1,231 research outputs found
Structural relaxation in a supercooled molecular liquid
We perform molecular-dynamics simulations of a molecular system in
supercooled states for different values of inertia parameters to provide
evidence that the long-time dynamics depends only on the equilibrium structure.
This observation is consistent with the prediction of the mode-coupling theory
for the glass transition and with the hypothesis that the potential
energy-landscape controls the slow dynamics. We also find that dynamical
properties at intermediate wavenumber depend on the spatial correlation of the
molecule's geometrical center.Comment: 7 pages, 4 figures, Europhys. Lett. in pres
A mode-coupling theory for the glassy dynamics of a diatomic probe molecule immersed in a simple liquid
Generalizing the mode-coupling theory for ideal liquid-glass transitions,
equations of motion are derived for the correlation functions describing the
glassy dynamics of a diatomic probe molecule immersed in a simple glass-forming
system. The molecule is described in the interaction-site representation and
the equations are solved for a dumbbell molecule consisting of two fused hard
spheres in a hard-sphere system. The results for the molecule's arrested
position in the glass state and the reorientational correlators for
angular-momentum index and near the glass transition are
compared with those obtained previously within a theory based on a
tensor-density description of the molecule in order to demonstrate that the two
approaches yield equivalent results. For strongly hindered reorientational
motion, the dipole-relaxation spectra for the -process can be mapped on
the dielectric-loss spectra of glycerol if a rescaling is performed according
to a suggestion by Dixon et al. [Phys. Rev. Lett. {\bf 65}, 1108 (1990)]. It is
demonstrated that the glassy dynamics is independent of the molecule's inertia
parameters.Comment: 19 pages, 10 figures, Phys. Rev. E, in prin
Structural Relaxation and Mode Coupling in a Simple Liquid: Depolarized Light Scattering in Benzene
We have measured depolarized light scattering in liquid benzene over the
whole accessible temperature range and over four decades in frequency. Between
40 and 180 GHz we find a susceptibility peak due to structural relaxation. This
peak shows stretching and time-temperature scaling as known from
relaxation in glass-forming materials. A simple mode-coupling model provides
consistent fits of the entire data set. We conclude that structural relaxation
in simple liquids and relaxation in glass-forming materials are
physically the same. A deeper understanding of simple liquids is reached by
applying concepts that were originally developed in the context of
glass-transition research.Comment: submitted to New J. Phy
Magneto-Transport in the Two-Dimensional Lorentz Gas
We consider the two-dimensional Lorentz gas with Poisson distributed hard
disk scatterers and a constant magnetic field perpendicular to the plane of
motion. The velocity autocorrelation is computed numerically over the full
range of densities and magnetic fields with particular attention to the
percolation threshold between hopping transport and pure edge currents. The
Ohmic and Hall conductance are compared with mode-coupling theory and a recent
generalized kinetic equation valid for low densities and small fields. We argue
that the long time tail as persists for non-zero magnetic field.Comment: 7 pages, 14 figures. Uses RevTeX and epsfig.sty. Submitted to
Physical Review
Structural relaxation in a system of dumbbell molecules
The interaction-site-density-fluctuation correlators, the dipole-relaxation
functions, and the mean-squared displacements of a system of symmetric
dumbbells of fused hard spheres are calculated for two representative
elongations of the molecules within the mode-coupling theory for the evolution
of glassy dynamics. For large elongations, universal relaxation laws for states
near the glass transition are valid for parameters and time intervals similar
to the ones found for the hard-sphere system. Rotation-translation coupling
leads to an enlarged crossover interval for the mean-squared displacement of
the constituent atoms between the end of the von Schweidler regime and the
beginning of the diffusion process. For small elongations, the superposition
principle for the reorientational -process is violated for parameters
and time intervals of interest for data analysis, and there is a strong
breaking of the coupling of the -relaxation scale for the diffusion
process with that for representative density fluctuations and for dipole
reorientations.Comment: 15 pages, 14 figures, Phys. Rev. E in pres
Frustrated spin- Heisenberg magnet on a square-lattice bilayer: High-order study of the quantum critical behavior of the ---- model
The zero-temperature phase diagram of the spin-
---- model on an -stacked square-lattice
bilayer is studied using the coupled cluster method implemented to very high
orders. Both nearest-neighbor (NN) and frustrating next-nearest-neighbor
Heisenberg exchange interactions, of strengths and , respectively, are included in each layer. The two layers are
coupled via a NN interlayer Heisenberg exchange interaction with a strength
. The magnetic order parameter (viz.,
the sublattice magnetization) is calculated directly in the thermodynamic
(infinite-lattice) limit for the two cases when both layers have
antiferromagnetic ordering of either the N\'{e}el or the striped kind, and with
the layers coupled so that NN spins between them are either parallel (when
) to one another. Calculations
are performed at th order in a well-defined sequence of approximations,
which exactly preserve both the Goldstone linked cluster theorem and the
Hellmann-Feynman theorem, with . The sole approximation made is to
extrapolate such sequences of th-order results for to the exact limit,
. By thus locating the points where vanishes, we calculate
the full phase boundaries of the two collinear AFM phases in the
-- half-plane with . In particular, we provide the
accurate estimate, (), for the
position of the quantum triple point (QTP) in the region . We also
show that there is no counterpart of such a QTP in the region ,
where the two quasiclassical phase boundaries show instead an ``avoided
crossing'' behavior, such that the entire region that contains the nonclassical
paramagnetic phases is singly connected
The mean-squared displacement of a molecule moving in a glassy system
The mean-squared displacement (MSD) of a hard sphere and of a dumbbell
molecule consisting of two fused hard spheres immersed in a dense hard-sphere
system is calculated within the mode-coupling theory for ideal liquid-glass
transitions. It is proven that the velocity correlator, which is the second
time derivative of the MSD, is the negative of a completely monotone function
for times within the structural-relaxation regime. The MSD is found to exhibit
a large time interval for structural relaxation prior to the onset of the
-process which cannot be described by the asymptotic formulas for the
mode-coupling-theory-bifurcation dynamics. The -process for molecules
with a large elongation is shown to exhibit an anomalously wide cross-over
interval between the end of the von-Schweidler decay and the beginning of
normal diffusion. The diffusivity of the molecule is predicted to vary
non-monotonically as function of its elongation.Comment: 18 pages, 12 figures, Phys. Rev. E, in prin
An exactly solvable toy model that mimics the mode coupling theory of supercooled liquid and glass transition
A toy model is proposed which incorporates the reversible mode coupling
mechanism responsible for ergodic-nonergodic transition with trivial
Hamiltonian in the mode coupling theory (MCT) of structural glass transition.
The model can be analyzed without relying on uncontrolled approximations
inevitable in the current MCT. The strength of hopping processes can be easily
tuned and the ideal glass transition is reproduced only in a certain range of
the strength. On the basis of the analyses of our model we discuss about a
sharp ergodic-nonergodic transition and its smearing out by "hopping".Comment: 5 pages, 2 ps-figures, inappropriate terms replace
Fragile to strong crossover coupled to liquid-liquid transition in hydrophobic solutions
Using discrete molecular dynamics simulations we study the relation between
the thermodynamic and diffusive behaviors of a primitive model of aqueous
solutions of hydrophobic solutes consisting of hard spheres in the Jagla
particles solvent, close to the liquid-liquid critical point of the solvent. We
find that the fragile-to-strong dynamic transition in the diffusive behavior is
always coupled to the low-density/high-density liquid transition. Above the
liquid-liquid critical pressure, the diffusivity crossover occurs at the Widom
line, the line along which the thermodynamic response functions show maxima.
Below the liquid-liquid critical pressure, the diffusivity crossover occurs
when the limit of mechanical stability lines are crossed, as indicated by the
hysteresis observed when going from high to low temperature and vice versa.
These findings show that the strong connection between dynamics and
thermodynamics found in bulk water persists in hydrophobic solutions for
concentrations from low to moderate, indicating that experiments measuring the
relaxation time in aqueous solutions represent a viable route for solving the
open questions in the field of supercooled water.Comment: 6 pages, 4 figures. Accepted for publication on Physical Review
Self-diffusion in sheared colloidal suspensions: violation of fluctuation-dissipation relation
Using memory-function formalism we show that in sheared colloidal suspensions
the fluctuation-dissipation theorem for self-diffusion, i.e. Einstein's
relation between self-diffusion and mobility tensors, is violated and propose a
new way to measure this violation in Brownian Dynamics simulations. We derive
mode-coupling expressions for the tagged particle friction tensor and for an
effective, shear-rate dependent temperature
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