197 research outputs found
Diffusion and jump-length distribution in liquid and amorphous CuZr
Using molecular dynamics simulation, we calculate the distribution of atomic
jum ps in CuZr in the liquid and glassy states. In both states
the distribution of jump lengths can be described by a temperature independent
exponential of the length and an effective activation energy plus a
contribution of elastic displacements at short distances. Upon cooling the
contribution of shorter jumps dominates. No indication of an enhanced
probability to jump over a nearest neighbor distance was found. We find a
smooth transition from flow in the liquid to jumps in the g lass. The
correlation factor of the diffusion constant decreases with decreasing
temperature, causing a drop of diffusion below the Arrhenius value, despite an
apparent Arrhenius law for the jump probability
Ac hopping conduction at extreme disorder takes place on the percolating cluster
Simulations of the random barrier model show that ac currents at extreme
disorder are carried almost entirely by the percolating cluster slightly above
threshold; thus contradicting traditional theories contributions from isolated
low-activation-energy clusters are negligible. The effective medium
approximation in conjunction with the Alexander-Orbach conjecture leads to an
excellent analytical fit to the universal ac conductivity with no nontrivial
fitting parameters
Isomorphs in model molecular liquids
Isomorphs are curves in the phase diagram along which a number of static and
dynamic quantities are invariant in reduced units. A liquid has good isomorphs
if and only if it is strongly correlating, i.e., the equilibrium
virial/potential energy fluctuations are more than 90% correlated in the NVT
ensemble. This paper generalizes isomorphs to liquids composed of rigid
molecules and study the isomorphs of two systems of small rigid molecules, the
asymmetric dumbbell model and the Lewis-Wahnstrom OTP model. In particular, for
both systems we find that the isochoric heat capacity, the excess entropy, the
reduced molecular center-of-mass self part of the intermediate scattering
function, the reduced molecular center-of-mass radial distribution function to
a good approximation are invariant along an isomorph. In agreement with theory,
we also find that an instantaneous change of temperature and density from an
equilibrated state point to another isomorphic state point leads to no
relaxation. The isomorphs of the Lewis-Wahnstrom OTP model were found to be
more approximative than those of the asymmetric dumbbell model, which is
consistent with the OTP model being less strongly correlating. For both models
we find "master isomorphs", i.e., isomorphs have identical shape in the
virial/potential energy phase diagram.Comment: 20 page
Communication: Studies of the Lennard-Jones fluid in 2, 3, and 4 dimensions highlight the need for a liquid-state 1/d expansion
The recent theoretical prediction by Maimbourg and Kurchan [arXiv:1603.05023]
that for regular pair-potential systems the virial potential-energy correlation
coefficient increases towards unity as the dimension goes to infinity is
investigated for the standard 12-6 Lennard-Jones fluid. This is done by
computer simulations for going from the critical point along the
critical isotherm/isochore to higher density/temperature. In all cases the
virial potential-energy correlation coefficient increases significantly. For a
given density and temperature relative to the critical point, with increasing
number of dimension the Lennard-Jones system conforms better to the
hidden-scale-invariance property characterized by high virial potential-energy
correlations (a property that leads to the existence of isomorphs in the
thermodynamic phase diagram, implying that it becomes effectively
one-dimensional in regard to structure and dynamics). The present paper also
gives the first numerical demonstration of isomorph invariance of structure and
dynamics in four dimensions. Our findings emphasize the need for a universally
applicable expansion in liquid-state theory; we conjecture that the
systems known to obey hidden scale invariance in three dimensions are those for
which the yet-to-be-developed expansion converges rapidly
Observation of Single Transits in Supercooled Monatomic Liquids
A transit is the motion of a system from one many-particle potential energy
valley to another. We report the observation of transits in molecular dynamics
(MD) calculations of supercooled liquid argon and sodium. Each transit is a
correlated simultaneous shift in the equilibrium positions of a small local
group of particles, as revealed in the fluctuating graphs of the particle
coordinates versus time. This is the first reported direct observation of
transit motion in a monatomic liquid in thermal equilibrium. We found transits
involving 2 to 11 particles, having mean shift in equilibrium position on the
order of 0.4 R_1 in argon and 0.25 R_1 in sodium, where R_1 is the nearest
neighbor distance. The time it takes for a transit to occur is approximately
one mean vibrational period, confirming that transits are fast.Comment: 19 pages, 8 figure
Energy landscape of a Lennard-Jones liquid: Statistics of stationary points
Molecular dynamics simulations are used to generate an ensemble of saddles of
the potential energy of a Lennard-Jones liquid. Classifying all extrema by
their potential energy u and number of unstable directions k, a well defined
relation k(u) is revealed. The degree of instability of typical stationary
points vanishes at a threshold potential energy, which lies above the energy of
the lowest glassy minima of the system. The energies of the inherent states, as
obtained by the Stillinger-Weber method, approach the threshold energy at a
temperature close to the mode-coupling transition temperature Tc.Comment: 4 RevTeX pages, 6 eps figures. Revised versio
Saddles in the energy landscape probed by supercooled liquids
We numerically investigate the supercooled dynamics of two simple model
liquids exploiting the partition of the multi-dimension configuration space in
basins of attraction of the stationary points (inherent saddles) of the
potential energy surface. We find that the inherent saddles order and potential
energy are well defined functions of the temperature T. Moreover, decreasing T,
the saddle order vanishes at the same temperature (T_MCT) where the inverse
diffusivity appears to diverge as a power law. This allows a topological
interpretation of T_MCT: it marks the transition from a dynamics between basins
of saddles (T>T_MCT) to a dynamics between basins of minima (T<T_MCT).Comment: 4 pages, 3 figures, to be published on PR
Growing Correlation Length on Cooling Below the Onset of Caging in a Simulated Glass-Forming Liquid
We present a calculation of a fourth-order, time-dependent density
correlation function that measures higher-order spatiotemporall correlations of
the density of a liquid. From molecular dynamics simulations of a glass-forming
Lennard-Jones liquid, we find that the characteristic length scale of this
function has a maximum as a function of time which increases steadily beyond
the characteristic length of the static pair correlation function in the
temperature range approaching the mode coupling temperature from above
Invariants in the Yukawa system’s thermodynamic phase diagram
This paper shows that several known properties of the Yukawa system can be
derived from the isomorph theory, which applies to any system that has strong
correlations between its virial and potential-energy equilibrium fluctuations.
Such "Roskilde-simple" systems have a simplified thermodynamic phase diagram
deriving from the fact that they have curves (isomorphs) along which structure
and dynamics in reduced units are invariant to a good approximation. We show
that the Yukawa system has strong virial potential-energy correlations and
identify its isomorphs by two different methods. One method, the so-called
direct isomorph check, identifies isomorphs numerically from jumps of
relatively small density changes (here 10%). The second method identifies
isomorphs analytically from the pair potential. The curves obtained by the two
methods are close to each other; these curves are confirmed to be isomorphs by
demonstrating the invariance of the radial distribution function, the static
structure factor, the mean-square displacement as a function of time, and the
incoherent intermediate scattering function. Since the melting line is
predicted to be an isomorph, the theory provides a derivation of a known
approximate analytical expression for this line in the temperature-density
phase diagram. The paper's results give the first demonstration that the
isomorph theory can be applied to systems like dense colloidal suspensions and
strongly coupled dusty plasmas.Comment: 12 pages, 12 figure
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