495 research outputs found
Critical line of an n-component cubic model
We consider a special case of the n-component cubic model on the square
lattice, for which an expansion exists in Ising-like graphs. We construct a
transfer matrix and perform a finite-size-scaling analysis to determine the
critical points for several values of n. Furthermore we determine several
universal quantities, including three critical exponents. For n<2, these
results agree well with the theoretical predictions for the critical O(n)
branch. This model is also a special case of the () model of
Domany and Riedel. It appears that the self-dual plane of the latter model
contains the exactly known critical points of the n=1 and 2 cubic models. For
this reason we have checked whether this is also the case for 1<n<2. However,
this possibility is excluded by our numerical results
Minimal vertex covers on finite-connectivity random graphs - a hard-sphere lattice-gas picture
The minimal vertex-cover (or maximal independent-set) problem is studied on
random graphs of finite connectivity. Analytical results are obtained by a
mapping to a lattice gas of hard spheres of (chemical) radius one, and they are
found to be in excellent agreement with numerical simulations. We give a
detailed description of the replica-symmetric phase, including the size and the
entropy of the minimal vertex covers, and the structure of the unfrozen
component which is found to percolate at connectivity . The
replica-symmetric solution breaks down at . We give a simple
one-step replica symmetry broken solution, and discuss the problems in
interpretation and generalization of this solution.Comment: 32 pages, 9 eps figures, to app. in PRE (01 May 2001
Spectra of Sparse Random Matrices
We compute the spectral density for ensembles of of sparse symmetric random
matrices using replica, managing to circumvent difficulties that have been
encountered in earlier approaches along the lines first suggested in a seminal
paper by Rodgers and Bray. Due attention is payed to the issue of localization.
Our approach is not restricted to matrices defined on graphs with Poissonian
degree distribution. Matrices defined on regular random graphs or on scale-free
graphs, are easily handled. We also look at matrices with row constraints such
as discrete graph Laplacians. Our approach naturally allows to unfold the total
density of states into contributions coming from vertices of different local
coordination.Comment: 22 papges, 8 figures (one on graph-Laplacians added), one reference
added, some typos eliminate
Minimizing Unsatisfaction in Colourful Neighbourhoods
Colouring sparse graphs under various restrictions is a theoretical problem
of significant practical relevance. Here we consider the problem of maximizing
the number of different colours available at the nodes and their
neighbourhoods, given a predetermined number of colours. In the analytical
framework of a tree approximation, carried out at both zero and finite
temperatures, solutions obtained by population dynamics give rise to estimates
of the threshold connectivity for the incomplete to complete transition, which
are consistent with those of existing algorithms. The nature of the transition
as well as the validity of the tree approximation are investigated.Comment: 28 pages, 12 figures, substantially revised with additional
explanatio
Towards finite-dimensional gelation
We consider the gelation of particles which are permanently connected by
random crosslinks, drawn from an ensemble of finite-dimensional continuum
percolation. To average over the randomness, we apply the replica trick, and
interpret the replicated and crosslink-averaged model as an effective molecular
fluid. A Mayer-cluster expansion for moments of the local static density
fluctuations is set up. The simplest non-trivial contribution to this series
leads back to mean-field theory. The central quantity of mean-field theory is
the distribution of localization lengths, which we compute for all
connectivities. The highly crosslinked gel is characterized by a one-to-one
correspondence of connectivity and localization length. Taking into account
higher contributions in the Mayer-cluster expansion, systematic corrections to
mean-field can be included. The sol-gel transition shifts to a higher number of
crosslinks per particle, as more compact structures are favored. The critical
behavior of the model remains unchanged as long as finite truncations of the
cluster expansion are considered. To complete the picture, we also discuss
various geometrical properties of the crosslink network, e.g. connectivity
correlations, and relate the studied crosslink ensemble to a wider class of
ensembles, including the Deam-Edwards distribution.Comment: 18 pages, 4 figures, version to be published in EPJ
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