21 research outputs found
Unconventional critical activated scaling of two-dimensional quantum spin-glasses
We study the critical behavior of two-dimensional short-range quantum spin
glasses by numerical simulations. Using a parallel tempering algorithm, we
calculate the Binder cumulant for the Ising spin glass in a transverse magnetic
field with two different short-range bond distributions, the bimodal and the
Gaussian ones. Through an exhaustive finite-size scaling analysis, we show that
the universality class does not depend on the exact form of the bond
distribution but, most important, that the quantum critical behavior is
governed by an infinite randomness fixed point.Comment: 6 pages, 6 figure
Adsorption preference reversal phenomenon from multisite-occupancy theory fortwo-dimensional lattices
The statistical thermodynamics of polyatomic species mixtures adsorbed on
two-dimensional substrates was developed on a generalization in the spirit of
the lattice-gas model and the classical Guggenheim-DiMarzio approximation. In
this scheme, the coverage and temperature dependence of the Helmholtz free
energy and chemical potential are given. The formalism leads to the exact
statistical thermodynamics of binary mixtures adsorbed in one dimension,
provides a close approximation for two-dimensional systems accounting multisite
occupancy and allows to discuss the dimensionality and lattice structure
effects on the known phenomenon of adsorption preference reversal.Comment: 13 pages, 4 figure
Critical behavior of long straight rigid rods on two-dimensional lattices: Theory and Monte Carlo simulations
The critical behavior of long straight rigid rods of length (-mers) on
square and triangular lattices at intermediate density has been studied. A
nematic phase, characterized by a big domain of parallel -mers, was found.
This ordered phase is separated from the isotropic state by a continuous
transition occurring at a intermediate density . Two analytical
techniques were combined with Monte Carlo simulations to predict the dependence
of on , being . The first involves
simple geometrical arguments, while the second is based on entropy
considerations. Our analysis allowed us also to determine the minimum value of
(), which allows the formation of a nematic phase on a
triangular lattice.Comment: 23 pages, 5 figures, to appear in The Journal of Chemical Physic
Quasi-chemical approximation for polyatomic mixtures
The statistical thermodynamics of binary mixtures of polyatomic species was
developed on a generalization in the spirit of the lattice-gas model and the
quasi-chemical approximation (QCA). The new theoretical framework is obtained
by combining: (i) the exact analytical expression for the partition function of
non-interacting mixtures of linear -mers and -mers (species occupying
sites and sites, respectively) adsorbed in one dimension, and its extension
to higher dimensions; and (ii) a generalization of the classical QCA for
multicomponent adsorbates and multisite-occupancy adsorption. The process is
analyzed through the partial adsorption isotherms corresponding to both species
of the mixture. Comparisons with analytical data from Bragg-Williams
approximation (BWA) and Monte Carlo simulations are performed in order to test
the validity of the theoretical model. Even though a good fitting is obtained
from BWA, it is found that QCA provides a more accurate description of the
phenomenon of adsorption of interacting polyatomic mixtures.Comment: 27 pages, 8 figure
PyMembrane: A flexible framework for efficient simulations of elastic and liquid membranes
PyMembrane is a software package for simulating liquid and elastic membranes
using a discretisation of the continuum description based on unstructured
triangulated two-dimensional meshes embedded in three-dimensional space. The
package is written in C++, with a flexible and intuitive Python interface,
allowing for a quick setup, execution and analysis of complex simulations.
PyMembrane follows modern software engineering principles and features a
modular design that allows for straightforward implementation of custom
extensions while ensuring consistency and enabling inexpensive maintenance. A
hallmark feature of this design is the use of a standardized C++ interface
which streamlines adding new functionalities. Furthermore, PyMembrane uses data
structures optimised for unstructured meshes, ensuring efficient mesh
operations and force calculations. By providing several templates for typical
simulations supplemented by extensive documentation, the users can seamlessly
set up and run research-level simulations and extend the package to integrate
additional features, underscoring PyMembrane's commitment to user-centric
design.Comment: 7 Figure
Entropy-driven phase transition in a system of long rods on a square lattice
The isotropic-nematic (I-N) phase transition in a system of long straight
rigid rods of length k on square lattices is studied by combining Monte Carlo
simulations and theoretical analysis. The process is analyzed by comparing the
configurational entropy of the system with the corresponding to a fully aligned
system, whose calculation reduces to the 1D case. The results obtained (1)
allow to estimate the minimum value of k which leads to the formation of a
nematic phase and provide an interesting interpretation of this critical value;
(2) provide numerical evidence on the existence of a second phase transition
(from a nematic to a non-nematic state) occurring at density close to 1 and (3)
allow to test the predictions of the main theoretical models developed to treat
the polymers adsorption problem.Comment: 14 pages, 6 figures. Accepted for publication in JSTA
Determination of the Critical Exponents for the Isotropic-Nematic Phase Transition in a System of Long Rods on Two-dimensional Lattices: Universality of the Transition
Monte Carlo simulations and finite-size scaling analysis have been carried
out to study the critical behavior and universality for the isotropic-nematic
phase transition in a system of long straight rigid rods of length
(-mers) on two-dimensional lattices. The nematic phase, characterized by a
big domain of parallel -mers, is separated from the isotropic state by a
continuous transition occurring at a finite density. The determination of the
critical exponents, along with the behavior of Binder cumulants, indicate that
the transition belongs to the 2D Ising universality class for square lattices
and the three-state Potts universality class for triangular lattices.Comment: 7 pages, 8 figures, uses epl2.cls, to appear in Europhysics Letter
Cell division and death inhibit glassy behaviour of confluent tissues
We investigate the effects of cell division and apopotosis on collective
dynamics in two-dimensional epithelial tissues. Our model includes three key
ingredients observed across many epithelia, namely cell-cell adhesion, cell
death and a cell division process that depends on the surrounding environment.
We show a rich non-equilibrium phase diagram depending on the ratio of cell
death to cell division and on the adhesion strength. For large apopotosis
rates, cells die out and the tissue disintegrates. As the death rate decreases,
however, we show, consecutively, the existence of a gas-like phase, a gel-like
phase, and a dense confluent (tissue) phase. Most striking is the observation
that the tissue is self-melting through its own internal activity, ruling out
the existence of any glassy phase.Comment: 9 pages, 10 figure
Jamming and percolation in random sequential adsorption of straight rigid rods on a two-dimensional triangular lattice
Monte Carlo simulations and finite-size scaling analysis have been performed
to study the jamming and percolation behavior of linear -mers (also known as
rods or needles) on the two-dimensional triangular lattice, considering an
isotropic RSA process on a lattice of linear dimension and periodic
boundary conditions. Extensive numerical work has been done to extend previous
studies to larger system sizes and longer -mers, which enables the
confirmation of a nonmonotonic size dependence of the percolation threshold and
the estimation of a maximum value of from which percolation would no longer
occurs. Finally, a complete analysis of critical exponents and universality
have been done, showing that the percolation phase transition involved in the
system is not affected, having the same universality class of the ordinary
random percolation.Comment: 6 figure