276 research outputs found
Stability of supercooled binary liquid mixtures
Recently the supercooled Wahnstrom binary Lennard-Jones mixture was partially
crystallized into phase crystals in lengthy Molecular Dynamics
simulations. We present Molecular Dynamics simulations of a modified
Kob-Andersen binary Lennard-Jones mixture that also crystallizes in lengthy
simulations, here however by forming pure fcc crystals of the majority
component. The two findings motivate this paper that gives a general
thermodynamic and kinetic treatment of the stability of supercooled binary
mixtures, emphasizing the importance of negative mixing enthalpy whenever
present. The theory is used to estimate the crystallization time in a
Kob-Andersen mixture from the crystallization time in a series of relared
systems. At T=0.40 we estimate this time to be 5 time units
(). A new binary Lennard-Jones mixture is proposed that is not
prone to crystallization and faster to simulate than the two standard binary
Lennard-Jones mixtures; this is obtained by removing the like-particle
attractions by switching to Weeks-Chandler-Andersen type potentials, while
maintaining the unlike-particle attraction
Crystallization of the Wahnstr\"om Binary Lennard-Jones Liquid
We report observation of crystallization of the glass-forming binary
Lennard-Jones liquid first used by Wahnstr\"om [G. Wahnstr\"om, Phys. Rev. A
44, 3752 (1991)]. Molecular dynamics simulations of the metastable liquid on a
timescale of microseconds were performed. The liquid crystallized
spontaneously. The crystal structure was identified as MgZn_2. Formation of
transient crystallites is observed in the liquid. The crystallization is
investigate at different temperatures and compositions. At high temperature the
rate of crystallite formation is the limiting factor, while at low temperature
the limiting factor is growth rate. The melting temperature of the crystal is
estimated to be T_m=0.93 at rho=0.82. The maximum crystallization rate of the
A_2B composition is T=0.60+/-0.02.Comment: 4 pages, 4 figures; corrected typo
The Geometry of Slow Structural Fluctuations in a Supercooled Binary Alloy
The liquid structure of a glass-forming binary alloy is studied using
molecular dynamics simulations. The analysis combines common neighbour analysis
with the geometrical approach of Frank and Kasper to establish that the
supercooled liquid contains extended clusters characterised by the same short
range order as the crystal. Fluctuations in these clusters exhibit strong
correlations with fluctuations in the inherent structure energy. The steep
increase in the heat capacity on cooling is, thus, directly coupled to the
growing fluctuations of the Frank-Kasper clusters. The relaxation of particles
in the clusters dominates the slow tail of the self-intermediate scattering
function
Strong pressure-energy correlations in liquids as a configuration space property: Simulations of temperature down jumps and crystallization
Computer simulations recently revealed that several liquids exhibit strong
correlations between virial and potential energy equilibrium fluctuations in
the NVT ensemble [U. R. Pedersen {\it et al.}, Phys. Rev. Lett. {\bf 100},
015701 (2008)]. In order to investigate whether these correlations are present
also far from equilibrium constant-volume aging following a temperature down
jump from equilibrium was simulated for two strongly correlating liquids, an
asymmetric dumbbell model and Lewis-Wahnstr{\"o}m OTP, as well as for SPC water
that is not strongly correlating. For the two strongly correlating liquids
virial and potential energy follow each other closely during the aging towards
equilibrium. For SPC water, on the other hand, virial and potential energy vary
with little correlation as the system ages towards equilibrium. Further proof
that strong pressure-energy correlations express a configuration space property
comes from monitoring pressure and energy during the crystallization (reported
here for the first time) of supercooled Lewis-Wahnstr{\"o}m OTP at constant
temperature
Extreme case of density scaling:The Weeks-Chandler-Andersen system at low temperatures
This paper studies numerically the Weeks-Chandler-Andersen (WCA) system,
which is shown to obey hidden scale invariance with a density-scaling exponent
that varies from below 5 to above 500. This unprecedented variation makes it
advantageous to use the fourth-order Runge-Kutta algorithm for tracing out
isomorphs. Good isomorph invariance of the structure and dynamics is observed
over more than three orders of magnitude temperature variation. For all state
points studied, the virial potential-energy correlation coefficient and the
density-scaling exponent are controlled mainly by the temperature. A mean-field
theory is presented based on the assumption of statistically independent pair
interactions, which rationalizes this finding
Estimating the density scaling exponent of viscous liquids from specific heat and bulk modulus data
It was recently shown by computer simulations that a large class of liquids
exhibits strong correlations in their thermal fluctuations of virial and
potential energy [Pedersen et al., Phys. Rev. Lett. 100, 015701 (2008)]. Among
organic liquids the class of strongly correlating liquids includes van der
Waals liquids, but excludes ionic and hydrogen-bonding liquids. The present
note focuses on the density scaling of strongly correlating liquids, i.e., the
fact their relaxation time tau at different densities rho and temperatures T
collapses to a master curve according to the expression tau propto
F(rho^gamma/T) [Schroder et al., arXiv:0803.2199]. We here show how to
calculate the exponent gamma from bulk modulus and specific heat data, either
measured as functions of frequency in the metastable liquid or extrapolated
from the glass and liquid phases to a common temperature (close to the glass
transition temperature). Thus an exponent defined from the response to highly
nonlinear parameter changes may be determined from linear response
measurements
A repulsive reference potential reproducing the dynamics of a liquid with attractions
A well-known result of liquid state theory is that the structure of dense
fluids is mainly determined by repulsive forces. The WCA potential, which cuts
intermolecular potentials at their minima, is therefore often used as a
reference. However, this reference gives quite wrong results for the viscous
dynamics of the Kob-Andersen binary Lennard-Jones liquid [Berthier and Tarjus,
Phys. Rev. Lett. 103, 170601 (2009)]. We show that repulsive inverse-power law
potentials provide a useful reference for this liquid by reproducing its
structure, dynamics, and isochoric heat capacity
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