641 research outputs found
Orientational and induced contributions to the depolarized Rayleigh spectra of liquid and supercooled ortho-terphenyl
The depolarized light scattering spectra of the glass forming liquid
ortho-terphenyl have been calculated in the low frequency region using
molecular dynamics simulation. Realistic system's configurations are produced
by using a recent flexible molecular model and combined with two limiting
polarizability schemes, both of them using the dipole-induced-dipole
contributions at first and second order. The calculated Raman spectral shape
are in good agreement with the experimental results in a large temperature
range. The analysis of the different contributions to the Raman spectra
emphasizes that the orientational and the collision-induced (translational)
terms lie on the same time-scale and are of comparable intensity. Moreover, the
cross terms are always found to be an important contribution to the scattering
intensity.Comment: RevTeX4, 7 pages, 8 eps figure
Locally preferred structure in simple atomic liquids
We propose a method to determine the locally preferred structure of model
liquids. This latter is obtained numerically as the global minimum of the
effective energy surface of clusters formed by small numbers of particles
embedded in a liquid-like environment. The effective energy is the sum of the
intra-cluster interaction potential and of an external field that describes the
influence of the embedding bulk liquid at a mean-field level. Doing so we
minimize the surface effects present in isolated clusters without introducing
the full blown geometrical frustration present in bulk condensed phases. We
find that the locally preferred structure of the Lennard-Jones liquid is an
icosahedron, and that the liquid-like environment only slightly reduces the
relative stability of the icosahedral cluster. The influence of the boundary
conditions on the nature of the ground-state configuration of Lennard-Jones
clusters is also discussed.Comment: RevTeX 4, 17 pages, 6 eps figure
Liquid stability in a model for ortho-terphenyl
We report an extensive study of the phase diagram of a simple model for
ortho-terphenyl, focusing on the limits of stability of the liquid state.
Reported data extend previous studies of the same model to both lower and
higher densities and to higher temperatures. We estimate the location of the
homogeneous liquid-gas nucleation line and of the spinodal locus. Within the
potential energy landscape formalism, we calculate the distributions of depth,
number, and shape of the potential energy minima and show that the statistical
properties of the landscape are consistent with a Gaussian distribution of
minima over a wide range of volumes. We report the volume dependence of the
parameters entering in the Gaussian distribution (amplitude, average energy,
variance). We finally evaluate the locus where the configurational entropy
vanishes, the so-called Kauzmann line, and discuss the relative location of the
spinodal and Kauzmann loci.Comment: RevTeX 4, 8 pages, 8 eps figure
Disentangling density and temperature effects in the viscous slowing down of glassforming liquids
We present a consistent picture of the respective role of density and
temperature in the viscous slowing down of glassforming liquids and polymers.
Specifically, based in part upon a new analysis of simulation and experimental
data on liquid ortho-terphenyl, we conclude that a zeroth-order description of
the approach to the glass transition should be formulated in terms of a
temperature-driven super-Arrhenius activated behavior rather than a
density-driven congestion or jamming phenomenon. The density plays a role at a
quantitative level, but its effect on the viscosity and the structural
relaxation time can be simply described via a single parameter, an effective
interaction energy that is characteristic of the high temperature liquid
regime; as a result, density does not affect the ``fragility'' of the
glassforming system.Comment: RevTeX4, 8 pages, 8 eps figure
Molecular dynamics simulation study of the high frequency sound waves in the fragile glass former ortho-terphenyl
Using a realistic flexible molecule model of the fragile glass former
orthoterphenyl, we calculate via molecular dynamics simulation the collective
dynamic structure factor, recently measured in this system by Inelastic X-ray
Scattering. The comparison of the simulated and measured dynamic structure
factor, and the study of its properties in an extended momentum, frequency and
temperature range allows: i) to conclude that the utilized molecular model
gives rise to a dynamic structure factor in agreement with the experimental
data, for those thermodynamic states and momentum values where the latter are
available; ii) to confirm the existence of a slope discontinuity on the
T-dependence of the sound velocity that, at finite Q, takes place at a
temperature T_x higher than the calorimetric glass transition temperature T_g;
iii) to find that the values of T_x is Q-dependent and that its vanishing Q
limit is consistent with T_g. The latter finding is interpreted within the
framework of the current description of the dynamics of supercooled liquids in
terms of exploration of the potential energy landscape.Comment: RevTex, 9 pages, 10 eps figure
Impact of elastic heterogeneity on the propagation of vibrations at finite temperatures in glasses
Some aspects of how sound waves travel through disordered solids are still
unclear. Recent work has characterized a feature of disordered solids which
seems to influence vibrational excitations at the mesoscales, local elastic
heterogeneity. Sound waves propagation has been demonstrated to be strongly
affected by inhomogeneous mechanical features of the materials which add to the
standard anharmonic couplings, amounting to extremely complex transport
properties at finite temperatures. Here, we address these issues for the case
of a simple atomic glass former, by Molecular Dynamics computer simulation. In
particular, we focus on the transverse components of the vibrational
excitations in terms of dynamic structure factors, and characterize the
temperature dependence of sound dispersion and attenuation in an extended
frequency range. We provide a complete picture of how elastic heterogeneity
determines transport of vibrational excitations, also based on a direct
comparison of the numerical data with the predictions of the heterogeneous
elastic theory.Comment: 14 pages, 5 figures, 1 tabl
Dynamic force spectroscopy of DNA hairpins. II. Irreversibility and dissipation
We investigate irreversibility and dissipation in single molecules that
cooperatively fold/unfold in a two state manner under the action of mechanical
force. We apply path thermodynamics to derive analytical expressions for the
average dissipated work and the average hopping number in two state systems. It
is shown how these quantities only depend on two parameters that characterize
the folding/unfolding kinetics of the molecule: the fragility and the
coexistence hopping rate. The latter has to be rescaled to take into account
the appropriate experimental setup. Finally we carry out pulling experiments
with optical tweezers in a specifically designed DNA hairpin that shows
two-state cooperative folding. We then use these experimental results to
validate our theoretical predictions.Comment: 28 pages, 12 figure
Vibrational origin of the fast relaxation processes in molecular glass-formers
We study the interaction of the relaxation processes with the density
fluctuations by molecular dynamics simulation of a flexible molecule model for
o-terphenyl (oTP) in the liquid and supercooled phases. We find evidence,
besides the structural relaxation, of a secondary vibrational relaxation whose
characteristic time, few ps, is slightly temperature dependent. This i)
confirms the result by Monaco et al. [Phys. Rev, E 62, 7595 (2000)] of the
vibrational nature of the fast relaxation observed in Brillouin Light
Scattering (BLS) experiments in oTP; and ii) poses a caveat on the
interpretation of the BLS spectra of molecular systems in terms of a purely
center of mass dynamics.Comment: RevTeX, 5 pages, 4 eps figure
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