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The equilibrium optical phonons of graphene are well characterized in terms of anharmonicity and electron–phonon interactions; however, their non-equilibrium properties in the presence of hot charge carriers are still not fully explored. Here we study the Raman spectrum of graphene under ultrafast laser excitation with 3 ps pulses, which trade off between impulsive stimulation and spectral resolution. We localize energy into hot carriers, generating non-equilibrium temperatures in the ~1700–3100 K range, far exceeding that of the phonon bath, while simultaneously detecting the Raman response. The linewidths of both G and 2D peaks show an increase as function of the electronic temperature. We explain this as a result of the Dirac cones’ broadening and electron–phonon scattering in the highly excited transient regime, important for the emerging field of graphene-based photonics and optoelectronics., Non-equilibrium ultrafast processes in graphene entail relaxation pathways involving electron–electron and electron–phonon scattering events. Here, the authors probe graphene optical phonons at high electronic temperatures by means of Raman spectroscopy under pulsed excitation

Summary form only given. Electron-phonon scattering and anharmonicity are the dominant mechanisms, that enable to describe the equilibrium phonon properties in graphene and Raman scattering is the main tool for their characterization. In the first tens fs after the photoexcitation, an out of equilibrium distribution of (hot) electron is induced with respect to the (cold) phonon bath. Within a few picoseconds, the fast electron-electron and electron-phonon non radiative recombination channels determine the equilibrium between the electronic distribution and the lattice. Therefore, on the laboratory timescale, continuous wave laser sources, commonly used for high resolution spontaneous Raman scattering, examine already equilibrated carrier-phonon distributions. A way to impulsively localize energy into graphene's electronic subsystem is provided by sub picosecond photoexcitation. While the behaviour of hot charge carriers to such ultrafast perturbation has been thoroughly elucidated unraveling the nature of optical phonons under a strongly out of equilibrium regime is a challenge. We perform spontaneous Raman measurements in graphene by using a 3-ps laser excitation, which is revealed to be a good agreement between impulsive stimulation and the necessity of spectral resolution. Furthermore, we show how the Raman response of graphene can be detected in presence of an electronic subsystem temperature largely exceeding that of the phonon bath. We find a peculiar behaviour of the period and lifetime of both G and 2D phonons as function of the carriers' temperature in the range 1700-3100 K, which is strongly suggestive of a smearing out of the Dirac cones. We rationalize such behaviour by accordingly revisiting the traditional theoretical modeling of the electron-phonon coupling in this highly excited transient scenario, which is critical in the emerging field of graphene-based nanophotonic and optoelectronic devices operating at THz rates.

We investigated by inelastic X-ray scattering the dynamical properties of molten potassium in a wide range of momentum transfer, Q, from 1 nm(-1) up to the main peak of the structure factor Q approximate to 17 nm(-1). The observed increase of sound velocity in the low Q region (1 < Q < 3 nm(-1)), has been described within a model characterized by two distinct relaxation processes for the collective dynamics. The obtained results are discussed and compared with those from previous neutron scattering experiments. In particular, we associate the speed-up of the sound velocity to the 'instantaneous' disorder of the liquid as opposed to the argument, supported by some neutron scattering studies, of a transition from a liquid to solid like response of the system. (c) 2007 Elsevier B.V. All rights reserved.

Recent progress in theoretical and simulation studies of collective excitations in supercritical fluids is reviewed. We discuss a methodology of fit-free estimation of dispersion of longitudinal and transverse excitations in simple fluids. The issue of vanishing positive sound dispersion--a viscoelastic increase of the speed of sound from adiabatic one to its high-frequency (elastic) value--with reduction of density, as it was observed in inelastic X-ray scattering experiments on supercritical Ar, is discussed from the point of view of finding distinctions in collective dynamics of low--and high-density supercritical fluids. On the basis of several theoretical models within the extended hydrodynamic description of density-density correlations in liquids analytical expressions for positive sound dispersion are obtained and applied for analysis of time correlation functions obtained from molecular dynamics simulations. A location of a crossover from the liquid-like to gas-like types of collective dynamics is discussed based on general findings for spectra of collective excitations in supercritical Ar and soft sphere fluids.

The idea of the timescale invariance of relaxation processes in liquids (Yulmetyevetal 2001Phys. Rev. E64 057101; 2002JETPLett.76 147) is used to analyse the short-wave collectiv ee xcitation in liquid sodium, as recently measured by means of very-high-energy-resolution inelastic x-ray scattering (Scopigno et al 2002 Phys. Rev. E 65 031205). The dynamic structure factor, S(Q ,ω ), calculated on the basis of this idea is in very good agreement with the experimental data in the wavevector range from 1.5 to 14. 6n m −1 ,w here pronounced collective excitations exist. The frequency dependence of the non∗

Acoustics dynamics of the molecular glass-former indomethacin (IMC) have been investigated by Brillouin light scattering (BLS) at GHz frequencies. Elastic response of the system has been tracked from the melting temperature down to the glass transition through the supercooled liquid. Both the structural arrest and the vibrational dynamics are described by modeling the experimentally determined dynamic structure factor within the framework of the Langevin equation, through a simplified choice of memory function which allows one to determine sound velocity and the acoustic attenuation coefficient as a function of temperature. The density fluctuation spectra in the glassy phase, as probed by BLS, are compared with time-domain results from photoacoustics experiments. The arising scenario is discussed in the context of current literature reporting inelastic X-ray scattering and BLS in platelet geometry. The link between the probed elastic properties and the non-ergodicity factor of the glass phase is finally scrutinized.

The sound attenuation in the THz region is studied down to T 16 K in glassy glycerol by inelastic x-ray scattering. At striking variance with the decrease found below 100 K in the GHz data, the attenuation in the THz range does not show any T dependence. This result (i) indicates the presence of two different attenuation mechanisms, active, respectively, in the high- and low-frequency limits, (ii) demonstrates the nondynamic origin of the attenuation of THz sound waves, and confirms a similar conclusion obtained in SiO2 glass by molecular dynamics, and (iii) supports the low-frequency attenuation mechanism proposed by Fabian and Allen [ Phys. Rev. Lett. 82, 1478 (1999)].

We present new inelastic X-ray scattering data for the dynamic structure factor S(Q, ω) of liquid lithium collected at two different temperatures (T = 475K and T = 600K) and in a wide range of exchanged wave-vectors (Q = 1 nm−1 to Q=110nm−1). The analysed Q range covers the transition from collective to single-particle regimes. The spectra have been put on an absolute scale using its first sum rules, and the result obtained has been tested on diffraction data and novel molecular dynamie results. A relaxation fingerprint is observed at low wave-vectors (the so-called positive dispersion of the apparent sound speed), while at higher Q the single particle behaviour gradually emerges.

Density-driven phase transformations are a known phenomenon in liquids. Pressure-driven transitions from an open low-density to a higher-density close-packed structure were observed for a number of systems. Here, we show a less intuitive, inverse behavior. We investigated the electronic, atomic, and dynamic structures of liquid Rb along an isothermal line at 573 K, at 1.2-27.4 GPa, by means of ab initio molecular dynamics simulations and inelastic x-ray scattering experiments. The excellent agreement of the simulations with experimental data performed up to 6.6 GPa validates the overall approach. Above 12.5 GPa, the breakdown of the nearly-free-electron model drives a transition of the pure liquid metal towards a less metallic, denser liquid, whose first coordination shell is less compact. Our study unveils the interplay between electronic, structural, and dynamic degrees of freedom along this liquid-liquid phase transition. In view of its electronic nature, we believe that this behavior is general for the first group elements, thus shedding new light into the high-pressure properties of alkali metals.

The high frequency dynamics of Indomethacin and Celecoxib glasses has been investigated by inelastic x-ray scattering, accessing a momentum-energy region still unexplored in amorphous pharmaceuticals. We find evidence of phonon-like acoustic dynamics and determine the THz behavior of sound velocity and acoustic attenuation. Connections with ordinary sound propagation are discussed, along with the relation between fast and slow degrees of freedom as represented by non-ergodicity factor and kinetic fragility, respectively.

The pressure evolution of the sound velocity in liquid water in the terahertz regime, ${c}_{\ensuremath{\infty}}$, between 0.05 and 0.88 GPa, at room temperature, has been investigated by synchrotron inelastic x-ray scattering in a diamond anvil cell. We confirm previous results showing that ${c}_{\ensuremath{\infty}}$ increases with density much less than the adiabatic sound velocity ${c}_{s}$, which is reasonably related to the known structural modifications in the hydrogen bond network. At variance with a previous study where an anomaly was found in the density evolution of ${c}_{\ensuremath{\infty}}$---most likely due to the nonisothermal character of the study---the present work reveals a smooth behavior of ${c}_{\ensuremath{\infty}}$, which could provide a useful constraint to the current theories on liquid water.

In contrast to crystalline solids in which structural order governs dynamics and thermodynamics, the lack of long-range periodicity in amorphous materials is responsible for several anomalies. Although the relation between these anomalies and the 'bulk structure' is generally understood, the surface structure and the corresponding vibrational spectrum of amorphous solids is practically an unexplored theme. In this study, we resolve the differences in vibrational dynamics and atomic structure between bulk and surface (top 5 nm) atoms of amorphous selenium. We combine experimental (grazing incidence inelastic X-ray scattering) and computational (ab initio and semiempirical molecular orbital theoretical calculations) methods to scrutinize a variety of possible structural models. We find that a high concentration of particular types of 'coordination defects' in the surface layer is responsible for the observed differences. Resolving the structure of amorphous surfaces is, for example, important for understanding nanoparticles' properties where the surface-to-bulk ratio has a crucial role.

According to textbook definitions1, there exists no physical observable able to distinguish a liquid from a gas beyond the critical point, and hence only a single fluid phase is defined. There are, however, some thermophysical quantities, having maxima that define a line emanating from the critical point, named 'the Widom line'2 in the case of the constant-pressure specific heat. We determined the velocity of nanometric acoustic waves in supercritical fluid argon at high pressures by inelastic X-ray scattering and molecular dynamics simulations. Our study reveals a sharp transition on crossing the Widom line demonstrating how the supercritical region is actually divided into two regions that, although not connected by a first-order singularity, can be identified by different dynamical regimes: gas-like and liquid-like, reminiscent of the subcritical domains. These findings will pave the way to a deeper understanding of hot dense fluids, which are of paramount importance in fundamental and applied sciences.

We report on the study of the slow dynamics of supercooled and molten Selenium by means of dynamic light scattering. To achieve this in the present case, where the opaqueness of Se prevents visible light scattering measurements, we have developed a novel experimental setup using infrared radiation as the light source. By measuring the scattered intensity autocorrelation function we have been able to extract the average size of the Se clusters in the liquid state and determine the temperature dependence of the related diffusion coefficient which was found to exhibit Arrhenius behavior.

We present an experimental setup to study terahertz dynamics in fluids under high pressure, employing inelastic x-ray scattering and diamond anvil cell techniques. The use of a carefully designed vacuum chamber and the minimization and control of sources of parasitic scattering allowed circumventing previous limitations due to important empty cell contributions to the scattering signal. The successful implementation of our setup is demonstrated in the case of supercritical fluid argon, for which a full viscoelastic analysis yields the dispersion relation of sound waves, the generalized heat capacity ratio, and longitudinal viscosity. Our results are in excellent agreement with available experimental observables and molecular dynamics simulations.

Inelastic x-ray scattering (IXS) measurements of the dynamic structure factor in liquid Na57K43, sensitive to the atomic-scale coarse graining, reveal a sound velocity value exceeding the long wavelength, continuum value and indicate the coexistence of two phonon-like modes. Applying Generalized Collective Mode (GCM) analysis scheme, we show that the positive dispersion of the sound velocity occurs in a wavelength region below the crossover from hydrodynamic to atom-type excitations and, therefore, it can not be explained as sound propagation over the light specie (Na) network. The present result experimentally proves the existence of positive dispersion in a binary mixture due to a relaxation process, as opposed to fast sound phenomena., Comment: 7 pages, 8 figures, to appear in "Physical Review B"

Owing to their large relatively thermal conductivity, peculiar, nonhydrodynamic features are expected to characterize the acousticlike 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 nonconductive fluids is observed. Implications for high frequency transport properties in metallic fluids are discussed. (C) 2008 American Institute of Physics.

A Comment on the Letter by B. Ruffl\'e et al., Phys. Rev. Lett. 96, 045502 (2006). The authors of the Letter offer a Reply.

The high frequency dynamics of fluid oxygen have been investigated by Inelastic X-ray Scattering. In spite of the markedly supercritical conditions ($T\approx 2 T_c$, $P>10^2 P_c$), the sound velocity exceeds the hydrodynamic value of about 20%, a feature which is the fingerprint of liquid-like dynamics. The comparison of the present results with literature data obtained in several fluids allow us to identify the extrapolation of the liquid vapor-coexistence line in the ($P/P_c$, $T/T_c$) plane as the relevant edge between liquid- and gas-like dynamics. More interestingly, this extrapolation is very close to the non metal-metal transition in hot dense fluids, at pressure and temperature values as obtained by shock wave experiments. This result points to the existence of a connection between structural modifications and transport properties in dense fluids., 4 pages, 3 figures, accepted by Phys. Rev. Lett

Using Inelastic X-ray Scattering we studied the collective dynamics of the glassy alloy Ni$_{33}$Zr$_{67}$ in the first pseudo Brillouin zone, an energy-momentum region still unexplored in metallic glasses. We determine key properties such as the momentum transfer dependence of the sound velocity and of the acoustic damping, discussing the results in the general context of recently proposed pictures for acoustic dynamics in glasses. Specifically, we demonstrate the existence in this strong glass of well defined (in the Ioffe Regel sense) acoustic-like excitations well above the Boson Peak energy., 4 pages, 4 .eps figures, accepted in Phys. Rev. Lett

The dynamic structure factor of the polycrystalline plastic crystal 1-cyanoadamantane has been measured in the THz frequency region by inelastic X-ray scattering as a function of the exchanged momentum Q in the range 1–10 nm � 1 and as a function of the temperature in the two solid disordered phases: plastic crystal and glassy crystal. We find that the dispersion of the acoustic excitations is crystal-like in the two phases and that the attenuation C(Q) shows a negligible T dependence similarly to structural glasses and a Q a behavior with 1.15 < a < 1.50 at variance with structural glasses characterized by a Q 2 dependence.

The collective dynamics of liquid Gallium close to the melting point has been studied using Inelastic X-ray Scattering to probe lengthscales smaller than the size of the first coordination shell. %(momentum transfers, $Q$, $>$15 nm$^{-1}$). Although the structural properties of this partially covalent liquid strongly deviate from a simple hard-sphere model, the dynamics, as reflected in the quasi-elastic scattering, are beautifully described within the framework of the extended heat mode approximation of Enskog's kinetic theory, analytically derived for a hard spheres system. The present work demonstrates the applicability of Enskog's theory to non hard- sphere and non simple liquids., 5 pages, 2 figures, accepted in Phys. Rev. Lett

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 neutron scattering experiments (INS). 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 $\approx 30 %$, is the higher limit of a speed up, located in the momentum region $1, Comment: 7 pages, 5 .eps figures. To appear in J. Chem. Phys

The dynamic structure factor, S(Q,w), of vitreous silica, has been measured by inelastic X-ray scattering in the exchanged wavevector (Q) region Q=4-16.5 nm-1 and up to energies hw=115 meV in the Stokes side. The unprecedented statistical accuracy in such an extended energy range allows to accurately determine the longitudinal current spectra, and the energies of the vibrational excitations. The simultaneous observation of two excitations in the acoustic region, and the persistence of propagating sound waves up to Q values comparable with the (pseudo-)Brillouin zone edge, allow to observe a positive dispersion in the generalized sound velocity that, around Q=5 nm-1, varies from 6500 to 9000 m/s: this phenomenon was never experimentally observed in a glass., Comment: 5 pages, 3 figures. To appear in Phys. Rev. B

The dynamic structure factor of glassy and liquid glycerol has been measured by inelastic X-ray scattering in the exchanged momentum Q region Q = 2-23 nm(-1) and in the temperature range 80-570 K. Besides the propagating longitudinal mode, at low temperatures the spectra show a second non-Q-dispersing peak at hOhm(T) approximate to 8.5 meV. This peak is the signature of the transverse dynamics which, in topologically disordered systems, acquire a small Q-dependent longitudinal symmetry component. This assignment is substantiated by a study of the temperature dependence of the intensity of the second peak. This study showed that, in the liquid, the second peak intensity vanishes when the structural relaxation time tau(alpha) approaches Ohm(T)(-1), a behaviour consistent with the condition tau(alpha)Ohm(T) much greater than1 required for the existence of a non-relaxational transverse-like dynamics in the liquid state.

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 $S(Q,\omega)$ to the thermal relaxation, as it holds in an ordinary adiabatic hydrodynamics valid for non-conductive liquids and in the $Q \to 0$ 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)

The recently developed inelastic X-ray scattering technique opens a new kinematic region at the observation of molecular liquids vibrational dynamics over the lengthscales of the interparticle separation. We illustrate the capabilities of this powerful technique through the results obtained from liquid glycerol. A detailed analysis of the high-frequency vibrational dynamics of this system at ambient temperature is reported. New results in the study of structural relaxational dynamics in the high-temperature range (up to T = 560 K) are also discussed.

The high frequency dynamics of glassy Selenium has been studied by Inelastic X-ray Scattering at beamline BL35XU (SPring-8). The high quality of the data allows one to pinpoint the existence of a dispersing acoustic mode for wavevectors ($Q$) of $1.5, Comment: Phys. Rev. Lett. (Accepted)

The improvement in performance of the inelastic x-ray scattering technique has led to accurate determination of the dynamic structure factors of glassy and liquid glycerol, in the region of exchanged momentum Q = 2-23 nm(-1) and in the temperature range 80-570 K. Thanks to the improved statistical accuracy, it has been possible to identify, in the spectra at the lower temperatures, besides the propagating longitudinal mode, a second non-Q-dispersing peak at (h) over bar Omega(T) approximate to 8.5 meV. We interpret this peak as a signature of the transverse dynamics that, in topologically disordered systems, acquires a longitudinal symmetry component. This assignment is substantiated by the observation that this peak still survives, across the glass transition, in the liquid state, and vanishes when the structural relaxation time tau(alpha) approaches Omega(T)(-1): a behaviour consistent with the condition tau(alpha)Omega(T) >> 1 required for the existence of a non-relaxational transverse-like dynamics in the liquid state.

Inelastic X-ray Scattering measurements of the dynamics structure factor have bene performed on vitreous Beryllium Fluoride ({\it v-}BeF$_2$) at $T$=297 K in the momentum transfer, $Q$, range $Q$=1.5$\div$10 nm$^{-1}$. We find evidence of well defined high frequency acoustic modes. The energy position and linewidth of the excitations disperse with $Q$ as $\propto Q$ and $\propto Q^2$, respectively up to about one half of the first maximum of the static structure factor. Their magnitude compare favorably with low-frequency sound velocity and absorption data. The results indicate worth mentioning similarities of the high frequency collective dynamics of different network forming glasses such as {\it v-}B$_2$O$_3$ and {\it v-}SiO$_2$., Comment: 17 pages, 5 .ps figs

The collective dynamics of the so-called ‘simple’ liquids (in particular, liquid metals) are a good paradigm to illustrate the capabilities of inelastic X-ray scattering (IXS). In particular, it is shown how the present accuracy of the data can be exploited to provide not only a precise determination of the main spectral features (dispersion relations), but also a clue for the physical assessment of the microscopic relaxation mechanisms underlying the dynamics of the liquid state. Starting from new IXS data on liquid sodium as an example, we apply a generalized hydrodynamic framework in order to describe the details of the experimental lineshapes. A comparison with recent data on liquid lithium and aluminium is also reported: although quantitative differences indicate that the principle of corresponding states seems not to be applicable, in all these systems a simple viscoelastic approach turns out to be inadequate to reproduce the experimental data.

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, $S(Q,\omega)$, in a wide wavevector range, $1.5 \div 15$ nm$^{-1}$, and to investigate on the dynamical processes underlying the collective dynamics. A detailed analysis of the lineshape of $S(Q,\omega)$, 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. E

Inelastic X-ray scattering (IXS) is a novel spectroscopic technique that recently came up in addition to inelastic neutron scattering (INS), allowing one a determination of the microscopic dynamics in the Thz frequency region and over lengthscales comparable to the mean interparticle separations. The experimental basis of this method will be illustrated, along with a recent selection of examples on its application to the study of the high-frequency collective modes in liquids and glasses, emphasizing the relative merits and drawbacks with respect of INS. In particular, it will be shown how on the snapshot timescale accessed by IXS both the dynamics of the glassy and of the liquid state exhibit similar features and can be described within the same formalism.

Umklapp processes are known to exist in cristalline materials, where they control important properties such as thermal conductivity, heat capacity and electrical conductivity. In this work we report the provocative observation of Umklapp processes in a non-periodical system, namely liquid Lithium. The lack of a well defined periodicity seems then not to prevent the existence of these scattering processes mechanisms provided that the local order of the systems i.e. the maxima of the static structure factor supply the equivalent of a reciprocal lattice vector in the case of cristalline materials., 13 pages PS

The dynamical structure factor (S(Q,E)) of vitreous silica has been measured by Inelastic X-ray Scattering varying the exchanged wavevector (Q) at fixed exchanged energy (E) - an experimental procedure that, contrary to the usual one at constant Q, provides spectra with much better identified inelastic features. This allows the first direct evidence of Brillouin peaks in the S(Q,E) of SiO_2 at energies above the Boson Peak (BP) energy, a finding that excludes the possibility that the BP marks the transition from propagating to localised dynamics in glasses., 4 pages, 3 Postscript figures. To appear in Physical Review Letters

The dynamic structure factor, S(Q,E), of liquid lithium (T=475 K) has been determined by inelastic x-ray scattering (IXS) in the momentum transfer region (Q = 1.4-110 nm-1). These data allow to observe how, in a simple liquid, a phonon-like collective mode evolves towards the single particle dynamics. As a function of Q, one finds: i) at low Q's, a sound mode with a positive dispersion of the sound velocity, ii) at intermediate Q's, excitations whose energy oscillates similarly to phonons in the crystal Brillouin zones, and iii) at high Q's, the S(Q,E) approaches a Gaussian shape, indicating that the single particle dynamics has been reached., Comment: 3 pages and 5 figures

New inelastic X-ray scattering experiments have been performed in liquid lithium at two different temperatures: T=475 K (slightly above the melting point) and 600 K. Taking advantage of the absence of any kinematical restriction and incoherent contribution, and pushing the instrumental resolution up to 1.5 meV, it was possible to perform an accurate investigation of the dynamic structure factor S(Q,E) in the wavevector range from 1 to 110 nm^-1. For Q smaller than Q_m=25 nm^-1, the position of the main peak of the static structure factor, a detailed analysis of the lineshapes shows that any picture of the relaxation mechanisms based on a simple viscoelastic model must be abandoned. All the spectral features can instead satisfactorily be accounted for by including both fast and slow relaxation processes. The physical origin of the slow relaxation is associated to the structural rearrangement, while the local nature of the fast one is extensively discussed. At larger Q values a gradual crossover from the strongly correlated to single particle dynamics occurs, with an important weight provided by quantum effects., Comment: 25 pages, 12 PS figures

Coherent inelastic neutron and light (Raman and Brillouin) scattering measurements are repbrted on silica xerogels over a wide range of densities and temperatures. Raman scattering results show the presence of a boson peak at temperatures where the quasielastic contribution is negligible. The boson peak of xerogels is at a frequency around 35cm−1 and has a shape similar to that of melted silica. At lower frequencies (7–16cm−1) another bump is visible in the Raman scattering, believed to be connected to the presence of the pores and its frequency connected to their mean sizes. This important result seems to indicate that the disorder introduced by the presence of the pores does not influence very much the density of states in the frequency range of the boson peak. A comparison is made between the Q dependence of the elastic and inelastic structure factors as a function of sample density and temperature. The absence of a peak in the inelastic structure factor for the Q value corresponding to the fi...

New temperature dependent inelastic x-ray (IXS) and Raman (RS) scattering data are compared to each other and with existing inelastic neutron scattering data in vitreous silica (v-SiO_2), in the 300 - 1775 K region. The IXS data show collective propagating excitations up to Q=3.5 nm^-1. The temperature behaviour of the excitations at Q=1.6 nm^-1 matches that of the boson peak found in INS and RS. This supports the acoustic origin of the excess of vibrational states giving rise to the boson peak in this glass., 10 pages and 4 figures

The collective dynamics of a simple metal (lithium) have been numerically investigated by molecular dynamics and normal mode analysis. A generalized hydrodynamic framework allowed us to describe the system in different thermodynamic phases by the same model based on two relaxation processes. At the melting point we find good agreement with recent inelastic X-ray scattering results, while below the melting point, and ultimately in the glassy state, we find evidences of a “harmonie” relaxation as recently found in Lennard-Jones systems.