127 research outputs found
Monte Carlo studies of antiferromagnetic spin models in three dimensions
We study several antiferromagnetic formulations of the O(3) spin model in
three dimensions by means of Monte Carlo simulations. We discuss about the
vacua properties and analyze the phase transitions. Using Finite Size Scaling
analysis we conclude that all phase transitions found are of first orderComment: 4 pages, 2 Postscript figures. Contribution to Lattice '9
Finite-size scaling study of the d=4 site-diluted Ising
We study the four dimensional site-diluted Ising model using finite-size
scaling techniques. We explore the whole parameter space (density-coupling) in
order to determine the Universality Class of the transition line. Our data are
compatible with Mean Field behavior plus logarithmic corrections.Comment: Contribution to LATTICE 9
Finite Size Scaling and ``perfect'' actions: the three dimensional Ising model
Using Finite-Size Scaling techniques, we numerically show that the first
irrelevant operator of the lattice theory in three dimensions
is (within errors) completely decoupled at . This interesting
result also holds in the Thermodynamical Limit, where the renormalized coupling
constant shows an extraordinary reduction of the scaling-corrections when
compared with the Ising model. It is argued that Finite-Size Scaling analysis
can be a competitive method for finding improved actions.Comment: 13 pages, 3 figure
New Universality Class in three dimensions: the Antiferromagnetic model
We present the results of a Monte Carlo simulation of the model in
three dimensions with negative coupling. We observe a second order phase
transition between the disordered phase and an antiferromagnetic, unfrustrated,
ordered one. We measure, with a Finite Size Scaling analysis, the thermal
exponent, obtaining . We have found two magnetic-type relevant
operators whose related exponents are and
respectively.Comment: 10 pages, 2 Postscript figures. Revised version: references adde
An in-depth view of the microscopic dynamics of Ising spin glasses at fixed temperature
Using the dedicated computer Janus, we follow the nonequilibrium dynamics of
the Ising spin glass in three dimensions for eleven orders of magnitude. The
use of integral estimators for the coherence and correlation lengths allows us
to study dynamic heterogeneities and the presence of a replicon mode and to
obtain safe bounds on the Edwards-Anderson order parameter below the critical
temperature. We obtain good agreement with experimental determinations of the
temperature-dependent decay exponents for the thermoremanent magnetization.
This magnitude is observed to scale with the much harder to measure coherence
length, a potentially useful result for experimentalists. The exponents for
energy relaxation display a linear dependence on temperature and reasonable
extrapolations to the critical point. We conclude examining the time growth of
the coherence length, with a comparison of critical and activated dynamics.Comment: 38 pages, 26 figure
Ising exponents in the two-dimensional site-diluted Ising model
We study the site-diluted Ising model in two dimensions with Monte Carlo
simulations. Using finite-size scaling techniques we compute the critical
exponents observing deviations from the pure Ising ones. The differences can be
explained as the effects of logarithmic corrections, without requiring to
change the Universality Class.Comment: 7 pages, 2 postscript figures. Reference correcte
Dynamical generation of a gauge symmetry in the Double-Exchange model
It is shown that a bosonic formulation of the double-exchange model, one of the classical models for magnetism, generates dynamically a gauge-invariant phase in a finite region of the phase diagram. We use analytical methods, Monte Carlo simulations and Finite-Size Scaling analysis. We study the transition line between that region and the paramagnetic phase. The numerical results show that this transition line belongs to the Universality Class of the Antiferromagnetic RP(2) model. The fact that one can define a Universality Class for the Antiferromagnetic RP(2) model, different from the one of the O(N) models, is puzzling and somehow contradicts naive expectations about Universality
Matching microscopic and macroscopic responses in glasses
We first reproduce on the Janus and Janus II computers a milestone experiment
that measures the spin-glass coherence length through the lowering of
free-energy barriers induced by the Zeeman effect. Secondly we determine the
scaling behavior that allows a quantitative analysis of a new experiment
reported in the companion Letter [S. Guchhait and R. Orbach, Phys. Rev. Lett.
118, 157203 (2017)]. The value of the coherence length estimated through the
analysis of microscopic correlation functions turns out to be quantitatively
consistent with its measurement through macroscopic response functions.
Further, non-linear susceptibilities, recently measured in glass-forming
liquids, scale as powers of the same microscopic length.Comment: 6 pages, 4 figure
Finite size effects on measures of critical exponents in d=3 O(N) models
We study the critical properties of three-dimensional O(N) models, for
N=2,3,4. Parameterizing the leading corrections-to-scaling for the
exponent, we obtain a reliable infinite volume extrapolation, incompatible with
previous Monte Carlo values, but in agreement with -expansions. We
also measure the critical exponent related with the tensorial magnetization as
well as the exponents and critical couplings.Comment: 12 pages, 2 postscript figure
The Mpemba effect in spin glasses is a persistent memory effect
The Mpemba effect occurs when a hot system cools faster than an initially
colder one, when both are refrigerated in the same thermal reservoir. Using the
custom built supercomputer Janus II, we study the Mpemba effect in spin glasses
and show that it is a non-equilibrium process, governed by the coherence length
\xi of the system. The effect occurs when the bath temperature lies in the
glassy phase, but it is not necessary for the thermal protocol to cross the
critical temperature. In fact, the Mpemba effect follows from a strong
relationship between the internal energy and \xi that turns out to be a
sure-tell sign of being in the glassy phase. Thus, the Mpemba effect presents
itself as an intriguing new avenue for the experimental study of the coherence
length in supercooled liquids and other glass formers.Comment: Version accepted for publication in PNAS. 6 pages, 7 figure
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