32 research outputs found
Structural relaxation in orthoterphenyl: a schematic mode coupling theory model analysis
Depolarized light scattering spectra of orthoterphenyl showing the emergence
of the structural relaxation below the oscillatory microscopic excitations are
described by solutions of a schematic mode--coupling--theory model
Acoustic and relaxation processes in supercooled o-ter-phenyl by optical-heterodyne transient grating experiment
The dynamics of the fragile glass-forming o-ter-phenyl is investigated by
time-resolved transient grating experiment with an heterodyne detection
technique in a wide temperature range. We investigated the dynamics processes
of this glass-former over more then 6 decades in time with an excellent
signal/noise. Acoustic, structural and thermal relaxations have been clearly
identify and measured in a time-frequency window not covered by previous
spectroscopic investigations. A detailed comparison with the density response
function, calculated on the basis of generalized hydrodynamics model, has been
worked out
Evidence of short time dynamical correlations in simple liquids
We report a molecular dynamics (MD) study of the collective dynamics of a
simple monatomic liquid -interacting through a two body potential that mimics
that of lithium- across the liquid-glass transition. In the glassy phase we
find evidences of a fast relaxation process similar to that recently found in
Lennard-Jones glasses. The origin of this process is ascribed to the
topological disorder, i.e. to the dephasing of the different momentum
Fourier components of the actual normal modes of vibration of the disordered
structure. More important, we find that the fast relaxation persists in the
liquid phase with almost no temperature dependence of its characteristic
parameters (strength and relaxation time). We conclude, therefore, that in the
liquid phase well above the melting point, at variance with the usual
assumption of {\it un-correlated} binary collisions, the short time particles
motion is strongly {\it correlated} and can be described via a normal mode
expansion of the atomic dynamics.Comment: 7 pages, 7 .eps figs. To appear in Phys. Rev.
Surface critical exponents at a uniaxial Lifshitz point
Using Monte Carlo techniques, the surface critical behaviour of
three-dimensional semi-infinite ANNNI models with different surface
orientations with respect to the axis of competing interactions is
investigated. Special attention is thereby paid to the surface criticality at
the bulk uniaxial Lifshitz point encountered in this model. The presented Monte
Carlo results show that the mean-field description of semi-infinite ANNNI
models is qualitatively correct. Lifshitz point surface critical exponents at
the ordinary transition are found to depend on the surface orientation. At the
special transition point, however, no clear dependency of the critical
exponents on the surface orientation is revealed. The values of the surface
critical exponents presented in this study are the first estimates available
beyond mean-field theory.Comment: 10 pages, 7 figures include
Propylene Carbonate Reexamined: Mode-Coupling Scaling without Factorisation ?
The dynamic susceptibility of propylene carbonate in the moderately viscous
regime above is reinvestigated by incoherent neutron and
depolarised light scattering, and compared to dielectric loss and solvation
response. Depending on the strength of relaxation, a more or less
extended scaling regime is found. Mode-coupling fits yield consistently
and K, although different positions of the
susceptibility minimum indicate that not all observables have reached the
universal asymptotics
Equilibration times in numerical simulation of structural glasses: Comparing parallel tempering and conventional molecular dynamics
Generation of equilibrium configurations is the major obstacle for numerical
investigation of the slow dynamics in supercooled liquid states. The parallel
tempering (PT) technique, originally proposed for the numerical equilibration
of discrete spin-glass model configurations, has recently been applied in the
study of supercooled structural glasses. We present an investigation of the
ability of parallel tempering to properly sample the liquid configuration space
at different temperatures, by mapping the PT dynamics into the dynamics of the
closest local potential energy minima (inherent structures). Comparing the PT
equilibration process with the standard molecular dynamics equilibration
process we find that the PT does not increase the speed of equilibration of the
(slow) configurational degrees of freedom.Comment: 5 pages, 3 figure
Critical slowing down near the Rayleigh-BĂ©nard convective instability
The decay time of temperature fluctuations in a thin horizontal fluid layer subjected to a downward temperature gradient has been studied by forced Rayleigh scattering. The decay time was found to increase markedly as the temperature gradient approached the critical value for the onset of the Rayleigh-Bénard convective instability and was most pronounced for spatial wavelengths close to twice the thickness of the fluid layer.On a mesuré. à l'aide de la technique de diffusion Rayleigh forcé. le temps de relaxation des fluctuations de température dans une mince couche horizontale de fluide soumise à un gradien de température orienté vers le bas. On a trouvé que ce temps de relaxation augmente beaucoup lorsque le gradient de température s'approche de la valeur critique correspondant à l'instabilité de convection de Rayleigh-Bénard. et que cette augmentation est la plus grande pour des longueurs d'onde spatiales proches du double de l'épaisseur de la lame de liquide