214 research outputs found
Wavenumber dependence of structural alpha relaxation in a molecular liquid
Structural alpha relaxation in liquid orthoterphenyl is studied by means of
coherent neutron time-of-flight and backscattering spectroscopy over a large
temperature range. Not only amplitude and relaxation time but also the spectral
line shape show a significant variation with wavenumber. As expected from mode
coupling theory, these variations are correlated with the static structure
factor. Even far above the melting point, alpha relaxation remains
non-exponential.Comment: 6 pages of LaTeX, 4 figure
Fast relaxation in a fragile liquid under pressure
The incoherent dynamic structure factor of ortho-terphenyl has been measured
by neutron time-of-flight and backscattering technique in the pressure range
from 0.1 MPa to 240 MPa for temperatures between 301 K and 335 K.
Tagged-particle correlations in the compressed liquid decay in two steps. The
alpha-relaxation lineshape is independent of pressure, and the relaxation time
proportional to viscosity. A kink in the amplitude f_Q(P) reveals the onset of
beta relaxation. The beta-relaxation regime can be described by the
mode-coupling scaling function; amplitudes and time scales allow a consistent
determination of the critical pressure P_c(T). alpha and beta relaxation depend
in the same way on the thermodynamic state; close to the mode-coupling
cross-over, this dependence can be parametrised by an effective coupling Gamma
~ n*T**{-1/4}.Comment: 4 Pages of RevTeX, 4 figures (submitted to Physical Review Letters
Test of the semischematic model for a liquid of linear molecules
We apply to a liquid of linear molecules the semischematic mode-coupling
model, previously introduced to describe the center of mass (COM) slow dynamics
of a network-forming molecular liquid. We compare the theoretical predictions
and numerical results from a molecular dynamics simulation, both for the time
and the wave-vector dependence of the COM density-density correlation function.
We discuss the relationship between the presented analysis and the results from
an approximate solution of the equations from molecular mode-coupling theory
[R. Schilling and T. Scheidsteger, Phys. Rev. E 56 2932 (1997)].Comment: Revtex, 10 pages, 4 figure
Molecular dynamics simulation of the fragile glass former ortho-terphenyl: a flexible molecule model
We present a realistic model of the fragile glass former orthoterphenyl and
the results of extensive molecular dynamics simulations in which we
investigated its basic static and dynamic properties. In this model the
internal molecular interactions between the three rigid phenyl rings are
described by a set of force constants, including harmonic and anharmonic terms;
the interactions among different molecules are described by Lennard-Jones
site-site potentials. Self-diffusion properties are discussed in detail
together with the temperature and momentum dependencies of the
self-intermediate scattering function. The simulation data are compared with
existing experimental results and with the main predictions of the Mode
Coupling Theory.Comment: 20 pages and 28 postscript figure
Crystal-like high frequency phonons in the amorphous phases of solid water
The high frequency dynamics of low- (LDA) and high-density amorphous-ice
(HDA) and of cubic ice (I_c) has been measured by inelastic X-ray Scattering
(IXS) in the 1-15 nm^{-1} momentum transfer (Q) range. Sharp phonon-like
excitations are observed, and the longitudinal acoustic branch is identified up
to Q = 8nm^{-1} in LDA and I_c and up to 5nm^{-1} in HDA. The narrow width of
these excitations is in sharp contrast with the broad features observed in all
amorphous systems studied so far. The "crystal-like" behavior of amorphous
ices, therefore, implies a considerable reduction in the number of decay
channels available to sound-like excitations which is assimilated to low local
disorder.Comment: 4 pages, 3 figure
Ice XII in its second regime of metastability
We present neutron powder diffraction results which give unambiguous evidence
for the formation of the recently identified new crystalline ice phase[Lobban
et al.,Nature, 391, 268, (1998)], labeled ice XII, at completely different
conditions. Ice XII is produced here by compressing hexagonal ice I_h at T =
77, 100, 140 and 160 K up to 1.8 GPa. It can be maintained at ambient pressure
in the temperature range 1.5 < T < 135 K. High resolution diffraction is
carried out at T = 1.5 K and ambient pressure on ice XII and accurate
structural properties are obtained from Rietveld refinement. At T = 140 and 160
K additionally ice III/IX is formed. The increasing amount of ice III/IX with
increasing temperature gives an upper limit of T ~ 150 K for the successful
formation of ice XII with the presented procedure.Comment: 3 Pages of RevTeX, 3 tables, 3 figures (submitted to Physical Review
Letters
Dynamics in a supercooled molecular liquid: Theory and Simulations
We report extensive simulations of liquid supercooled states for a simple
three-sites molecular model, introduced by Lewis and Wahnstr"om [L. J. Lewis
and G. Wahnstr"om, Phys. Rev. E 50, 3865 (1994)] to mimic the behavior of
ortho-terphenyl. The large system size and the long simulation length allow to
calculate very precisely --- in a large q-vector range --- self and collective
correlation functions, providing a clean and simple reference model for
theoretical descriptions of molecular liquids in supercooled states. The time
and wavevector dependence of the site-site correlation functions are compared
with detailed predictions based on ideal mode-coupling theory, neglecting the
molecular constraints. Except for the wavevector region where the dynamics is
controlled by the center of mass (around 9 nm-1), the theoretical predictions
compare very well with the simulation data.
Neutron scattering and molecular correlations in a supercooled liquid
We show that the intermediate scattering function for neutron
scattering (ns) can be expanded naturely with respect to a set of molecular
correlation functions that give a complete description of the translational and
orientational two-point correlations in the liquid. The general properties of
this expansion are discussed with special focus on the -dependence and hints
for a (partial) determination of the molecular correlation functions from
neutron scattering results are given. The resulting representation of the
static structure factor is studied in detail for a model system using
data from a molecular dynamics simulation of a supercooled liquid of rigid
diatomic molecules. The comparison between the exact result for and
different approximations that result from a truncation of the series
representation demonstrates its good convergence for the given model system. On
the other hand it shows explicitly that the coupling between translational
(TDOF) and orientational degrees of freedom (ODOF) of each molecule and
rotational motion of different molecules can not be neglected in the
supercooled regime.Further we report the existence of a prepeak in the
ns-static structure factor of the examined fragile glassformer, demonstrating
that prepeaks can occur even in the most simple molecular liquids. Besides
examining the dependence of the prepeak on the scattering length and the
temperature we use the expansion of into molecular correlation
functions to point out intermediate range orientational order as its principle
origin.Comment: 13 pages, 7 figure
Asymptotic laws for tagged-particle motion in glassy systems
Within the mode-coupling theory for structural relaxation in simple systems
the asymptotic laws and their leading-asymptotic correction formulas are
derived for the motion of a tagged particle near a glass-transition
singularity. These analytic results are compared with numerical ones of the
equations of motion evaluated for a tagged hard sphere moving in a hard-sphere
system. It is found that the long-time part of the two-step relaxation process
for the mean-squared displacement can be characterized by the -relaxation-scaling law and von Schweidler's power-law decay while the
critical-decay regime is dominated by the corrections to the leading power-law
behavior. For parameters of interest for the interpretations of experimental
data, the corrections to the leading asymptotic laws for the non-Gaussian
parameter are found to be so large that the leading asymptotic results are
altered qualitatively by the corrections. Results for the non-Gaussian
parameter are shown to follow qualitatively the findings reported in the
molecular-dynamics-simulations work by Kob and Andersen [Phys. Rev. E 51, 4626
(1995)]
Atomic Transport in Dense, Multi-Component Metallic Liquids
Pd43Ni10Cu27P0 has been investigated in its equilibrium liquid state with
incoherent, inelastic neutron scattering. As compared to simple liquids, liquid
PdNiCuP is characterized by a dense packing with a packing fraction above 0.5.
The intermediate scattering function exhibits a fast relaxation process that
precedes structural relaxation. Structural relaxation obeys a time-temperature
superposition that extends over a temperature range of 540K. The mode-coupling
theory of the liquid to glass transition (MCT) gives a consistent description
of the dynamics which governs the mass transport in liquid PdNiCuP alloys. MCT
scaling laws extrapolate to a critical temperature Tc at about 20% below the
liquidus temperature. Diffusivities derived from the mean relaxation times
compare well with Co diffusivities from recent tracer diffusion measurements
and diffsuivities calculated from viscosity via the Stokes-Einstein relation.
In contrast to simple metallic liquids, the atomic transport in dense, liquid
PdNiCuP is characterized by a drastical slowing down of dynamics on cooling, a
q^{-2} dependence of the mean relaxation times at intermediate q and a
vanishing isotope effect as a result of a highly collective transport
mechanism. At temperatures as high as 2Tc diffusion in liquid PdNiCuP is as
fast as in simple liquids at the melting point. However, the difference in the
underlying atomic transport mechanism indicates that the diffusion mechanism in
liquids is not controlled by the value of the diffusivity but rather by that of
the packing fraction
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