128 research outputs found
The dynamical structure factor in disordered systems
We study the spectral width as a function of the external momentum for the
dynamical structure factor of a disordered harmonic solid, considered as a toy
model for supercooled liquids and glasses. Both in the context of single-link
coherent potential approximation and of a single-defect approximation, two
different regimes are clearly identified: if the density of states at zero
energy is zero, the Rayleigh law is recovered for small momentum. On the
contrary, if the disorder induces a non vanishing density of states at zero
energy, a linear behaviour is obtained. The dynamical structure factor is
numerically calculated in lattices as large as , and satisfactorily
agrees with the analytical computations.Comment: 7 pages plus 4 postscript figure
Critical exponents and unusual properties of the broken phase in the 3d-RP(2) antiferromagnetic model
We present the results of a Monte Carlo simulation of the antiferromagnetic
RP(2) model in three dimensions. With finite-size scaling techniques we
accurately measure the critical exponents and compare them with those of O(N)
models. We are able to parameterize the corrections-to-scaling. The symmetry
properties of the broken phase are also studied.Comment: 4 pages, TeX type, Poster session contribution to "Lattice96"
conference, Washington University, StLoui
A lattice field theoretical model for high- superconductivity
We present a 2+1-dimensional lattice model for the copper oxide
superconductors and their parent compounds, in which both the charge and spin
degrees of freedom are treated dynamically. The spin-charge coupling parameter
is associated to the doping fraction in the cuprates. The model is able to
account for the various phases of the cuprates and their properties, not only
at low and intermediate doping but also for (highly) over-doped compounds. We
acquire a qualitative understanding of high- superconductivity as a
Bose-Einstein condensation of bound charge pairs.Comment: talk presented in the Lattice 97 conferenc
Phase Transitions in Disordered Systems: The Example of the Random-Field Ising Model in Four Dimensions
By performing a high-statistics simulation of the D=4 random-field Ising model at zero temperature for different shapes of the random-field distribution, we show that the model is ruled by a single universality class. We compute to a high accuracy the complete set of critical exponents for this class, including the correction-to-scaling exponent. Our results indicate that in four dimensions (i) dimensional reduction as predicted by the perturbative renormalization group does not hold and (ii) three independent critical exponents are needed to describe the transition
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
Equilibrium fluid-solid coexistence of hard spheres
We present a tethered Monte Carlo simulation of the crystallization of hard
spheres. Our method boosts the traditional umbrella sampling to the point of
making practical the study of constrained Gibb's free energies depending on
several crystalline order-parameters. We obtain high-accuracy estimates of the
fluid-crystal coexistence pressure for up to 2916 particles (enough to
accommodate fluid-solid interfaces). We are able to extrapolate to infinite
volume the coexistence pressure (p_{co}=11.5727(10) k_B T/\sigma^3) and the
interfacial free energy (\gamma_{100}=0.636(11) k_B T/\sigma^2).Comment: 6 pages, 4 pdf figures. Version to be published in PRL. Appendices
contain Supplemental Materia
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
Phase diagram of a polydisperse soft-spheres model for liquids and colloids
The phase diagram of soft spheres with size dispersion has been studied by
means of an optimized Monte Carlo algorithm which allows to equilibrate below
the kinetic glass transition for all sizes distribution. The system
ubiquitously undergoes a first order freezing transition. While for small size
dispersion the frozen phase has a crystalline structure, large density
inhomogeneities appear in the highly disperse systems. Studying the interplay
between the equilibrium phase diagram and the kinetic glass transition, we
argue that the experimentally found terminal polydispersity of colloids is a
purely kinetic phenomenon.Comment: Version to be published in Physical Review Letter
Separation and fractionation of order and disorder in highly polydisperse systems
Microcanonical Monte Carlo simulations of a polydisperse soft-spheres model
for liquids and colloids have been performed for very large polydispersity, in
the region where a phase-separation is known to occur when the system (or part
of it) solidifies. By studying samples of different sizes, from N=256 to N=864,
we focus on the nature of the two distinct coexisting phases. Measurements of
crystalline order in particles of different size reveal that the solid phase
segregates between a crystalline solid with cubic symmetry and a disordered
phase. This phenomenon is termed fractionation.Comment: 8 pages, 5 figure
Anderson Localization in Euclidean Random Matrices
We study spectra and localization properties of Euclidean random matrices.
The problem is approximately mapped onto that of a matrix defined on a random
graph. We introduce a powerful method to find the density of states and the
localization threshold. We solve numerically an exact equation for the
probability distribution function of the diagonal element of the the resolvent
matrix, with a population dynamics algorithm, and we show how this can be used
to find the localization threshold. An application of the method in the context
of the Instantaneous Normal Modes of a liquid system is given.Comment: 4 page
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