354 research outputs found
Evidence against a glass transition in the 10-state short range Potts glass
We present the results of Monte Carlo simulations of two different 10-state
Potts glasses with random nearest neighbor interactions on a simple cubic
lattice. In the first model the interactions come from a \pm J distribution and
in the second model from a Gaussian one, and in both cases the first two
moments of the distribution are chosen to be equal to J_0=-1 and Delta J=1. At
low temperatures the spin autocorrelation function for the \pm J model relaxes
in several steps whereas the one for the Gaussian model shows only one. In both
systems the relaxation time increases like an Arrhenius law. Unlike the
infinite range model, there are only very weak finite size effects and there is
no evidence that a dynamical or a static transition exists at a finite
temperature.Comment: 9 pages of Latex, 4 figure
Finite-size scaling at the dynamical transition of the mean-field 10-state Potts glass
We use Monte Carlo simulations to study the static and dynamical properties
of a Potts glass with infinite range Gaussian distributed exchange interactions
for a broad range of temperature and system size up to N=2560 spins. The
results are compatible with a critical divergence of the relaxation time tau at
the theoretically predicted dynamical transition temperature T_D, tau \propto
(T-T_D)^{-\Delta} with Delta \approx 2. For finite N a further power law at
T=T_D is found, tau(T=T_D) \propto N^{z^\star} with z^\star \approx 1.5 and for
T>T_D dynamical finite-size scaling seems to hold. The order parameter
distribution P(q) is qualitatively compatible with the scenario of a first
order glass transition as predicted from one-step replica symmetry breaking
schemes.Comment: 8 pages of Latex, 4 figure
The Glassy Potts Model
We introduce a Potts model with quenched, frustrated disorder, that enjoys of
a gauge symmetry that forbids spontaneous magnetization, and allows the glassy
phase to extend from down to T=0. We study numerical the 4 dimensional
model with states. We show the existence of a glassy phase, and we
characterize it by studying the probability distributions of an order
parameter, the binder cumulant and the divergence of the overlap
susceptibility. We show that the dynamical behavior of the system is
characterized by aging.Comment: 4 pages including 4 (color) ps figures (all on page 4
Critical Behavior of Three-Dimensional Disordered Potts Models with Many States
We study the 3D Disordered Potts Model with p=5 and p=6. Our numerical
simulations (that severely slow down for increasing p) detect a very clear spin
glass phase transition. We evaluate the critical exponents and the critical
value of the temperature, and we use known results at lower values to
discuss how they evolve for increasing p. We do not find any sign of the
presence of a transition to a ferromagnetic regime.Comment: 9 pages and 9 Postscript figures. Final version published in J. Stat.
Mec
Synthesis of Potential Anti-Cancer Agents. XVI. Nitrogen Mustards from 1-Aminophenazine and 8-Aminoquinoline
The synthesis of nitrogen mustard amides from 1-aminophenazine
and 8-aminoquinoline is described
Partially and Fully Frustrated Coupled Oscillators With Random Pinning Fields
We have studied two specific models of frustrated and disordered coupled
Kuramoto oscillators, all driven with the same natural frequency, in the
presence of random external pinning fields. Our models are structurally
similar, but differ in their degree of bond frustration and in their finite
size ground state properties (one has random ferro- and anti-ferromagnetic
interactions; the other has random chiral interactions). We have calculated the
equilibrium properties of both models in the thermodynamic limit using the
replica method, with emphasis on the role played by symmetries of the pinning
field distribution, leading to explicit predictions for observables,
transitions, and phase diagrams. For absent pinning fields our two models are
found to behave identically, but pinning fields (provided with appropriate
statistical properties) break this symmetry. Simulation data lend satisfactory
support to our theoretical predictions.Comment: 37 pages, 7 postscript figure
How the Replica-Symmetry-Breaking Transition Looks Like in Finite-Size Simulations
Finite-size effects in the mean-field Ising spin glass and the mean-field
three-state Potts glass are investigated by Monte Carlo simulations. In the
thermodynamic limit, each model is known to exhibit a continuous phase
transition into the ordered state with a full and a one-step replica-symmetry
breaking (RSB), respectively. In the Ising case, Binder parameter g calculated
for various finite sizes remains positive at any temperature and crosses at the
transition point, while in the Potts case g develops a negative dip without
showing a crossing in the g>0 region. By contrast, non-self averaging
parameters always remain positive and show a clear crossing at the transition
temperature in both cases. Our finding suggests that care should be taken in
interpreting the numerical data of the Binder parameter, particularly when the
system exhibits a one-step-like RSB.Comment: 7 pages, 8 figure
Numerical Study of a Field Theory for Directed Percolation
A numerical method is devised for study of stochastic partial differential
equations describing directed percolation, the contact process, and other
models with a continuous transition to an absorbing state. Owing to the
heightened sensitivity to fluctuationsattending multiplicative noise in the
vicinity of an absorbing state, a useful method requires discretization of the
field variable as well as of space and time. When applied to the field theory
for directed percolation in 1+1 dimensions, the method yields critical
exponents which compare well against accepted values.Comment: 18 pages, LaTeX, 6 figures available upon request LC-CM-94-00
Warm climates of the pastâa lesson for the future? This is one article from the Discussion Meeting Issue âWarm climates of the pastâa lesson for the future?â compiled and edited by Daniel J. Lunt, Harry Elderfield, Richard Pancost and Andy Ridgwell
This Discussion Meeting Issue of the Philosophical Transactions A had its genesis in a Discussion Meeting of the Royal Society which took place on 10â11 October 2011. The Discussion Meeting, entitled âWarm climates of the past: a lesson for the future?â, brought together 16 eminent international speakers from the field of palaeoclimate, and was attended by over 280 scientists and members of the public. Many of the speakers have contributed to the papers compiled in this Discussion Meeting Issue. The papers summarize the talks at the meeting, and present further or related work. This Discussion Meeting Issue asks to what extent information gleaned from the study of past climates can aid our understanding of future climate change. Climate change is currently an issue at the forefront of environmental science, and also has important sociological and political implications. Most future predictions are carried out by complex numerical models; however, these models cannot be rigorously tested for scenarios outside of the modern, without making use of past climate data. Furthermore, past climate data can inform our understanding of how the Earth system operates, and can provide important contextual information related to environmental change. All past time periods can be useful in this context; here, we focus on past climates that were warmer than the modern climate, as these are likely to be the most similar to the future. This introductory paper is not meant as a comprehensive overview of all work in this field. Instead, it gives an introduction to the important issues therein, using the papers in this Discussion Meeting Issue, and other works from all the Discussion Meeting speakers, as exemplars of the various ways in which past climates can inform projections of future climate. Furthermore, we present new work that uses a palaeo constraint to quantitatively inform projections of future equilibrium ice sheet change
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