1,911 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
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
Conclusions and perspectives: Perspectives for future research-and-development projects on biological
The review of published scientific literature on the biological control of selected pests and diseases has lead to the identification of clear knowledge gaps highlighted in previous chapters. Further bottlenecks were revealed by seeking the possible reasons for the striking discrepancy between the rich inventory of potential biocontrol agents described by scientists and a very small number of commercial products on the market. To complement these analyses, the participants of Research Activity 4.3 of the European Network ENDURE organized consultations of experts (scientists, extension specialists and representatives of the Biocontrol industry) at the occasion of scientific meetings of three Working Groups of IOBC-wpr
High frequency acoustic modes in liquid gallium at the melting point
The microscopic dynamics in liquid gallium (l-Ga) at melting (T=315 K) has
been studied by inelastic x-ray scattering. We demonstrate the existence of
collective acoustic-like modes up to wave-vectors above one half of the first
maximum of the static structure factor, at variance with earlier results from
inelastic neutron scattering data [F.J. Bermejo et al. Phys. Rev. E 49, 3133
(1994)]. Despite the structural (an extremely rich polymorphism and rather
complex phase diagram) and electronic (mixed valence) peculiarity of l-Ga, its
collective dynamics is strikingly similar to the one of Van der Walls and
alkali metals liquids. This result speaks in favor of the universality of the
short time dynamics in monatomic liquids rather than of system-specific
dynamics.Comment: LaTex format, 11 pages, 4 EncapsulatedPostScript figure
General features of the energy landscape in Lennard-Jones like model liquids
Features of the energy landscape sampled by supercooled liquids are
numerically analyzed for several Lennard-Jones like model systems. The
properties of quasisaddles (minima of the square gradient of potential energy
W=|grad V|^2), are shown to have a direct relationship with the dynamical
behavior, confirming that the quasisaddle order extrapolates to zero at the
mode-coupling temperature T_MCT. The same result is obtained either analyzing
all the minima of W or the saddles (absolute minima of W), supporting the
conjectured similarity between quasisaddles and saddles, as far as the
temperature dependence of the properties influencing the slow dynamics is
concerned. We find evidence of universality in the shape of the landscape:
plots for different systems superimpose into master curves, once energies and
temperatures are scaled by T_MCT. This allows to establish a quantitative
relationship between T_MCT and potential energy barriers for LJ-like systems,
and suggests a possible generalization to different model liquids.Comment: 8 pages, 5 figure
High frequency dynamics in liquid nickel: an IXS study
Owing to their large relatively thermal conductivity, peculiar,
non-hydrodynamic features are expected to characterize the acoustic-like
excitations observed in liquid metals. We report here an experimental study of
collective modes in molten nickel, a case of exceptional geophysical interest
for its relevance in Earth interior science. Our result shed light on
previously reported contrasting evidences: in the explored energy-momentum
region no deviation from the generalized hydrodynamic picture describing non
conductive fluids are observed. Implications for high frequency transport
properties in metallic fluids are discussed.Comment: 6 pages, 4 figures, to appear in "Journal of Chemical Physics
ORIGIN OF LIGHT SCATTERING FROM DISORDERED SYSTEMS
Anelastic light scattering is computed numerically for model disordered
systems (linear chains and 2-dimensional site and bond percolators), with and
without electrical disorder. A detailed analysis of the vibrational modes and
of their Raman activity evidences that two extreme mechanisms for scattering
may be singled out. One of these resembles scattering from finite size systems,
while the other mechanisms originates from spatial fluctuations of the
polarizability and is such that modes in even small frequency intervals may
have very different Raman activities. As a consequence, the average coupling
coefficient is the variance of a zero-average quantity. Our
analysis shows that for both linear chains and 2-dimensional percolators the
second mechanism dominates over the first, and therefore Raman scattering from
disordered systems is essentially due to spatial fluctuations.Comment: 12 pages, Latex, 7 figures available on request
Landscapes and Fragilities
The concept of fragility provides a possibility to rank different supercooled
liquids on the basis of the temperature dependence of dynamic and/or
thermodynamic quantities. We recall here the definitions of kinetic and
thermodynamic fragility proposed in the last years and discuss their
interrelations. At the same time we analyze some recently introduced models for
the statistical properties of the potential energy landscape. Building on the
Adam-Gibbs relation, which connects structural relaxation times to
configurational entropy, we analyze the relation between statistical properties
of the landscape and fragility. We call attention to the fact that the
knowledge of number, energy depth and shape of the basins of the potential
energy landscape may not be sufficient for predicting fragility. Finally, we
discuss two different possibilities for generating strong behavior.Comment: 17 pages, 10 figures; accepted version, minor correction
Frustration and sound attenuation in structural glasses
Three classes of harmonic disorder systems (Lennard-Jones like glasses,
percolators above threshold, and spring disordered lattices) have been
numerically investigated in order to clarify the effect of different types of
disorder on the mechanism of high frequency sound attenuation. We introduce the
concept of frustration in structural glasses as a measure of the internal
stress, and find a strong correlation between the degree of frustration and the
exponent alpha that characterizes the momentum dependence of the sound
attenuation . In particular, alpha decreases from
about d+1 in low-frustration systems (where d is the spectral dimension), to
about 2 for high frustration systems like the realistic glasses examined.Comment: Revtex, 4 pages including 4 figure
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