280 research outputs found
An Analytic Equation of State for Ising-like Models
Using an Environmentally Friendly Renormalization we derive, from an
underlying field theory representation, a formal expression for the equation of
state, , that exhibits all desired asymptotic and analyticity
properties in the three limits , and . The only
necessary inputs are the Wilson functions , and
, associated with a renormalization of the transverse vertex
functions. These Wilson functions exhibit a crossover between the Wilson-Fisher
fixed point and the fixed point that controls the coexistence curve.
Restricting to the case N=1, we derive a one-loop equation of state for naturally parameterized by a ratio of non-linear scaling fields. For
we show that a non-parameterized analytic form can be deduced. Various
asymptotic amplitudes are calculated directly from the equation of state in all
three asymptotic limits of interest and comparison made with known results. By
positing a scaling form for the equation of state inspired by the one-loop
result, but adjusted to fit the known values of the critical exponents, we
obtain better agreement with known asymptotic amplitudes.Comment: 10 pages, 2 figure
Fractal and Transfractal Recursive Scale-Free Nets
We explore the concepts of self-similarity, dimensionality, and
(multi)scaling in a new family of recursive scale-free nets that yield
themselves to exact analysis through renormalization techniques. All nets in
this family are self-similar and some are fractals - possessing a finite
fractal dimension - while others are small world (their diameter grows
logarithmically with their size) and are infinite-dimensional. We show how a
useful measure of "transfinite" dimension may be defined and applied to the
small world nets. Concerning multiscaling, we show how first-passage time for
diffusion and resistance between hub (the most connected nodes) scale
differently than for other nodes. Despite the different scalings, the Einstein
relation between diffusion and conductivity holds separately for hubs and
nodes. The transfinite exponents of small world nets obey Einstein relations
analogous to those in fractal nets.Comment: Includes small revisions and references added as result of readers'
feedbac
Novel glassy behavior in a ferromagnetic p-spin model
Recent work has suggested the existence of glassy behavior in a ferromagnetic
model with a four-spin interaction. Motivated by these findings, we have
studied the dynamics of this model using Monte Carlo simulations with
particular attention being paid to two-time quantities. We find that the system
shares many features in common with glass forming liquids. In particular, the
model exhibits: (i) a very long-lived metastable state, (ii) autocorrelation
functions that show stretched exponential relaxation, (iii) a non-equilibrium
timescale that appears to diverge at a well defined temperature, and (iv) low
temperature aging behaviour characteristic of glasses.Comment: 6 pages, 5 figure
Critical Fluctuations and Disorder at the Vortex Liquid to Crystal Transition in Type-II Superconductors
We present a functional renormalization group (FRG) analysis of a
Landau-Ginzburg model of type-II superconductors (generalized to complex
fields) in a magnetic field, both for a pure system, and in the presence of
quenched random impurities. Our analysis is based on a previous FRG treatment
of the pure case [E.Br\'ezin et. al., Phys. Rev. B, {\bf 31}, 7124 (1985)]
which is an expansion in . If the coupling functions are
restricted to the space of functions with non-zero support only at reciprocal
lattice vectors corresponding to the Abrikosov lattice, we find a stable FRG
fixed point in the presence of disorder for , identical to that of the
disordered model in dimensions. The pure system has a stable fixed
point only for and so the physical case () is likely to have a
first order transition. We speculate that the recent experimental findings that
disorder removes the apparent first order transition are consistent with these
calculations.Comment: 4 pages, no figures, typeset using revtex (v3.0
Random field Ising systems on a general hierarchical lattice: Rigorous inequalities
Random Ising systems on a general hierarchical lattice with both, random
fields and random bonds, are considered. Rigorous inequalities between
eigenvalues of the Jacobian renormalization matrix at the pure fixed point are
obtained. These inequalities lead to upper bounds on the crossover exponents
.Comment: LaTeX, 13 pages, figs. 1a,1b,2. To be published in PR
Charge Transport in the Dense Two-Dimensional Coulomb Gas
The dynamics of a globally neutral system of diffusing Coulomb charges in two
dimensions, driven by an applied electric field, is studied in a wide
temperature range around the Berezinskii-Kosterlitz-Thouless transition. I
argue that the commonly accepted ``free particle drift'' mechanism of charge
transport in this system is limited to relatively low particle densities. For
higher densities, I propose a modified picture involving collective ``partner
transfer'' between bound pairs. The new picture provides a natural explanation
for recent experimental and numerical findings which deviate from standard
theory. It also clarifies the origin of dynamical scaling in this context.Comment: 4 pages, RevTeX, 2 eps figures included; some typos corrected, final
version to be published in Phys. Rev. Let
Universal Critical Behavior of Aperiodic Ferromagnetic Models
We investigate the effects of geometric fluctuations, associated with
aperiodic exchange interactions, on the critical behavior of -state
ferromagnetic Potts models on generalized diamond hierarchical lattices. For
layered exchange interactions according to some two-letter substitutional
sequences, and irrelevant geometric fluctuations, the exact recursion relations
in parameter space display a non-trivial diagonal fixed point that governs the
universal critical behavior. For relevant fluctuations, this fixed point
becomes fully unstable, and we show the apperance of a two-cycle which is
associated with a novel critical behavior. We use scaling arguments to
calculate the critical exponent of the specific heat, which turns out
to be different from the value for the uniform case. We check the scaling
predictions by a direct numerical analysis of the singularity of the
thermodynamic free-energy. The agreement between scaling and direct
calculations is excellent for stronger singularities (large values of ). The
critical exponents do not depend on the strengths of the exchange interactions.Comment: 4 pages, 1 figure (included), RevTeX, submitted to Phys. Rev. E as a
Rapid Communicatio
Nonuniversal Correlations and Crossover Effects in the Bragg-Glass Phase of Impure Superconductors
The structural correlation functions of a weakly disordered Abrikosov lattice
are calculated in a functional RG-expansion in dimensions. It is
shown, that in the asymptotic limit the Abrikosov lattice exhibits still
quasi-long-range translational order described by a {\it nonuniversal} exponent
which depends on the ratio of the renormalized elastic constants
of the flux line (FL) lattice. Our calculations
clearly demonstrate three distinct scaling regimes corresponding to the Larkin,
the random manifold and the asymptotic Bragg-glass regime. On a wide range of
{\it intermediate} length scales the FL displacement correlation function
increases as a power law with twice the manifold roughness exponent , which is also {\it nonuniversal}. Correlation functions in the
asymptotic regime are calculated in their full anisotropic dependencies and
various order parameters are examined. Our results, in particular the
-dependency of the exponents, are in variance with those of the
variational treatment with replica symmetry breaking which allows in principle
an experimental discrimination between the two approaches.Comment: 17 pages, 10 figure
Critical behaviour of the Random--Bond Ashkin--Teller Model, a Monte-Carlo study
The critical behaviour of a bond-disordered Ashkin-Teller model on a square
lattice is investigated by intensive Monte-Carlo simulations. A duality
transformation is used to locate a critical plane of the disordered model. This
critical plane corresponds to the line of critical points of the pure model,
along which critical exponents vary continuously. Along this line the scaling
exponent corresponding to randomness varies continuously
and is positive so that randomness is relevant and different critical behaviour
is expected for the disordered model. We use a cluster algorithm for the Monte
Carlo simulations based on the Wolff embedding idea, and perform a finite size
scaling study of several critical models, extrapolating between the critical
bond-disordered Ising and bond-disordered four state Potts models. The critical
behaviour of the disordered model is compared with the critical behaviour of an
anisotropic Ashkin-Teller model which is used as a refference pure model. We
find no essential change in the order parameters' critical exponents with
respect to those of the pure model. The divergence of the specific heat is
changed dramatically. Our results favor a logarithmic type divergence at
, for the random bond Ashkin-Teller and four state Potts
models and for the random bond Ising model.Comment: RevTex, 14 figures in tar compressed form included, Submitted to
Phys. Rev.
Short-Range Ising Spin Glass: Multifractal Properties
The multifractal properties of the Edwards-Anderson order parameter of the
short-range Ising spin glass model on d=3 diamond hierarchical lattices is
studied via an exact recursion procedure. The profiles of the local order
parameter are calculated and analysed within a range of temperatures close to
the critical point with four symmetric distributions of the coupling constants
(Gaussian, Bimodal, Uniform and Exponential). Unlike the pure case, the
multifractal analysis of these profiles reveals that a large spectrum of the
-H\"older exponent is required to describe the singularities of the
measure defined by the normalized local order parameter, at and below the
critical point. Minor changes in these spectra are observed for distinct
initial distributions of coupling constants, suggesting an universal spectra
behavior. For temperatures slightly above T_{c}, a dramatic change in the
function is found, signalizing the transition.Comment: 8 pages, LaTex, PostScript-figures included but also available upon
request. To be published in Physical Review E (01/March 97
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