829 research outputs found
Warm and Cold Denaturation in the Phase Diagram of a Protein Lattice Model
Studying the properties of the solvent around proteins, we propose a much
more sophisticated model of solvation than temperature-independent pairwise
interactions between monomers, as is used commonly in lattice representations.
We applied our model of solvation to a 16-monomer chain constrained on a
two-dimensional lattice. We compute a phase diagram function of the temperature
and a solvent parameter which is related to the pH of the solution. It exhibits
a native state in which the chain coalesces into a unique compact conformation
as well as a denatured state. Under certain solvation conditions, both warm and
cold denaturations occur between the native and the denatured states. A good
agreement is found with the data obtained from calorimetric experiments,
thereby validating the proposed model.Comment: 7 pages, 2 figure
The superheated Melting of Grain Boundary
Based on a model of the melting of Grain Boundary (GB), we discuss the
possibility of the existence of superheated GB state. A Molecular Dynamics
simulation presented here shows that the superheated GB state can realized in
the high symmetric tilt GB. Whether the sizes of liquid nuclei exceed a
critical size determined the superheating grain boundary melting or not. Our
results also indicate that the increase of melting point due to pressure is
smaller than the superheating due to nucleation mechanism.Comment: Accepted by PRB, 7 pages and 5 figure
Elasticity and metastability limit in supercooled liquids: a lattice model
We present Monte Carlo simulations on a lattice system that displays a first
order phase transition between a disordered phase (liquid) and an ordered phase
(crystal). The model is augmented by an interaction that simulates the effect
of elasticity in continuum models. The temperature range of stability of the
liquid phase is strongly increased in the presence of the elastic interaction.
We discuss the consequences of this result for the existence of a kinetic
spinodal in real systems.Comment: 8 pages, 5 figure
Kinetic glass behavior in a diffusive model
Three properties of the Edwards-Anderson model with mobile bonds are
investigated which are characteristic of kinetic glasses. First is two-time
relaxation in aged systems, where a significant difference is observed between
spin and bond autocorrelation functions. The spin subsystem does not show
two-time behavior, and the relaxation is stretched exponential. The bond
subsystem shows two-time behavior, with the first relaxation nearly exponential
and the second similar to the spin one. Second is the two-temperature behavior,
which can be tuned by bond dilution through the full range reported in the
literature. Third is the rigid-to-floppy transition, identified as a function
of bond dilution. Simple Glauber Monte Carlo evolution without extraneous
constraints reproduces the behavior of classical kinetic simulations, with the
bond (spin) degree of freedom corresponding to configurational (orientational)
disorder.Comment: 4 pages, 3 figures, minimal corrections, to appear in Phys. Rev. B
(RC
Dynamical heterogeneity in a glass forming ideal gas
We conduct a numerical study of the dynamical behavior of a system of
three-dimensional crosses, particles that consist of three mutually
perpendicular line segments rigidly joined at their midpoints. In an earlier
study [W. van Ketel et al., Phys. Rev. Lett. 94, 135703 (2005)] we showed that
this model has the structural properties of an ideal gas, yet the dynamical
properties of a strong glass former. In the present paper we report an
extensive study of the dynamical heterogeneities that appear in this system in
the regime where glassy behavior sets in. On the one hand, we find that the
propensity of a particle to diffuse is determined by the structure of its local
environment. The local density around mobile particles is significantly less
than the average density, but there is little clustering of mobile particles,
and the clusters observed tend to be small. On the other hand, dynamical
susceptibility results indicate that a large dynamical length scale develops
even at moderate densities. This suggests that propensity and other mobility
measures are an incomplete measure of dynamical length scales in this system.Comment: 11 pages, 7 figure
Glassy Mean-Field Dynamics of the Backgammon model
In this paper we present an exact study of the relaxation dynamics of the
backgammon model. This is a model of a gas of particles in a discrete space
which presents glassy phenomena as a result of {\it entropy barriers} in
configuration space. The model is simple enough to allow for a complete
analytical treatment of the dynamics in infinite dimensions. We first derive a
closed equation describing the evolution of the occupation number
probabilities, then we generalize the analysis to the study the autocorrelation
function. We also consider possible variants of the model which allow to study
the effect of energy barriers.Comment: 21 pages, revtex, 4 uuencoded figure
Solidity of viscous liquids. IV. Density fluctuations
This paper is the fourth in a series exploring the physical consequences of
the solidity of highly viscous liquids. It is argued that the two basic
characteristics of a flow event (a jump between two energy minima in
configuration space) are the local density change and the sum of all particle
displacements. Based on this it is proposed that density fluctuations are
described by a time-dependent Ginzburg-Landau equation with rates in k-space of
the form with where is the average
intermolecular distance. The inequality expresses a long-wavelength dominance
of the dynamics which implies that the Hamiltonian (free energy) may be taken
to be ultra local. As an illustration of the theory the case with the simplest
non-trivial Hamiltonian is solved to second order in the Gaussian
approximation, where it predicts an asymmetric frequency dependence of the
isothermal bulk modulus with Debye behavior at low frequencies and an
decay of the loss at high frequencies. Finally, a general
formalism for the description of viscous liquid dynamics, which supplements the
density dynamics by including stress fields, a potential energy field, and
molecular orientational fields, is proposed
Free Energy Landscape Of Simple Liquids Near The Glass Transition
Properties of the free energy landscape in phase space of a dense hard sphere
system characterized by a discretized free energy functional of the
Ramakrishnan-Yussouff form are investigated numerically. A considerable number
of glassy local minima of the free energy are located and the distribution of
an appropriately defined ``overlap'' between minima is calculated. The process
of transition from the basin of attraction of a minimum to that of another one
is studied using a new ``microcanonical'' Monte Carlo procedure, leading to a
determination of the effective height of free energy barriers that separate
different glassy minima. The general appearance of the free energy landscape
resembles that of a putting green: deep minima separated by a fairly flat
structure. The growth of the effective free-energy barriers with increasing
density is consistent with the Vogel-Fulcher law, and this growth is primarily
driven by an entropic mechanism.Comment: 10 pages, 6 postscript figures, uses iopart.cls and iopart10.clo
(included). Invited talk at the ICTP Trieste Conference on "Unifying Concepts
in Glass Physics", September 1999. To be published in J. Phys. Cond. Ma
Replica symmetry breaking in long-range glass models without quenched disorder
We discuss mean field theory of glasses without quenched disorder focusing on
the justification of the replica approach to thermodynamics. We emphasize the
assumptions implicit in this method and discuss how they can be verified. The
formalism is applied to the long range Ising model with orthogonal coupling
matrix. We find the one step replica-symmetry breaking solution and show that
it is stable in the intermediate temperature range that includes the glass
state but excludes very low temperatures. At very low temperatures this
solution becomes unstable and this approach fails.Comment: 6 pages, 2 figure
Properties of a continuous-random-network model for amorphous systems
We use a Monte Carlo bond-switching method to study systematically the
thermodynamic properties of a "continuous random network" model, the canonical
model for such amorphous systems as a-Si and a-SiO. Simulations show
first-order "melting" into an amorphous state, and clear evidence for a glass
transition in the supercooled liquid. The random-network model is also extended
to study heterogeneous structures, such as the interface between amorphous and
crystalline Si.Comment: Revtex file with 4 figure
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