2,289 research outputs found
Saddles in the energy landscape: extensivity and thermodynamic formalism
We formally extend the energy landscape approach for the thermodynamics of
liquids to account for saddle points. By considering the extensive nature of
macroscopic potential energies, we derive the scaling behavior of saddles with
system size, as well as several approximations for the properties of low-order
saddles (i.e., those with only a few unstable directions). We then cast the
canonical partition function in a saddle-explicit form and develop, for the
first time, a rigorous energy landscape approach capable of reproducing trends
observed in simulations, in particular the temperature dependence of the energy
and fractional order of sampled saddles.Comment: 4 pages, 1 figur
Variation of the glass transition temperature with rigidity and chemical composition
The effects of flexibility and chemical composition in the variation of the
glass transition temperature are obtained by using the Lindemann criteria, that
relates melting temperature with atomic vibrations. Using this criteria and
that floppy modes at low frequencies enhance in a considerable way the average
cuadratic displacement, we show that the consequence is a modified glass
transition temperature. This approach allows to obtain in a simple way the
empirically modified Gibbs-DiMarzio law, which has been widely used in
chalcogenide glasses to fit the changes in the glass transition temperature
with the chemical composition . The method predicts that the constant that
appears in the law depends upon the ratio of two characteristic frequencies (or
temperatures). Then, the constant for the Se-Ge-As glass is estimated by using
the experimental density of vibrational states, and the result shows a very
good agreement with the experimental fit from glass transition temperature
variation
Scaling properties of critical bubble of homogeneous nucleation in stretched fluid of square-gradient density-functional model with triple-parabolic free energy
The square-gradient density-functional model with triple-parabolic free
energy is used to study homogeneous bubble nucleation in a stretched liquid to
check the scaling rule for the work of formation of the critical bubble as a
function of scaled undersaturation , the
difference in chemical potential between the bulk undersaturated
and saturated liquid divided by between the liquid
spinodal and saturated liquid. In contrast to our study, a similar
density-functional study for a Lennard-Jones liquid by Shen and Debenedetti [J.
Chem. Phys. {\bf 114}, 4149 (2001)] found that not only the work of formation
but other various quantities related to the critical bubble show the scaling
rule, however, we found virtually no scaling relationships in our model near
the coexistence. Although some quantities show almost perfect scaling relations
near the spinodal, the work of formation divided by the value deduced from the
classical nucleation theory shows no scaling in this model even though it
correctly vanishes at the spinodal. Furthermore, the critical bubble does not
show any anomaly near the spinodal as predicted many years ago. In particular,
our model does not show diverging interfacial width at the spinodal, which is
due to the fact that compressibility remains finite until the spinodal is
reached in our parabolic models.Comment: 10 pages, 10 figures, Journal of Chemical Physics accepted for
publicatio
Cooperative Origin of Low-Density Domains in Liquid Water
We study the size of clusters formed by water molecules possessing large
enough tetrahedrality with respect to their nearest neighbors. Using Monte
Carlo simulation of the SPC/E model of water, together with a geometric
analysis based on Voronoi tessellation, we find that regions of lower density
than the bulk are formed by accretion of molecules into clusters exceeding a
minimum size. Clusters are predominantly linear objects and become less compact
as they grow until they reach a size beyond which further accretion is not
accompanied by a density decrease. The results suggest that the formation of
"ice-like" regions in liquid water is cooperative.Comment: 16 pages, 6 figure
Energy landscape and rigidity
The effects of floppy modes in the thermodynamical properties of a system are
studied. From thermodynamical arguments, we deduce that floppy modes are not at
zero frequency and thus a modified Debye model is used to take into account
this effect. The model predicts a deviation from the Debye law at low
temperatures. Then, the connection between the topography of the energy
landscape, the topology of the phase space and the rigidity of a glass is
explored. As a result, we relate the number of constraints and floppy modes
with the statistics of the landscape. We apply these ideas to a simple model
for which we provide an approximate expression for the number of energy basins
as a function of the rigidity. This allows to understand certains features of
the glass transition, like the jump in the specific heat or the reversible
window observed in chalcogenide glasses.Comment: 1 text+3 eps figure
Potential Energy Landscape Equation of State
Depth, number, and shape of the basins of the potential energy landscape are
the key ingredients of the inherent structure thermodynamic formalism
introduced by Stillinger and Weber [F. H. Stillinger and T. A. Weber, Phys.
Rev. A 25, 978 (1982)]. Within this formalism, an equation of state based only
on the volume dependence of these landscape properties is derived. Vibrational
and configurational contributions to pressure are sorted out in a transparent
way. Predictions are successfully compared with data from extensive molecular
dynamics simulations of a simple model for the fragile liquid orthoterphenyl.Comment: RevTeX4, 4 pages, 5 figure
A closer look at arrested spinodal decomposition in protein solutions
Concentrated aqueous solutions of the protein lysozyme undergo a liquid solid
transition upon a temperature quench into the unstable spinodal region below a
characteristic arrest temperature of Tf=15C. We use video microscopy and
ultra-small angle light scattering in order to investigate the arrested
structures as a function of initial concentration, quench temperature and rate
of the temperature quench. We find that the solid-like samples show all the
features of a bicontinuous network that is formed through an arrested spinodal
decomposition process. We determine the correlation length Xi and demonstrate
that Xi exhibits a temperature dependence that closely follows the critical
scaling expected for density fluctuations during the early stages of spinodal
decomposition. These findings are in agreement with an arrest scenario based on
a state diagram where the arrest or gel line extends far into the unstable
region below the spinodal line. Arrest then occurs when during the early stage
of spinodal decomposition the volume fraction phi2 of the dense phase
intersects the dynamical arrest threshold phi2Glass, upon which phase
separation gets pinned into a space-spanning gel network with a characteristic
length Xi
A test of non-equilibrium thermodynamics in glassy systems: the soft-sphere case
The scaling properties of the soft-sphere potential allow the derivation of
an exact expression for the pressure of a frozen liquid, i.e., the pressure
corresponding to configurations which are local minima in its multidimensional
potential energy landscape. The existence of such a relation offers the unique
possibility for testing the recently proposed extension of the liquid free
energy to glassy out-of-equilibrium conditions and the associated expression
for the temperature of the configurational degrees of freedom. We demonstrate
that the non-equilibrium free energy provides an exact description of the
soft-sphere pressure in glass states
Crystallization Mechanism of Hard Sphere Glasses
In supercooled liquids, vitrification generally suppresses crystallization.
Yet some glasses can still crystallize despite the arrest of diffusive motion.
This ill-understood process may limit the stability of glasses, but its
microscopic mechanism is not yet known. Here we present extensive computer
simulations addressing the crystallization of monodisperse hard-sphere glasses
at constant volume (as in a colloid experiment). Multiple crystalline patches
appear without particles having to diffuse more than one diameter. As these
patches grow, the mobility in neighbouring areas is enhanced, creating dynamic
heterogeneity with positive feedback. The future crystallization pattern cannot
be predicted from the coordinates alone: crystallization proceeds by a sequence
of stochastic micro-nucleation events, correlated in space by emergent dynamic
heterogeneity.Comment: 4 pages 4 figures Accepted for publication in Phys. Rev. Lett., April
201
Thermodynamic behaviour and structural properties of an aqueous sodium chloride solution upon supercooling
We present the results of a molecular dynamics simulation study of
thermodynamic and structural properties upon supercooling of a low
concentration sodium chloride solution in TIP4P water and the comparison with
the corresponding bulk quantities. We study the isotherms and the isochores for
both the aqueous solution and bulk water. The comparison of the phase diagrams
shows that thermodynamic properties of the solution are not merely shifted with
respect to the bulk. Moreover, from the analysis of the thermodynamic curves,
both the spinodal line and the temperatures of maximum density curve can be
calculated. The spinodal line appears not to be influenced by the presence of
ions at the chosen concentration, while the temperatures of maximum density
curve displays both a mild shift in temperature and a shape modification with
respect to bulk. Signatures of the presence of a liquid-liquid critical point
are found in the aqueous solution. By analysing the water-ion radial
distribution functions of the aqueous solution we observe that upon changing
density, structural modifications appear close to the spinodal. For low
temperatures additional modifications appear also for densities close to that
corresponding to a low density configurational energy minimum.Comment: 10 pages, 13 figures, 2 tables. To be published in J. Chem. Phy
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