
The electronic, structural and vibrational properties of small carbon
interstitial and antisite clusters are investigated by ab initio methods in 3C
and 4H-SiC. The defects possess sizable dissociation energies and may be formed
via condensation of carbon interstitials, e.g. generated in the course of ion
implantation. All considered defect complexes possess localized vibrational
modes (LVM's) well above the SiC bulk phonon spectrum. In particular, the
compact antisite clusters exhibit high-frequency LVM's up to 250meV. The
isotope shifts resulting from a_{13}C enrichment are analyzed. In the light of
these results, the photoluminescence centers D_{II} and P-U are discussed. The
dicarbon antisite is identified as a plausible key ingredient of the
D_{II}-center, whereas the carbon split-interstitial is a likely origin of the
P-T centers. The comparison of the calculated and observed high-frequency modes
suggests that the U-center is also a carbon-antisite based defect.Comment: 15 pages, 6 figures, accepted by Phys. Rev.
Infrared Behavior of Interacting Bosons at Zero Temperature
We exploit the symmetries associated with the stability of the superfluid
phase to solve the long-standing problem of interacting bosons in the presence
of a condensate at zero temperature. Implementation of these symmetries poses
strong conditions on the renormalizations that heal the singularities of
perturbation theory. The renormalized theory gives: For d>3 the Bogoliubov
quasiparticles as an exact result; for 1<d<=3 a nontrivial solution with the
exact exponent for the singular longitudinal correlation function, with phonons
again as low-lying excitations.Comment: Minor Changes. 4 pages, RevTeX, no figures, uses multicol.sty e-mail:
[email protected]
Scaling of thermal conductivity of helium confined in pores
We have studied the thermal conductivity of confined superfluids on a
bar-like geometry. We use the planar magnet lattice model on a lattice with . We have applied open boundary conditions on the bar
sides (the confined directions of length ) and periodic along the long
direction. We have adopted a hybrid Monte Carlo algorithm to efficiently deal
with the critical slowing down and in order to solve the dynamical equations of
motion we use a discretization technique which introduces errors only
in the time step . Our results demonstrate the
validity of scaling using known values of the critical exponents and we
obtained the scaling function of the thermal resistivity. We find that our
results for the thermal resistivity scaling function are in very good agreement
with the available experimental results for pores using the tempComment: 5 two-column pages, 3 figures, Revtex
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