137 research outputs found
Multicanonical analysis of the plaquette-only gonihedric Ising model and its dual
The three-dimensional purely plaquette gonihedric Ising model and its dual
are investigated to resolve inconsistencies in the literature for the values of
the inverse transition temperature of the very strong temperature-driven
first-order phase transition that is apparent in the system. Multicanonical
simulations of this model allow us to measure system configurations that are
suppressed by more than 60 orders of magnitude compared to probable states.
With the resulting high-precision data, we find excellent agreement with our
recently proposed nonstandard finite-size scaling laws for models with a
macroscopic degeneracy of the low-temperature phase by challenging the
prefactors numerically. We find an overall consistent inverse transition
temperature of 0.551334(8) from the simulations of the original model both with
periodic and fixed boundary conditions, and the dual model with periodic
boundary conditions. For the original model with periodic boundary conditions,
we obtain the first reliable estimate of the interface tension, 0.12037(18),
using the statistics of suppressed configurations
Macroscopic Degeneracy and order in the 3d plaquette Ising model
The purely plaquette 3d Ising Hamiltonian with the spins living at the
vertices of a cubic lattice displays several interesting features. The
symmetries of the model lead to a macroscopic degeneracy of the low-temperature
phase and prevent the definition of a standard magnetic order parameter.
Consideration of the strongly anisotropic limit of the model suggests that a
layered, "fuki-nuke" order still exists and we confirm this with multicanonical
simulations. The macroscopic degeneracy of the low-temperature phase also
changes the finite-size scaling corrections at the first-order transition in
the model and we see this must be taken into account when analysing our
measurements.Comment: arXiv admin note: text overlap with arXiv:1412.442
Exact solutions to plaquette Ising models with free and periodic boundaries
An anisotropic limit of the 3d plaquette Ising model, in which the plaquette
couplings in one direction were set to zero, was solved for free boundary
conditions by Suzuki (Phys. Rev. Lett. 28 (1972) 507), who later dubbed it the
fuki-nuke, or "no-ceiling", model. Defining new spin variables as the product
of nearest-neighbour spins transforms the Hamiltonian into that of a stack of
(standard) 2d Ising models and reveals the planar nature of the magnetic order,
which is also present in the fully isotropic 3d plaquette model. More recently,
the solution of the fuki-nuke model was discussed for periodic boundary
conditions, which require a different approach to defining the product spin
transformation, by Castelnovo et al. (Phys. Rev. B 81 (2010) 184303).
We clarify the exact relation between partition functions with free and
periodic boundary conditions expressed in terms of original and product spin
variables for the 2d plaquette and 3d fuki-nuke models, noting that the
differences are already present in the 1d Ising model. In addition, we solve
the 2d plaquette Ising model with helical boundary conditions. The various
exactly solved examples illustrate how correlations can be induced in finite
systems as a consequence of the choice of boundary conditions.Comment: v5 - The title is changed to better reflect the contents and the
exposition is streamlined. Version accepted for publicatio
Lattice Gerbe Theory
-form abelian and non-abelian gauge fields on -dimensional hypercubic
lattices have been discussed in the past by various authors and most recently
by Lipstein and Reid-Edwards. In this note we recall that the Hamiltonian of a
variant of such theories is one of the family of generalized
Ising models originally considered by Wegner. For such " lattice
gerbe theories" general arguments can be used to show that a phase transition
for Wilson surfaces will occur for between volume and area scaling
behaviour. In the model is equivalent under duality to an infinite
coupling model and no transition is seen, whereas in the model is dual to
the Ising model and displays a continuous transition. In the
lattice gerbe theory is self-dual in the presence of an external
field and in it is self-dual in zero external field.Comment: v2: references added, abstract tweaked, (at least some of the)
timeline clarifie
The Wrong Kind of Gravity
The KPZ formula shows that coupling central charge less than one spin models
to 2D quantum gravity dresses the conformal weights to get new critical
exponents, where the relation between the original and dressed weights depends
only on the central charge. At the discrete level the coupling to 2D gravity is
effected by putting the spin models on annealed ensembles of planar random
graphs or their dual triangulations, where the connectivity fluctuates on the
same time-scale as the spins.
Since the sole determining factor in the dressing is the central charge, one
could contemplate putting a spin model on a quenched ensemble of 2D gravity
graphs with the ``wrong'' central charge. We might then expect to see the
critical exponents appropriate to the central charge used in generating the
graphs. In such cases the KPZ formula could be interpreted as giving a
continuous line of critical exponents which depend on this central charge. We
note that rational exponents other than the KPZ values can be generated using
this procedure for the Ising, tricritical Ising and 3-state Potts models.Comment: 8 pages, no figure
On Random Allocation Models in the Thermodynamic Limit
We discuss the phase transition and critical exponents in the random
allocation model (urn model) for different statistical ensembles. We provide a
unified presentation of the statistical properties of the model in the
thermodynamic limit, uncover new relationships between the thermodynamic
potentials and fill some lacunae in previous results on the singularities of
these potentials at the critical point and behaviour in the thermodynamic
limit.
The presentation is intended to be self-contained, so we carefully derive all
formulae step by step throughout. Additionally, we comment on a
quasi-probabilistic normalisation of configuration weights which has been
considered in some recent studie
Finite-size scaling and latent heat at the gonihedric first-order phase transition
A well-known feature of first-order phase transitions is that fixed boundary
conditions can strongly influence finite-size corrections, modifying the leading corrections for an L3 lattice in 3d from order 1/L3 under periodic boundary conditions to 1/L. A rather similar effect, albeit of completely different origin, occurs when the system possesses an exponential low-temperature phase degeneracy of the form 23L which causes for periodic boundary conditions a leading correction of order 1/L2
in 3d. We discuss a 3d plaquette Hamiltonian (“gonihedric”) Ising model, which displays such a degeneracy and manifests the modified scaling behaviour. We also investigate an apparent discrepancy between the fixed and periodic boundary condition
latent heats for the model when extrapolating to the thermodynamic limit
Softening of First-Order Phase Transition on Quenched Random Gravity Graphs
We perform extensive Monte Carlo simulations of the 10-state Potts model on
quenched two-dimensional gravity graphs to study the effect of
quenched coordination number randomness on the nature of the phase transition,
which is strongly first order on regular lattices. The numerical data provides
strong evidence that, due to the quenched randomness, the discontinuous
first-order phase transition of the pure model is softened to a continuous
transition, representing presumably a new universality class. This result is in
striking contrast to a recent Monte Carlo study of the 8-state Potts model on
two-dimensional Poissonian random lattices of Voronoi/Delaunay type, where the
phase transition clearly stayed of first order, but is in qualitative agreement
with results for quenched bond randomness on regular lattices. A precedent for
such softening with connectivity disorder is known: in the 10-state Potts model
on annealed Phi3 gravity graphs a continuous transition is also observed.Comment: Latex + 5 postscript figures, 10 pages of text, figures appende
Effect of rosiglitazone on progression of atherosclerosis: insights using 3D carotid cardiovascular magnetic resonance
<p>Abstract</p> <p>Background</p> <p>There is recent evidence suggesting that rosiglitazone increases death from cardiovascular causes. We investigated the direct effect of this drug on atheroma using 3D carotid cardiovascular magnetic resonance.</p> <p>Results</p> <p>A randomized, placebo-controlled, double-blind study was performed to evaluate the effect of rosiglitazone treatment on carotid atherosclerosis in subjects with type 2 diabetes and coexisting vascular disease or hypertension. The primary endpoint of the study was the change from baseline to 52 weeks of carotid arterial wall volume, reflecting plaque burden, as measured by carotid cardiovascular magnetic resonance. Rosiglitazone or placebo was allocated to 28 and 29 patients respectively. Patients were managed to have equivalent glycemic control over the study period, but in fact the rosiglitazone group lowered their HbA1c by 0.88% relative to placebo (P < 0.001). Most patients received a statin or fibrate as lipid control medication (rosiglitazone 78%, controls 83%). Data are presented as mean ± SD. At baseline, the carotid arterial wall volume in the placebo group was 1146 ± 550 mm<sup>3 </sup>and in the rosiglitazone group was 1354 ± 532 mm<sup>3</sup>. After 52 weeks, the respective volumes were 1134 ± 523 mm<sup>3 </sup>and 1348 ± 531 mm<sup>3</sup>. These changes (-12.1 mm<sup>3 </sup>and -5.7 mm<sup>3 </sup>in the placebo and rosiglitazone groups, respectively) were not statistically significant between groups (P = 0.57).</p> <p>Conclusion</p> <p>Treatment with rosiglitazone over 1 year had no effect on progression of carotid atheroma in patients with type 2 diabetes mellitus compared to placebo.</p
Ising and Potts Models on Quenched Random Gravity Graphs
We report on single-cluster Monte Carlo simulations of the Ising, 4-state
Potts and 10-state Potts models on quenched ensembles of planar, tri-valent
random graphs. We confirm that the first-order phase transition of the 10-state
Potts model on regular 2D lattices is softened by the quenched connectivity
disorder represented by the random graphs and that the exponents of the Ising
and 4-state Potts models are altered from their regular lattice counterparts.
The behaviour of spin models on such graphs is thus more analogous to models
with quenched bond disorder than to Poisonnian random lattices, where regular
lattice critical behaviour persists.
Using a wide variety of estimators we measure the critical exponents for all
three models, and compare the exponents with predictions derived from taking a
quenched limit in the KPZ formula for the Ising and 4-state Potts models.
Earlier simulations suggested that the measured values for the 10-state Potts
model were very close to the predicted quenched exponents of the {\it
four}-state Potts models. The analysis here, which employs a much greater range
of estimators and also benefits from greatly improved statistics, still
supports these numerical values.Comment: 14 pages (latex) + 6 latex tables + 5 figure
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