201 research outputs found
Intra-molecular coupling as a mechanism for a liquid-liquid phase transition
We study a model for water with a tunable intra-molecular interaction
, using mean field theory and off-lattice Monte Carlo simulations.
For all , the model displays a temperature of maximum
density.For a finite intra-molecular interaction ,our
calculations support the presence of a liquid-liquid phase transition with a
possible liquid-liquid critical point for water, likely pre-empted by
inevitable freezing. For J=0 the liquid-liquid critical point disappears at
T=0.Comment: 8 pages, 4 figure
Potential energy landscape-based extended van der Waals equation
The inherent structures ({\it IS}) are the local minima of the potential
energy surface or landscape, , of an {\it N} atom system.
Stillinger has given an exact {\it IS} formulation of thermodynamics. Here the
implications for the equation of state are investigated. It is shown that the
van der Waals ({\it vdW}) equation, with density-dependent and
coefficients, holds on the high-temperature plateau of the averaged {\it IS}
energy. However, an additional ``landscape'' contribution to the pressure is
found at lower . The resulting extended {\it vdW} equation, unlike the
original, is capable of yielding a water-like density anomaly, flat isotherms
in the coexistence region {\it vs} {\it vdW} loops, and several other desirable
features. The plateau energy, the width of the distribution of {\it IS}, and
the ``top of the landscape'' temperature are simulated over a broad reduced
density range, , in the Lennard-Jones fluid. Fits to the
data yield an explicit equation of state, which is argued to be useful at high
density; it nevertheless reproduces the known values of and at the
critical point
Landscape equivalent of the shoving model
It is shown that the shoving model expression for the average relaxation time
of viscous liquids follows largely from a classical "landscape" estimation of
barrier heights from curvature at energy minima. The activation energy involves
both instantaneous bulk and shear moduli, but the bulk modulus contributes less
than 8% to the temperature dependence of the activation energy. This reflects
the fact that the physics of the two models are closely related.Comment: 4 page
Entropy Crisis, Ideal Glass Transition and Polymer Melting: Exact Solution on a Husimi Cactus
We introduce an extension of the lattice model of melting of semiflexible
polymers originally proposed by Flory. Along with a bending penalty, present in
the original model and involving three sites of the lattice, we introduce an
interaction energy that corresponds to the presence of a pair of parallel bonds
and a second interaction energy associated with the presence of a hairpin turn.
Both these new terms represent four-site interactions. The model is solved
exactly on a Husimi cactus, which approximates a square lattice. We study the
phase diagram of the system as a function of the energies. For a proper choice
of the interaction energies, the model exhibits a first-order melting
transition between a liquid and a crystalline phase. The continuation of the
liquid phase below this temperature gives rise to a supercooled liquid, which
turns continuously into a new low-temperature phase, called metastable liquid.
This liquid-liquid transition seems to have some features that are
characteristic of the critical transition predicted by the mode-coupling
theory.Comment: To be published in Physical Review E, 68 (2) (2003
The Potential Energy Landscape and Mechanisms of Diffusion in Liquids
The mechanism of diffusion in supercooled liquids is investigated from the
potential energy landscape point of view, with emphasis on the crossover from
high- to low-T dynamics. Molecular dynamics simulations with a time dependent
mapping to the associated local mininum or inherent structure (IS) are
performed on unit-density Lennard-Jones (LJ). New dynamical quantities
introduced include r2_{is}(t), the mean-square displacement (MSD) within a
basin of attraction of an IS, R2(t), the MSD of the IS itself, and g_{loc}(t)
the mean waiting time in a cooperative region. At intermediate T, r2_{is}(t)
posesses an interval of linear t-dependence allowing calculation of an
intrabasin diffusion constant D_{is}. Near T_{c} diffusion is intrabasin
dominated with D = D_{is}. Below T_{c} the local waiting time tau_{loc} exceeds
the time, tau_{pl}, needed for the system to explore the basin, indicating the
action of barriers. The distinction between motion among the IS below T_{c} and
saddle, or border dynamics above T_{c} is discussed.Comment: submitted to pr
Non-Arrhenius Behavior of Secondary Relaxation in Supercooled Liquids
Dielectric relaxation spectroscopy (1 Hz - 20 GHz) has been performed on
supercooled glass-formers from the temperature of glass transition (T_g) up to
that of melting. Precise measurements particularly in the frequencies of
MHz-order have revealed that the temperature dependences of secondary
beta-relaxation times deviate from the Arrhenius relation in well above T_g.
Consequently, our results indicate that the beta-process merges into the
primary alpha-mode around the melting temperature, and not at the dynamical
transition point T which is approximately equal to 1.2 T_g.Comment: 4 pages, 4 figures, revtex
The Potts Fully Frustrated model: Thermodynamics, percolation and dynamics in 2 dimensions
We consider a Potts model diluted by fully frustrated Ising spins. The model
corresponds to a fully frustrated Potts model with variables having an integer
absolute value and a sign. This model presents precursor phenomena of a glass
transition in the high-temperature region. We show that the onset of these
phenomena can be related to a thermodynamic transition. Furthermore this
transition can be mapped onto a percolation transition. We numerically study
the phase diagram in 2 dimensions (2D) for this model with frustration and {\em
without} disorder and we compare it to the phase diagram of the model with
frustration {\em and} disorder and of the ferromagnetic model.
Introducing a parameter that connects the three models, we generalize the exact
expression of the ferromagnetic Potts transition temperature in 2D to the other
cases. Finally, we estimate the dynamic critical exponents related to the Potts
order parameter and to the energy.Comment: 10 pages, 10 figures, new result
Local influence of boundary conditions on a confined supercooled colloidal liquid
We study confined colloidal suspensions as a model system which approximates
the behavior of confined small molecule glass-formers. Dense colloidal
suspensions become glassier when confined between parallel glass plates. We use
confocal microscopy to study the motion of confined colloidal particles. In
particular, we examine the influence particles stuck to the glass plates have
on nearby free particles. Confinement appears to be the primary influence
slowing free particle motion, and proximity to stuck particles causes a
secondary reduction in the mobility of free particles. Overall, particle
mobility is fairly constant across the width of the sample chamber, but a
strong asymmetry in boundary conditions results in a slight gradient of
particle mobility.Comment: For conference proceedings, "Dynamics in Confinement", Grenoble,
March 201
Glass transition in the quenched and annealed version of the frustrated lattice gas model
In this paper we study the 3d frustrated lattice gas model in the annealed
version, where the disorder is allowed to evolve in time with a suitable
kinetic constraint. Although the model does not exhibit any thermodynamic
transition it shows a diverging peak at some characteristic time in the
dynamical non-linear susceptibility, similar to the results on the p-spin model
in mean field and Lennard-Jones mixture recently found by Donati et al.
[cond-mat/9905433]. Comparing these results to those obtained in the model with
quenched interactions, we conclude that the critical behavior of the dynamical
susceptibility is reminiscent of the thermodynamic transition present in the
quenched model, and signaled by the divergence of the static non-linear
susceptibility, suggesting therefore a similar mechanism also in supercooled
glass-forming liquids.Comment: 8 pages, 14 figure
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