1,052 research outputs found
Microcanonical Approach to the Simulation of First-Order Phase Transitions
A generalization of the microcanonical ensemble suggests a simple strategy
for the simulation of first order phase transitions. At variance with
flat-histogram methods, there is no iterative parameters optimization, nor long
waits for tunneling between the ordered and the disordered phases. We test the
method in the standard benchmark: the Q-states Potts model (Q=10 in 2
dimensions and Q=4 in 3 dimensions), where we develop a cluster algorithm. We
obtain accurate results for systems with more than one million of spins,
outperforming flat-histogram methods that handle up to tens of thousands of
spins.Comment: 4 pages, 3 postscript figure
Unraveling Quantum Annealers using Classical Hardness
Recent advances in quantum technology have led to the development and
manufacturing of experimental programmable quantum annealing optimizers that
contain hundreds of quantum bits. These optimizers, named `D-Wave' chips,
promise to solve practical optimization problems potentially faster than
conventional `classical' computers. Attempts to quantify the quantum nature of
these chips have been met with both excitement and skepticism but have also
brought up numerous fundamental questions pertaining to the distinguishability
of quantum annealers from their classical thermal counterparts. Here, we
propose a general method aimed at answering these, and apply it to
experimentally study the D-Wave chip. Inspired by spin-glass theory, we
generate optimization problems with a wide spectrum of `classical hardness',
which we also define. By investigating the chip's response to classical
hardness, we surprisingly find that the chip's performance scales unfavorably
as compared to several analogous classical algorithms. We detect, quantify and
discuss purely classical effects that possibly mask the quantum behavior of the
chip.Comment: 12 pages, 9 figure
Mean-value identities as an opportunity for Monte Carlo error reduction
In the Monte Carlo simulation of both Lattice field-theories and of models of
Statistical Mechanics, identities verified by exact mean-values such as
Schwinger-Dyson equations, Guerra relations, Callen identities, etc., provide
well known and sensitive tests of thermalization bias as well as checks of
pseudo random number generators. We point out that they can be further
exploited as "control variates" to reduce statistical errors. The strategy is
general, very simple, and almost costless in CPU time. The method is
demonstrated in the two dimensional Ising model at criticality, where the CPU
gain factor lies between 2 and 4.Comment: 10 pages, 2 tables. References updated and typos correcte
Testing statics-dynamics equivalence at the spin-glass transition in three dimensions
The statics-dynamics correspondence in spin glasses relate non-equilibrium
results on large samples (the experimental realm) with equilibrium quantities
computed on small systems (the typical arena for theoretical computations).
Here we employ statics-dynamics equivalence to study the Ising spin-glass
critical behavior in three dimensions. By means of Monte Carlo simulation, we
follow the growth of the coherence length (the size of the glassy domains), on
lattices too large to be thermalized. Thanks to the large coherence lengths we
reach, we are able to obtain accurate results in excellent agreement with the
best available equilibrium computations. To do so, we need to clarify the
several physical meanings of the dynamic exponent close to the critical
temperature.Comment: Version to appear in Physical Review
Lattice-Spin Mechanism in Colossal Magnetoresistant Manganites
We present a single-orbital double-exchange model, coupled with cooperative
phonons (the so called breathing-modes of the oxygen octahedra in manganites).
The model is studied with Monte Carlo simulations. For a finite range of doping
and coupling constants, a first-order Metal-Insulator phase transition is
found, that coincides with the Paramagnetic-Ferromagnetic phase transition. The
insulating state is due to the self-trapping of every carrier within an oxygen
octahedron distortion.Comment: 4 pages, 5 figures, ReVTeX macro, accepted for publication in PR
Optimized Monte Carlo Method for glasses
A new Monte Carlo algorithm is introduced for the simulation of supercooled
liquids and glass formers, and tested in two model glasses. The algorithm is
shown to thermalize well below the Mode Coupling temperature and to outperform
other optimized Monte Carlo methods. Using the algorithm, we obtain finite size
effects in the specific heat. This effect points to the existence of a large
correlation length measurable in equal time correlation functions.Comment: Proceedings of "X International workshop on Disordered Systems" held
in Molveno (Italy), March 200
Finite size effects in the specific heat of glass-formers
We report clear finite size effects in the specific heat and in the
relaxation times of a model glass former at temperatures considerably smaller
than the Mode Coupling transition. A crucial ingredient to reach this result is
a new Monte Carlo algorithm which allows us to reduce the relaxation time by
two order of magnitudes. These effects signal the existence of a large
correlation length in static quantities.Comment: Proceeding of "3rd International Workshop on Complex Systems". Sendai
(Japan). To appear on AIP Conference serie
Rejuvenation and Memory in model Spin Glasses in 3 and 4 dimensions
We numerically study aging for the Edwards-Anderson Model in 3 and 4
dimensions using different temperature-change protocols. In D=3, time scales a
thousand times larger than in previous work are reached with the SUE machine.
Deviations from cumulative aging are observed in the non monotonic time
behavior of the coherence length. Memory and rejuvenation effects are found in
a temperature-cycle protocol, revealed by vanishing effective waiting times.
Similar effects are reported for the D=3$site-diluted ferromagnetic Ising model
(without chaos). However, rejuvenation is reduced if off-equilibrium
corrections to the fluctuation-dissipation theorem are considered. Memory and
rejuvenation are quantitatively describable in terms of the growth regime of
the spin-glass coherence length.Comment: Extended protocols. Accepted in Phys. Rev. B. 10 postscript figure
On the critical behavior of the specific heat in glass-formers
We show numeric evidence that, at low enough temperatures, the potential
energy density of a glass-forming liquid fluctuates over length scales much
larger than the interaction range. We focus on the behavior of translationally
invariant quantities. The growing correlation length is unveiled by studying
the Finite Size effects. In the thermodynamic limit, the specific heat and the
relaxation time diverge as a power law. Both features point towards the
existence of a critical point in the metastable supercooled liquid phase.Comment: Version to be published in Phys. Rev.
An Ising Model for Metal-Organic Frameworks
We present a three-dimensional Ising model where lines of equal spins are
frozen in such that they form an ordered framework structure. The frame spins
impose an external field on the rest of the spins (active spins). We
demonstrate that this "porous Ising model" can be seen as a minimal model for
condensation transitions of gas molecules in metal-organic frameworks. Using
Monte Carlo simulation techniques, we compare the phase behavior of a porous
Ising model with that of a particle-based model for the condensation of methane
(CH) in the isoreticular metal-organic framework IRMOF-16. For both models,
we find a line of first-order phase transitions that end in a critical point.
We show that the critical behavior in both cases belongs to the 3D Ising
universality class, in contrast to other phase transitions in confinement such
as capillary condensation.Comment: 11 pages, 9 figure
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