9,908 research outputs found
Lattice gas with ``interaction potential''
We present an extension of a simple automaton model to incorporate non-local
interactions extending over a spatial range in lattice gases. {}From the
viewpoint of Statistical Mechanics, the lattice gas with interaction range may
serve as a prototype for non-ideal gas behavior. {}From the density
fluctuations correlation function, we obtain a quantity which is identified as
a potential of mean force. Equilibrium and transport properties are computed
theoretically and by numerical simulations to establish the validity of the
model at macroscopic scale.Comment: 12 pages LaTeX, figures available on demand ([email protected]
Mean field and corrections for the Euclidean Minimum Matching problem
Consider the length of the minimum matching of N points in
d-dimensional Euclidean space. Using numerical simulations and the finite size
scaling law , we obtain
precise estimates of for . We then consider
the approximation where distance correlations are neglected. This model is
solvable and gives at an excellent ``random link'' approximation to
. Incorporation of three-link correlations further improves
the accuracy, leading to a relative error of 0.4% at d=2 and 3. Finally, the
large d behavior of this expansion in link correlations is discussed.Comment: source and one figure. Submitted to PR
Statistical mechanics of error exponents for error-correcting codes
Error exponents characterize the exponential decay, when increasing message
length, of the probability of error of many error-correcting codes. To tackle
the long standing problem of computing them exactly, we introduce a general,
thermodynamic, formalism that we illustrate with maximum-likelihood decoding of
low-density parity-check (LDPC) codes on the binary erasure channel (BEC) and
the binary symmetric channel (BSC). In this formalism, we apply the cavity
method for large deviations to derive expressions for both the average and
typical error exponents, which differ by the procedure used to select the codes
from specified ensembles. When decreasing the noise intensity, we find that two
phase transitions take place, at two different levels: a glass to ferromagnetic
transition in the space of codewords, and a paramagnetic to glass transition in
the space of codes.Comment: 32 pages, 13 figure
Conformal diagrams for the gravitational collapse of a spherical dust cloud
We present an algorithm for the construction of conformal coordinates in the
interior of a spherically symmetric, collapsing matter cloud in general
relativity. This algorithm is based on the numerical integration of the radial
null geodesics and a local analysis of their behavior close to the singularity.
As an application, we consider a collapsing spherical dust cloud, generate the
corresponding conformal diagram and analyze the structure of the resulting
singularity. A new bound on the initial data which guarantees that the
singularity is visible from future null infinity is also obtained.Comment: added a new subsection with a phase space analysis, 23 pages, 8
figure
Kerr non-linearity in a superconducting Josephson metamaterial
We present a detailed experimental and theoretical analysis of the dispersion
and non-linear Kerr frequency shifts of plasma modes in a one-dimensional
Josephson junction chain containing 500 SQUIDs in the regime of weak
nonlinearity. The measured low-power dispersion curve agrees perfectly with the
theoretical model if we take into account the Kerr renormalisation of the bare
frequencies and the long-range nature of the island charge screening by a
remote ground plane. We measured the self- and cross-Kerr shifts for the
frequencies of the eight lowest modes in the chain. We compare the measured
Kerr coefficients with theory and find good agreement
Quantum interference of ultrastable twin optical beams
We report the first measurement of the quantum phase-difference noise of an
ultrastable nondegenerate optical parametric oscillator that emits twin beams
classically phase-locked at exact frequency degeneracy. The measurement
illustrates the property of a lossless balanced beam-splitter to convert
number-difference squeezing into phase-difference squeezing and, thus, provides
indirect evidence for Heisenberg-limited interferometry using twin beams. This
experiment is a generalization of the Hong-Ou-Mandel interference effect for
continuous variables and constitutes a milestone towards continuous-variable
entanglement of bright, ultrastable nondegenerate beams.Comment: 4 pages, 4 figs, accepted by Phys. Rev. Let
Star Formation Near Photodissociation Regions: Detection of a Peculiar Protostar Near Ced 201
We present the detection and characterization of a peculiar low-mass
protostar (IRAS 22129+7000) located ~0.4 pc from Ced 201 Photodissociation
Region (PDR) and ~0.2 pc from the HH450 jet. The cold circumstellar envelope
surrounding the object has been mapped through its 1.2 mm dust continuum
emission with IRAM-30m/MAMBO. The deeply embedded protostar is clearly detected
with Spitzer/MIPS (70 um), IRS (20-35 um) and IRAC (4.5, 5.8, and 8 um) but
also in the K_s band (2.15 um). Given the large "near- and mid-IR excess" in
its spectral energy distribution, but large submillimeter-to-bolometric
luminosity ratio (~2%), IRAS 22129+7000 must be a transition Class 0/I source
and/or a multiple stellar system. Targeted observations of several molecular
lines from CO, 13CO, C18O, HCO+ and DCO+ have been obtained. The presence of a
collimated molecular outflow mapped with the CSO telescope in the CO J=3-2 line
suggests that the protostar/disk system is still accreting material from its
natal envelope. Indeed, optically thick line profiles from high density tracers
such as HCO+ J=1-0 show a red-shifted-absorption asymmetry reminiscent of
inward motions. We construct a preliminary physical model of the circumstellar
envelope (including radial density and temperature gradients, velocity field
and turbulence) that reproduces the observed line profiles and estimates the
ionization fraction. The presence of both mechanical and (non-ionizing)
FUV-radiative input makes the region an interesting case to study triggered
star formation
Finite geometries and diffractive orbits in isospectral billiards
Several examples of pairs of isospectral planar domains have been produced in
the two-dimensional Euclidean space by various methods. We show that all these
examples rely on the symmetry between points and blocks in finite projective
spaces; from the properties of these spaces, one can derive a relation between
Green functions as well as a relation between diffractive orbits in isospectral
billiards.Comment: 10 page
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