1,583 research outputs found
Phase Transitions in the Multicomponent Widom-Rowlinson Model and in Hard Cubes on the BCC--Lattice
We use Monte Carlo techniques and analytical methods to study the phase
diagram of the M--component Widom-Rowlinson model on the bcc-lattice: there are
M species all with the same fugacity z and a nearest neighbor hard core
exclusion between unlike particles. Simulations show that for M greater or
equal 3 there is a ``crystal phase'' for z lying between z_c(M) and z_d(M)
while for z > z_d(M) there are M demixed phases each consisting mostly of one
species. For M=2 there is a direct second order transition from the gas phase
to the demixed phase while for M greater or equal 3 the transition at z_d(M)
appears to be first order putting it in the Potts model universality class. For
M large, Pirogov-Sinai theory gives z_d(M) ~ M-2+2/(3M^2) + ... . In the
crystal phase the particles preferentially occupy one of the sublattices,
independent of species, i.e. spatial symmetry but not particle symmetry is
broken. For M to infinity this transition approaches that of the one component
hard cube gas with fugacity y = zM. We find by direct simulations of such a
system a transition at y_c ~ 0.71 which is consistent with the simulation
z_c(M) for large M. This transition appears to be always of the Ising type.Comment: 11 pages, 4 postscript figures (added in replacement), Physica A (in
press
Kinetics of Joint Ordering and Decomposition in Binary Alloys
We study phase segregation in a model alloy undergoing both ordering and
decomposition, using computer simulations of Kawasaki exchange dynamics on a
square lattice. Following a quench into the miscibility gap we observe an early
stage in which ordering develops while the composition remains almost uniform.
Then decomposition starts with segregation into ordered and disordered phases.
The two spherically averaged structure functions, related to decomposition and
to ordering, were both observed to obey scaling rules in the late coarsening
stage where the time increase of the characteristic lengths was consistent with
. While was similar for ordering and decomposition at low
concentration of the minority component, it showed an increase (decrease) with
concentration for ordering (decomposition). The domain morphology was found to
depend on the concentration of the minority component, in a way that suggests a
wetting of antiphase boundaries in the ordered domains by the disordered phase.Comment: 23 pages, in TeX, figues available upon reques
Modelling of Phase Separation in Alloys with Coherent Elastic Misfit
Elastic interactions arising from a difference of lattice spacing between two
coherent phases can have a strong influence on the phase separation
(coarsening) of alloys. If the elastic moduli are different in the two phases,
the elastic interactions may accelerate, slow down or even stop the phase
separation process. If the material is elastically anisotropic, the
precipitates can be shaped like plates or needles instead of spheres and can
form regular precipitate superlattices. Tensions or compressions applied
externally to the specimen may have a strong effect on the shapes and
arrangement of the precipitates. In this paper, we review the main theoretical
approaches that have been used to model these effects and we relate them to
experimental observations. The theoretical approaches considered are (i)
`macroscopic' models treating the two phases as elastic media separated by a
sharp interface (ii) `mesoscopic' models in which the concentration varies
continuously across the interface (iii) `microscopic' models which use the
positions of individual atoms.Comment: 106 pages, in Latex, figures available upon request, e-mail
addresses: [email protected], [email protected],
[email protected], submitted to the Journal of Statistical Physic
Effect of phonon-phonon interactions on localization
We study the heat current J in a classical one-dimensional disordered chain
with on-site pinning and with ends connected to stochastic thermal reservoirs
at different temperatures. In the absence of anharmonicity all modes are
localized and there is a gap in the spectrum. Consequently J decays
exponentially with system size N. Using simulations we find that even a small
amount of anharmonicity leads to a J~1/N dependence, implying diffusive
transport of energy.Comment: 4 pages, 2 figures, Published versio
On time's arrow in Ehrenfest models with reversible deterministic dynamics
We introduce a deterministic, time-reversible version of the Ehrenfest urn
model. The distribution of first-passage times from equilibrium to
non-equilibrium states and vice versa is calculated. We find that average times
for transition to non-equilibrium always scale exponentially with the system
size, whereas the time scale for relaxation to equilibrium depends on
microscopic dynamics. To illustrate this, we also look at deterministic and
stochastic versions of the Ehrenfest model with a distribution of microscopic
relaxation times.Comment: 6 pages, 7 figures, revte
Diffusion Effects on the Breakdown of a Linear Amplifier Model Driven by the Square of a Gaussian Field
We investigate solutions to the equation , where is a Gaussian stochastic field
with covariance , and . It is shown that the
coupling at which the -th moment
diverges at time $t$, is always less or equal for ${\cal D}>0$ than for ${\cal
D}=0$. Equality holds under some reasonable assumptions on $C$ and, in this
case, $\lambda_{cN}(t)=N\lambda_c(t)$ where $\lambda_c(t)$ is the value of
$\lambda$ at which diverges.
The case is solved for a class of . The dependence of
on is analyzed. Similar behavior is conjectured when
diffusion is replaced by diffraction, , the case of
interest for backscattering instabilities in laser-plasma interaction.Comment: 19 pages, in LaTeX, e-mail addresses: [email protected],
[email protected], [email protected],
[email protected]
The promoter of the human interleukin-2 gene contains two octamer-binding sites and is partially activated by the expression of Oct-2
The gene encoding interleukin-2 (IL-2) contains a sequence 52 to 326 nucleotides upstream of its transcriptional initiation site that promotes transcription in T cells that have been activated by costimulation with tetradecanoyl phorbol myristyl acetate (TPA) and phytohemagglutinin (PHA). We found that the ubiquitous transcription factor, Oct-1, bound to two previously identified motifs within the human IL-2 enhancer, centered at nucleotides -74 and -251. Each site in the IL-2 enhancer that bound Oct-1 in vitro was also required to achieve a maximal transcriptional response to TPA plus PHA in vivo. Point mutations within either the proximal or distal octamer sequences reduced the response of the enhancer to activation by 54 and 34%, respectively. Because the murine T-cell line EL4 constitutively expresses Oct-2 and requires only TPA to induce transcription of the IL-2 gene, the effect of Oct-2 expression on activation of the IL-2 promoter in Jurkat T cells was determined. Expression of Oct-2 potentiated transcription 13-fold in response to TPA plus PHA and permitted the enhancer to respond to the single stimulus of TPA. Therefore, both the signal requirements and the magnitude of the transcription response of the IL-2 promoter can be modulated by Oct-2
Comment on ``Scaling Laws for a System with Long-Range Interactions within Tsallis Statistics''
In their recent Letter [Phys. Rev. Lett. 83, 4233 (1999)], Salazar and Toral
(ST) study numerically a finite Ising chain with non-integrable interactions
decaying like 1/r^(d+sigma) where -d <= sigma <= 0 (like ST, we discuss general
dimensionality d). In particular, they explore a presumed connection between
non-integrable interactions and Tsallis's non-extensive statistics. We point
out that (i) non-integrable interactions provide no more motivation for Tsallis
statistics than do integrable interactions, i.e., Gibbs statistics remain
meaningful for the non-integrable case, and in fact provide a {\em complete and
exact treatment}; and (ii) there are undesirable features of the method ST use
to regulate the non-integrable interactions.Comment: Accepted for publication in Phys. Rev. Let
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