297 research outputs found
Competing density-wave orders in a one-dimensional hard-boson model
We describe the zero-temperature phase diagram of a model of bosons,
occupying sites of a linear chain, which obey a hard-exclusion constraint: any
two nearest-neighbor sites may have at most one boson. A special case of our
model was recently proposed as a description of a ``tilted'' Mott insulator of
atoms trapped in an optical lattice. Our quantum Hamiltonian is shown to
generate the transfer matrix of Baxter's hard-square model. Aided by exact
solutions of a number of special cases, and by numerical studies, we obtain a
phase diagram containing states with long-range density-wave order with period
2 and period 3, and also a floating incommensurate phase. Critical theories for
the various quantum phase transitions are presented. As a byproduct, we show
how to compute the Luttinger parameter in integrable theories with
hard-exclusion constraints.Comment: 16 page
Order Parameters of the Dilute A Models
The free energy and local height probabilities of the dilute A models with
broken \Integer_2 symmetry are calculated analytically using inversion and
corner transfer matrix methods. These models possess four critical branches.
The first two branches provide new realisations of the unitary minimal series
and the other two branches give a direct product of this series with an Ising
model. We identify the integrable perturbations which move the dilute A models
away from the critical limit. Generalised order parameters are defined and
their critical exponents extracted. The associated conformal weights are found
to occur on the diagonal of the relevant Kac table. In an appropriate regime
the dilute A model lies in the universality class of the Ising model in a
magnetic field. In this case we obtain the magnetic exponent
directly, without the use of scaling relations.Comment: 53 pages, LaTex, ITFA 93-1
A descriptive study of the usage of spinal manipulative therapy techniques within a randomised clinical trial in acute low back pain
Vertex Models on Feynman Diagrams
The statistical mechanics of spin models, such as the Ising or Potts models,
on generic random graphs can be formulated economically by considering the N
--> 1 limit of Hermitian matrix models. In this paper we consider the N --> 1
limit in complex matrix models, which describes vertex models of different
sorts living on random graphs. From the graph theoretic perspective one is
using matrix model and field theory inspired methods to count various classes
of directed graphs.
We also make some remarks on vertex models on planar random graphs (the N -->
infinity limit) where the resulting matrix models are not generally soluble
using currently known methods. Nonetheless, some particular cases may be mapped
onto known models and hence solved.Comment: 10 Pages text (LaTeX), 4 eps figure
Microstructure modelling of hot deformation of Al–1%Mg alloy
This study presents the application of the finite elementmethod and intelligent systems techniques to the
prediction of microstructural mapping for aluminium alloys. Here, the material within each finite element
is defined using a hybrid model. The hybrid model is based on neuro-fuzzy and physically based components
and it has been combined with the finite element technique. The model simulates the evolution of
the internal state variables (i.e. dislocation density, subgrain size and subgrain boundary misorientation)
and their effect on the recrystallisation behaviour of the stock. This paper presents the theory behind
the model development, the integration between the numerical techniques, and the application of the
technique to a hot rolling operation using aluminium, 1 wt% magnesium alloy. Furthermore, experimental
data from plane strain compression (PSC) tests and rolling are used to validate the modelling outcome.
The results show that the recrystallisation kinetics agree well with the experimental results for different
annealing times. This hybrid approach has proved to be more accurate than conventional methods using empirical equations
Dgsos on DTRS
We perform simulations of a discrete gaussian solid on solid (DGSOS) model on
dynamical graphs, which is equivalent to coupling the model to 2d
quantum gravity, using the cluster algorithms recently developed by Evertz
et.al.for use on fixed lattices. We find evidence from the growth of the
width-squared in the rough phase of KT-like behaviour, which is consistent with
theoretical expectations. We also investigate the cluster statistics, dynamical
critical exponent and lattice properties, and compare these with the dual XY
model.Comment: 9 pages, COLO-HEP-32
Geometric effects on T-breaking in p+ip and d+id superconductors
Superconducting order parameters that change phase around the Fermi surface
modify Josephson tunneling behavior, as in the phase-sensitive measurements
that confirmed order in the cuprates. This paper studies Josephson coupling
when the individual grains break time-reversal symmetry; the specific cases
considered are and , which may appear in SrRuO and
NaCoO(HO) respectively. -breaking order parameters
lead to frustrating phases when not all grains have the same sign of
time-reversal symmetry breaking, and the effects of these frustrating phases
depend sensitively on geometry for 2D arrays of coupled grains. These systems
can show perfect superconducting order with or without macroscopic
-breaking. The honeycomb lattice of superconducting grains has a
superconducting phase with no spontaneous breaking of but instead power-law
correlations. The superconducting transition in this case is driven by binding
of fractional vortices, and the zero-temperature criticality realizes a
generalization of Baxter's three-color model.Comment: 8 page
Study of Percolative Transitions with First-Order Characteristics in the Context of CMR Manganites
The unusual magneto-transport properties of manganites are widely believed to
be caused by mixed-phase tendencies and concomitant percolative processes.
However, dramatic deviations from "standard" percolation have been unveiled
experimentally. Here, a semi-phenomenological description of Mn oxides is
proposed based on coexisting clusters with smooth surfaces, as suggested by
Monte Carlo simulations of realistic models for manganites, also briefly
discussed here. The present approach produces fairly abrupt percolative
transitions and even first-order discontinuities, in agreement with
experiments. These transitions may describe the percolation that occurs after
magnetic fields align the randomly oriented ferromagnetic clusters believed to
exist above the Curie temperature in Mn oxides. In this respect, part of the
manganite phenomenology could belong to a new class of percolative processes
triggered by phase competition and correlations.Comment: 4 pages, 4 eps figure
Ising Spins on Thin Graphs
The Ising model on ``thin'' graphs (standard Feynman diagrams) displays
several interesting properties. For ferromagnetic couplings there is a mean
field phase transition at the corresponding Bethe lattice transition point. For
antiferromagnetic couplings the replica trick gives some evidence for a spin
glass phase. In this paper we investigate both the ferromagnetic and
antiferromagnetic models with the aid of simulations. We confirm the Bethe
lattice values of the critical points for the ferromagnetic model on
and graphs and examine the putative spin glass phase in the
antiferromagnetic model by looking at the overlap between replicas in a
quenched ensemble of graphs. We also compare the Ising results with those for
higher state Potts models and Ising models on ``fat'' graphs, such as those
used in 2D gravity simulations.Comment: LaTeX 13 pages + 9 postscript figures, COLO-HEP-340,
LPTHE-Orsay-94-6
Transfer Matrices and Partition-Function Zeros for Antiferromagnetic Potts Models. V. Further Results for the Square-Lattice Chromatic Polynomial
We derive some new structural results for the transfer matrix of
square-lattice Potts models with free and cylindrical boundary conditions. In
particular, we obtain explicit closed-form expressions for the dominant (at
large |q|) diagonal entry in the transfer matrix, for arbitrary widths m, as
the solution of a special one-dimensional polymer model. We also obtain the
large-q expansion of the bulk and surface (resp. corner) free energies for the
zero-temperature antiferromagnet (= chromatic polynomial) through order q^{-47}
(resp. q^{-46}). Finally, we compute chromatic roots for strips of widths 9 <=
m <= 12 with free boundary conditions and locate roughly the limiting curves.Comment: 111 pages (LaTeX2e). Includes tex file, three sty files, and 19
Postscript figures. Also included are Mathematica files data_CYL.m and
data_FREE.m. Many changes from version 1: new material on series expansions
and their analysis, and several proofs of previously conjectured results.
Final version to be published in J. Stat. Phy
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