218 research outputs found
Comment on "Collective dynamics in liquid lithium, sodium, and aluminum"
In a recent paper, S. Singh and K. Tankeshwar (ST), [Phys. Rev. E
\textbf{67}, 012201 (2003)], proposed a new interpretation of the collective
dynamics in liquid metals, and, in particular, of the relaxation mechanisms
ruling the density fluctuations propagation. At variance with both the
predictions of the current literature and the results of recent Inelastic X-ray
Scattering (IXS) experiments, ST associate the quasielastic component of the
to the thermal relaxation, as it holds in an ordinary adiabatic
hydrodynamics valid for non-conductive liquids and in the limit. We
show here that this interpretation leads to a non-physical behaviour of
different thermodynamic and transport parameters.Comment: 4 pages, 1 figure, to appear in PRE (scheduled in 1 June issue
Heat capacity of liquids: A hydrodynamic approach
We study autocorrelation functions of energy, heat and entropy densities
obtained by molecular dynamics simulations of supercritical Ar and compare them
with the predictions of the hydrodynamic theory. It is shown that the predicted
by the hydrodynamic theory single-exponential shape of the entropy density
autocorrelation functions is perfectly reproduced for small wave numbers by the
molecular dynamics simulations and permits the calculation of the
wavenumber-dependent specific heat at constant pressure. The estimated
wavenumber-dependent specific heats at constant volume and pressure,
and , are shown to be in the long-wavelength limit in good agreement
with the macroscopic experimental values of and for the studied
thermodynamic points of supercritical Ar.Comment: 8 pages, 5 figure
Acoustic attenuation in glasses and its relation with the boson peak
A theory for the vibrational dynamics in disordered solids [W. Schirmacher,
Europhys. Lett. {\bf 73}, 892 (2006)], based on the random spatial variation of
the shear modulus, has been applied to determine the wavevector ()
dependence of the Brillouin peak position ( and width (),
as well as the density of vibrational states (), in disordered
systems. As a result, we give a firm theoretical ground to the ubiquitous
dependence of observed in glasses. Moreover, we derive a
quantitative relation between the excess of the density of states (the boson
peak) and , two quantities that were not considered related before.
The successful comparison of this relation with the outcome of experiments and
numerical simulations gives further support to the theory.Comment: To appear on PR
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.
Collective dynamics of liquid aluminum probed by Inelastic X-ray Scattering
An inelastic X-ray scattering experiment has been performed in liquid
aluminum with the purpose of studying the collective excitations at wavevectors
below the first sharp diffraction peak. The high instrumental resolution (up to
1.5 meV) allows an accurate investigation of the dynamical processes in this
liquid metal on the basis of a generalized hydrodynamics framework. The
outcoming results confirm the presence of a viscosity relaxation scenario ruled
by a two timescale mechanism, as recently found in liquid lithium.Comment: 8 pages, 7 figure
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
Inelastic X-ray scattering study of the collective dynamics in liquid sodium
Inelastic X-ray scattering data have been collected for liquid sodium at
T=390 K, i.e. slightly above the melting point. Owing to the very high
instrumental resolution, pushed up to 1.5 meV, it has been possible to
determine accurately the dynamic structure factor, , in a wide
wavevector range, nm, and to investigate on the dynamical
processes underlying the collective dynamics. A detailed analysis of the
lineshape of , similarly to other liquid metals, reveals the
co-existence of two different relaxation processes with slow and fast
characteristic timescales respectively. The present data lead to the conclusion
that: i) the picture of the relaxation mechanism based on a simple viscoelastic
model fails; ii) although the comparison with other liquid metals reveals
similar behavior, the data do not exhibit an exact scaling law as the principle
of corresponding state would predict.Comment: RevTex, 7 pages, 6 eps figures. Accepted by Phys. Rev.
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
Accessing Excited State Molecular Vibrations by Femtosecond Stimulated Raman Spectroscopy
Excited state vibrations are crucial for determining the photophysical and photochemical properties of molecular compounds. Stimulated Raman scattering can coherently stimulate and probe molecular vibrations with optical pulses, but it is generally restricted to ground state properties. Working under resonance conditions enables cross-section enhancement and selective excitation to a targeted electronic level but is hampered by an increased signal complexity due to the presence of overlapping spectral contributions. Here, we show how detailed information about ground and excited state vibrations can be disentangled by exploiting the relative time delay between Raman and probe pulses to control the excited state population, combined with a diagrammatic formalism to dissect the pathways concurring with the signal generation. The proposed method is then exploited to elucidate the vibrational properties of the ground and excited electronic states in the paradigmatic case of cresyl violet. We anticipate that the presented approach holds the potential for selective mapping of the reaction coordinates pertaining to transient electronic stages implied in photoactive compounds
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