1,249 research outputs found
Instability of the O(5) multicritical behavior in the SO(5) theory of high-Tc superconductors
We study the nature of the multicritical point in the three-dimensional
O(3)+O(2) symmetric Landau-Ginzburg-Wilson theory, which describes the
competition of two order parameters that are O(3) and O(2) symmetric,
respectively. This study is relevant for the SO(5) theory of high-Tc
superconductors, which predicts the existence of a multicritical point in the
temperature-doping phase diagram, where the antiferromagnetic and
superconducting transition lines meet.
We investigate whether the O(3)+O(2) symmetry gets effectively enlarged to
O(5) approaching the multicritical point. For this purpose, we study the
stability of the O(5) fixed point. By means of a Monte Carlo simulation, we
show that the O(5) fixed point is unstable with respect to the spin-4 quartic
perturbation with the crossover exponent , in substantial
agreement with recent field-theoretical results. This estimate is much larger
than the one-loop -expansion estimate , which has
often been used in the literature to discuss the multicritical behavior within
the SO(5) theory. Therefore, no symmetry enlargement is generically expected at
the multicritical transition.
We also perform a five-loop field-theoretical analysis of the
renormalization-group flow. It shows that bicritical systems are not in the
attraction domain of the stable decoupled fixed point. Thus, in these
systems--high-Tc cuprates should belong to this class--the multicritical point
corresponds to a first-order transition.Comment: 18 page
A Model for Force Fluctuations in Bead Packs
We study theoretically the complex network of forces that is responsible for
the static structure and properties of granular materials. We present detailed
calculations for a model in which the fluctuations in the force distribution
arise because of variations in the contact angles and the constraints imposed
by the force balance on each bead of the pile. We compare our results for force
distribution function for this model, including exact results for certain
contact angle probability distributions, with numerical simulations of force
distributions in random sphere packings. This model reproduces many aspects of
the force distribution observed both in experiment and in numerical simulations
of sphere packings
Effective Potential for Scalar Field in Three Dimensions: Ising Model in the Ferromagnetic Phase
We compute the effective potential for one-component real
scalar field in three Euclidean dimensions (3D) in the case of
spontaneously broken symmetry, from the Monte Carlo simulation of the 3D Ising
model in external field at temperatures approaching the phase transition from
below. We study probability distributions of the order parameter on the
lattices from to , at . We find that, in close
analogy with the symmetric case, plays an important role: is very well approximated by the sum of , and
terms. An unexpected feature is the negative sign of the
term. As close to the continuum limit as we can get (), we
obtain
We also compute several universal coupling constants and ratios, including
the combination of critical amplitudes .Comment: 13 pages, 5 Postscript figures, uses epsf.st
New supersymmetric solutions of N=2, D=5 gauged supergravity with hyperscalars
We construct new supersymmetric solutions, including AdS bubbles, in an N=2
truncation of five-dimensional N=8 gauged supergravity. This particular
truncation is given by N=2 gauged supergravity coupled to two vector multiples
and three incomplete hypermultiplets, and was originally investigated in the
context of obtaining regular AdS bubble geometries with multiple active
R-charges. We focus on cohomogeneity-one solutions corresponding to objects
with two equal angular momenta and up to three independent R-charges.
Curiously, we find a new set of zero and negative mass solitons asymptotic to
AdS_5/Z_k, for k \ge 3, which are everywhere regular without closed timelike
curves.Comment: Latex 3 times, 42 page
Properties of Interfaces in the two and three dimensional Ising Model
To investigate order-order interfaces, we perform multimagnetical Monte Carlo
simulations of the and Ising model. Following Binder we extract the
interfacial free energy from the infinite volume limit of the magnetic
probability density. Stringent tests of the numerical methods are performed by
reproducing with high precision exact results. In the physically more
interesting case we estimate the amplitude of the critical
interfacial tension to be . This
result is in good agreement with a previous MC calculation by Mon, as well as
with experimental results for related amplitude ratios. In addition, we study
in some details the shape of the magnetic probability density for temperatures
below the Curie point.Comment: 25 pages; sorry no figures include
Field theory of bi- and tetracritical points: Statics
We calculate the static critical behavior of systems of symmetry by renormalization group method within the minimal
subtraction scheme in two loop order. Summation methods lead to fixed points
describing multicritical behavior. Their stability boarder lines in the space
of order parameter components and and spatial dimension
are calculated. The essential features obtained already in two loop order for
the interesting case of an antiferromagnet in a magnetic field (,
) are the stability of the biconical fixed point and the
neighborhood of the stability border lines to the other fixed points leading to
very small transient exponents. We are also able to calculate the flow of
static couplings, which allows to consider the attraction region. Depending on
the nonuniversal background parameters the existence of different multicritical
behavior (bicritical or tetracritical) is possible including a triple point.Comment: 6 figure
Plane waves with weak singularities
We study a class of time dependent solutions of the vacuum Einstein equations
which are plane waves with weak null singularities. This singularity is weak in
the sense that though the tidal forces diverge at the singularity, the rate of
divergence is such that the distortion suffered by a freely falling observer
remains finite. Among such weak singular plane waves there is a sub-class which
do not exhibit large back reaction in the presence of test scalar probes.
String propagation in these backgrounds is smooth and there is a natural way to
continue the metric beyond the singularity. This continued metric admits string
propagation without the string becoming infinitely excited. We construct a one
parameter family of smooth metrics which are at a finite distance in the space
of metrics from the extended metric and a well defined operator in the string
sigma model which resolves the singularity.Comment: 22 pages, Added references and clarifying comment
Critical specific heats of the N-vector spin models on the sc and the bcc lattices
We have computed through order the high-temperature expansions
for the nearest-neighbor spin correlation function of the
classical N-vector model, with general N, on the simple-cubic and on the
body-centered-cubic lattices.
For this model, also known in quantum field theory as the lattice O(N)
nonlinear sigma model, we have presented in previous papers extended expansions
of the susceptibility, of its second field derivative and of the second moment
of the correlation function.
Here we study the internal specific energy and the specific heat
, obtaining new estimates of the critical parameters and therefore
a more accurate direct test of the hyperscaling relation on a range of values of the spin dimensionality N, including N=0
[the self-avoiding walk model], N=1 [the Ising spin 1/2 model], N=2 [the XY
model], N=3 [the classical Heisenberg model]. By the newly extended series, we
also compute the universal combination of critical amplitudes usually denoted
by , in fair agreement with renormalization group estimates.Comment: 15 pages, latex, no figure
Renormalized couplings and scaling correction amplitudes in the N-vector spin models on the sc and the bcc lattices
For the classical N-vector model, with arbitrary N, we have computed through
order \beta^{17} the high temperature expansions of the second field derivative
of the susceptibility \chi_4(N,\beta) on the simple cubic and on the body
centered cubic lattices. (The N-vector model is also known as the O(N)
symmetric classical spin Heisenberg model or, in quantum field theory, as the
lattice
O(N) nonlinear sigma model.) By analyzing the expansion of \chi_4(N,\beta) on
the two lattices, and by carefully allowing for the corrections to scaling, we
obtain updated estimates of the critical parameters and more accurate tests of
the hyperscaling relation d\nu(N) +\gamma(N) -2\Delta_4(N)=0 for a range of
values of the spin dimensionality N, including
N=0 [the self-avoiding walk model], N=1 [the Ising spin 1/2 model],
N=2 [the XY model], N=3 [the classical Heisenberg model]. Using the recently
extended series for the susceptibility and for the second correlation moment,
we also compute the dimensionless renormalized four point coupling constants
and some universal ratios of scaling correction amplitudes in fair agreement
with recent renormalization group estimates.Comment: 23 pages, latex, no figure
Critical adsorption on curved objects
A systematic fieldtheoretic description of critical adsorption on curved
objects such as spherical or rodlike colloidal particles immersed in a fluid
near criticality is presented. The temperature dependence of the corresponding
order parameter profiles and of the excess adsorption are calculated
explicitly. Critical adsorption on elongated rods is substantially more
pronounced than on spherical particles. It turns out that, within the context
of critical phenomena in confined geometries, critical adsorption on a
microscopically thin `needle' represents a distinct universality class of its
own. Under favorable conditions the results are relevant for the flocculation
of colloidal particles.Comment: 52 pages, 10 figure
- …