4,835 research outputs found
A Proposal of a Renormalization Group Transformation for Lattice Field Theories
We propose a new Real Space Renormalization Group transformation useful for
Monte Carlo calculations in theories with global or local symmetries. From
relaxation arguments we define the block-spin transformation with two tunable
free parameters, adapted to the system's action. Varying them it is possible to
place the fixed point very close to the simulation point. We show how the
method works in a simple model with global symmetry: the three dimensional XY
model.Comment: 26 pages, uuencoded compressed postscript single file, 8 figures
include
Numerical test of the Cardy-Jacobsen conjecture in the site-diluted Potts model in three dimensions
We present a microcanonical Monte Carlo simulation of the site-diluted Potts
model in three dimensions with eight internal states, partly carried out in the
citizen supercomputer Ibercivis. Upon dilution, the pure model's first-order
transition becomes of the second-order at a tricritical point. We compute
accurately the critical exponents at the tricritical point. As expected from
the Cardy-Jacobsen conjecture, they are compatible with their Random Field
Ising Model counterpart. The conclusion is further reinforced by comparison
with older data for the Potts model with four states.Comment: Final version. 9 pages, 9 figure
The out-equilibrium 2D Ising spin glass: almost, but not quite, a free-field theory
We consider the spatial correlation function of the two-dimensional Ising
spin glass under out-equilibrium conditions. We pay special attention to the
scaling limit reached upon approaching zero temperature. The field-theory of a
non-interacting field makes a surprisingly good job at describing the spatial
shape of the correlation function of the out-equilibrium Edwards-Anderson Ising
model in two dimensions.Comment: 20 pages + 5 Figure
Effect of Dilution on First Order Transitions: The Three Dimensional Three States Potts Model
We have studied numerically the effect of quenched site dilution on a first
order phase transition in three dimensions. We have simulated the site diluted
three states Potts model studying in detail the second order region of its
phase diagram. We have found that the exponent is compatible with the one
of the three dimensional diluted Ising model whereas the exponent is
definitely different.Comment: RevTex. 6 pages and 6 postscript figure
Tempering Dynamics and Relaxation Times in the Ising Model
We discuss the tempering Monte Carlo method, and its critical slowing down in
the Ising model. We show that at the tempering does not change the
critical slowing down exponent . We also discuss the exponential slowing
down for the transition from the plus to the minus state in the cold phase, and
we show that tempering reduces it to a power law slowing down. We discuss the
relation of the flip-flop rate to the surface tension for the local dynamical
schemes.Comment: 10 pages, compressed ps-file (uufiles
Microcanonical finite-size scaling in specific heat diverging 2nd order phase transitions
A Microcanonical Finite Site Ansatz in terms of quantities measurable in a
Finite Lattice allows to extend phenomenological renormalization (the so called
quotients method) to the microcanonical ensemble. The Ansatz is tested
numerically in two models where the canonical specific-heat diverges at
criticality, thus implying Fisher-renormalization of the critical exponents:
the 3D ferromagnetic Ising model and the 2D four-states Potts model (where
large logarithmic corrections are known to occur in the canonical ensemble). A
recently proposed microcanonical cluster method allows to simulate systems as
large as L=1024 (Potts) or L=128 (Ising). The quotients method provides
extremely accurate determinations of the anomalous dimension and of the
(Fisher-renormalized) thermal exponent. While in the Ising model the
numerical agreement with our theoretical expectations is impressive, in the
Potts case we need to carefully incorporate logarithmic corrections to the
microcanonical Ansatz in order to rationalize our data.Comment: 13 pages, 8 figure
Comment on "Evidence of Non-Mean-Field-Like Low-Temperature Behavior in the Edwards-Anderson Spin-Glass Model"
A recent interesting paper [Yucesoy et al. Phys. Rev. Lett. 109, 177204
(2012), arXiv:1206:0783] compares the low-temperature phase of the 3D
Edwards-Anderson (EA) model to its mean-field counterpart, the
Sherrington-Kirkpatrick (SK) model. The authors study the overlap distributions
P_J(q) and conclude that the two models behave differently. Here we notice that
a similar analysis using state-of-the-art, larger data sets for the EA model
(generated with the Janus computer) leads to a very clear interpretation of the
results of Yucesoy et al., showing that the EA model behaves as predicted by
the replica symmetry breaking (RSB) theory.Comment: Version accepted for publication in PRL. 1 page, 1 figur
Study of star-forming galaxies in SDSS up to redshift 0.4: I. Metallicity evolution
The chemical composition of the gas in galaxies versus cosmic time provides a
very important tool for understanding galaxy evolution. Although there are many
studies at high redshift, they are rather scarce at lower redshifts. However,
low redshift studies can provide important clues about the evolution of
galaxies, furnishing the required link between local and high redshift
universe. In this work we focus on the metallicity of the gas of star-forming
galaxies at low redshift, looking for signs of chemical evolution.
To analyze the metallicity contents star-forming galaxies of similar
luminosities and masses at different redshifts. With this purpose, we present a
study of the metallicity of relatively massive (log(M_star/M_sun)>10.5) star
forming galaxies from SDSS--DR5 (Sloan Digital Sky Survey--Data Release 5),
using different redshift intervals from 0.04 to 0.4.
We used data processed with the STARLIGHT spectral synthesis code, correcting
the fluxes for dust extinction, estimating metallicities using the R_23 method,
and segregating the samples with respect to the value of the
[NII]6583/[OII]3727 line ratio in order to break the R_23 degeneracy selecting
the upper branch. We analyze the luminosity and mass-metallicity relations, and
the effect of the Sloan fiber diameter looking for possible biases.
By dividing our redshift samples in intervals of similar magnitude and
comparing them, significant signs of metallicity evolution are found.
Metallicity correlates inversely with redshift: from redshift 0 to 0.4 a
decrement of ~0.1 dex in 12+log(O/H) is found.Comment: 11 pages, 9 figures, Accepted for publication in A&
An experiment-oriented analysis of 2D spin-glass dynamics: a twelve time-decades scaling study
Recent high precision experimental results on spin-glass films ask for a
detailed understanding of the domain-growth dynamics of two-dimensional spin
glasses. To achieve this goal, we numerically simulate the out-equilibrium
dynamics of the Ising spin glass for a time that spans close to twelve orders
of magnitude (from picoseconds to order of a second), in systems large enough
to avoid finite-size effects. We find that the time-growth of the size of the
glassy domains is excellently described by a single scaling function. A single
time-scale controls the dynamics. diverges upon approaching
the critical point. The divergence of is Arrhenius-like,
with a barrier height that depends very mildly on temperature. The growth of
this barrier-height is best described by critical dynamics. As a side product
we obtain an impressive confirmation of universality of the equilibrium
behavior of two-dimensional spin-glasses.Comment: 21 pages, 9 figures. Updated references. Added DOI and Journal re
Small-angle neutron scattering and Molecular Dynamics structural study of gelling DNA nanostars
DNA oligomers with properly designed sequences self-assemble into well
defined constructs. Here, we exploit this methodology to produce bulk
quantities of tetravalent DNA nanostars (each one composed by 196 nucleotides)
and to explore the structural signatures of their aggregation process. We
report small-angle neutron scattering experiments focused on the evaluation of
both the form factor and the temperature evolution of the scattered intensity
at a nano star concentration where the system forms a tetravalent equilibrium
gel. We also perform molecular dynamics simulations of one isolated tetramer to
evaluate the form factor theoretically, without resorting to any approximate
shape. The numerical form factor is found to be in very good agreement with the
experimental one. Simulations predict an essentially temperature independent
form factor, offering the possibility to extract the effective structure factor
and its evolution during the equilibrium gelation.Comment: 9 pages, 5 figure
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