3,411 research outputs found
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
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
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.
Temperature chaos in 3D Ising Spin Glasses is driven by rare events
Temperature chaos has often been reported in literature as a rare-event
driven phenomenon. However, this fact has always been ignored in the data
analysis, thus erasing the signal of the chaotic behavior (still rare in the
sizes achieved) and leading to an overall picture of a weak and gradual
phenomenon. On the contrary, our analysis relies on a large-deviations
functional that allows to discuss the size dependencies. In addition, we had at
our disposal unprecedentedly large configurations equilibrated at low
temperatures, thanks to the Janus computer. According to our results, when
temperature chaos occurs its effects are strong and can be felt even at short
distances.Comment: 5 pages, 5 figure
Temperature chaos is a non-local effect
Temperature chaos plays a role in important effects, like for example memory
and rejuvenation, in spin glasses, colloids, polymers. We numerically
investigate temperature chaos in spin glasses, exploiting its recent
characterization as a rare-event driven phenomenon. The peculiarities of the
transformation from periodic to anti-periodic boundary conditions in spin
glasses allow us to conclude that temperature chaos is non-local: no bounded
region of the system causes it. We precise the statistical relationship between
temperature chaos and the free-energy changes upon varying boundary conditions.Comment: 15 pages, 8 figures. Version accepted for publication in JSTA
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Ontological realism, concepts and classification in molecular biology: Development and application of the gene ontology
Purpose – The purpose of this article is to evaluate the development and use of the gene ontology (GO), a scientific vocabulary widely used in molecular biology databases, with particular reference to the relation between the theoretical basis of the GO, and the pragmatics of its application.
Design/methodology/approach – The study uses a combination of bibliometric analysis, content analysis and discourse analysis. These analyses focus on details of the ways in which the terms of the ontology are amended and deleted, and in which they are applied by users.
Findings – Although the GO is explicitly based on an objective realist epistemology, a considerable extent of subjectivity and social factors are evident in its development and use. It is concluded that bio-ontologies could beneficially be extended to be pluralist, while remaining objective, taking a view of concepts closer to that of more traditional controlled vocabularies.
Originality/value – This is one of very few studies which evaluate the development of a formal ontology in relation to its conceptual foundations, and the first to consider the GO in this way
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