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M Sampoli

Publications and source records attributed to M Sampoli.

11 recordsLinked to original sources
PubMed2005

Signatures of quantum behavior in the microscopic dynamics of liquid hydrogen and deuterium.

We discuss the microscopic dynamics and structure of liquid hydrogen and deuterium, as probed by inelastic x-ray scattering measurements. Samples are kept in corresponding thermodynamic conditions, at which classical systems are expected to exhibit the same dynamic and static responses. On the contrary, we observe clear differences revealing the onset of quantum deviations, both in the broadening of inelastic excitations and in the position of the first sharp diffraction peak. These features are discussed, compared to path-integral Monte Carlo simulations, and finally associated with the different de Broglie wavelengths of the two isotopes.

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PubMed2004

Collective dynamics in molten potassium: an inelastic x-ray scattering study.

The high-frequency collective dynamics of molten potassium has been investigated by inelastic x-ray scattering, disclosing an energy/momentum transfer region unreachable by previous inelastic neutron scattering (INS) experiments. We find that a two-step relaxation scenario, similar to that found in other liquid metals, applies to liquid potassium. In particular, we show how the sound velocity determined by INS experiments, exceeding the hydrodynamic value by approximately 30%, is the higher limit of a speedup, located in the momentum region 1 < Q < 3 nm(-1), which marks the departure from the isothermal value. We point out how this phenomenology is the consequence of a microscopic relaxation process that, in turn, can be traced back to the presence of "instantaneous" disorder, rather than to the crossover from a liquid to solidlike response.

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PubMed2003

Nonequilibrium thermodynamic description of the coupling between structural and entropic modes in supercooled liquids.

The density response of supercooled glycerol to an impulsive stimulated thermal grating (q=0.63 microm(-1)) has been studied in the temperature range (T=200-340 K) where the structure rearrangement (alpha relaxation) and the thermal diffusion occur on the same time scale. A strong interaction between the two modes occurs giving rise to a dip in the T dependence of the apparent thermal conductivity and a flattening of the apparent alpha-relaxation time upon cooling. A nonequilibrium thermodynamic model for the long time response has been developed. The model is capable to reproduce the experimental data and to explain the observed phenomenology.

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PubMed2001

Acoustic and relaxation processes in supercooled orthoterphenyl by optical-heterodyne transient grating experiment.

The dynamics of the fragile glass-forming orthoterphenyl have been investigated by transient grating experiments with an heterodyne detection technique. We measured the relaxation processes of this glass former over more than six decades in time with an excellent signal-to-noise ratio. Acoustic, structural, and thermal relaxations have been clearly identified in a time-frequency window not covered by previous spectroscopic studies and their characteristic dynamic parameters have been measured as a function of temperature and wave vector. A detailed comparison with the density response function, calculated on the basis of generalized hydrodynamic model, has been worked out.

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PubMed2001

Molecular dynamics simulation of the fragile glass former orthoterphenyl: a flexible molecule model. II. Collective dynamics.

We present a molecular dynamics study of the collective dynamics of a model for the fragile glass former orthoterphenyl. In this model, introduced by Mossa, Di Leonardo, Ruocco, and Sampoli [Phys. Rev. E 62, 612 (2000)], the intramolecular interaction among the three rigid phenyl rings is described by a set of force constants whose value has been fixed in order to obtain a realistic isolated molecule spectrum. The interaction between different molecules is described by a Lennard Jones site-site potential. We study the behavior of the coherent scattering functions F(t)(q,t), considering the density fluctuations of both molecular and phenyl-ring centers of mass; moreover we directly simulate the neutron scattering spectra taking into account both the contributions due to carbon and hydrogens atoms. We compare our results with the main predictions of the mode-coupling theory and with the available coherent neutron scattering experimental data.

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PubMed2000

Relaxation processes in harmonic glasses?

A relaxation process, with the associated phenomenology of sound attenuation and sound velocity dispersion, is found in a simulated harmonic Lennard-Jones glass. We propose to identify this process with the so-called microscopic (or, instantaneous) relaxation process observed in real glasses and supercooled liquids. A model based on the memory function approach accounts for the observation and allows one to relate to each other (1) the characteristic time and strength of this process, (2) the low frequency limit of the dynamic structure factor of the glass, and (3) the high frequency sound attenuation coefficient, with its observed quadratic dependence on the momentum transfer.

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PubMed2000

Molecular dynamics simulation of the fragile glass-former orthoterphenyl: 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.

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