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Density fluctuations in simple liquids are analysed in the context of three different and widely used formalisms, whose equivalence in the hydrodynamic limit is shown. We, furthermore, address the issue of the dispersion of the propagating modes outside the hydrodynamics, by comparing three different definitions of the generalized sound velocity. The first definition is standard in statistical mechanics. It relates the sound velocity to the imaginary part of the complex conjugate poles of the so-called intermediate scattering function. Other definitions, frequently used in the literature, identify the characteristic frequencies of the inelastic excitations with the maxima of the inelastic features of the dynamic structure factor, or with the maxima of the current function. The behaviour of these three quantities in the hydrodynamic limit is discussed. Deviations from hydrodynamic dispersion law are also considered with particular emphasis given to the analysis of different sound propagation regimes related to different density fluctuations decay channels.

Coherent artefact is a well-known nonlinear process plaguing ultrafast time-resolved experiments. Here, a Femtosecond-Stimulated-Raman-Scattering based approach, able to disentangle genuine dynamics from coherent artefacts, is introduced for demonstrating optically-driven femtosecond enhancement of the exchange interaction. © 2020 The Author(s)

Monitoring the interactions between electronic and vibrational degrees of freedom in molecules is critical to our understanding of their structural dynamics. This is typically hampered by the lack of spectroscopic probes able to detect different energy scales with high temporal and frequency resolution. Coherent Raman spectroscopy can combine the capabilities of multidimensional spectroscopy with structural sensitivity at ultrafast timescales. Here, we develop a three-color-based 2D impulsive stimulated Raman technique that can selectively probe vibrational mode couplings between different active sites in molecules by taking advantage of resonance Raman enhancement. Three temporally delayed pulses generate nuclear wave packets whose evolution reports on the underlying potential energy surface, which we decipher using a diagrammatic approach enabling us to assign the origin of the spectroscopic signatures. We benchmark the method by revealing vibronic couplings in the ultrafast dynamics following photoexcitation of the green fluorescent protein.

The "Frenkel line" (FL), the thermodynamic locus where the time for a particle to move by its size equals the shortest transverse oscillation period, has been proposed as a boundary between recently discovered liquid-like and gas-like regions in supercritical fluids. We report a simulation study of isothermal supercritical neon in a range of densities intersecting the FL. Specifically, structural properties and single-particle and collective dynamics are scrutinized to unveil the onset of any anomalous behavior at the FL. We find that (i) the pair distribution function smoothly evolves across the FL displaying medium-range order, (ii) low-frequency transverse excitations are observed below the "Frenkel frequency", and (iii) the high-frequency shear modulus does not vanish even for low-density fluids, indicating that positive sound dispersion characterizing the liquid-like region of the supercritical state is unrelated to transverse dynamics. These facts critically undermine the definition of the FL and its significance for any relevant partition of the supercritical phase.

Author(s): Batignani, G; Pontecorvo, E; Bossini, D; Ferrante, C; Fumero, G; Cerullo, G; Mukamel, S; Scopigno, T | Abstract: Illuminating a magnetic material with femtosecond laser pulses induces complex ultrafast dynamical processes. The resulting optically detectable response usually contains contributions from both the optical properties and the magnetic degrees of freedom. Disentangling all the different components concurring to the generation of the total signal is a major challenge of contemporary experimental solid-state physics. Here, this problem is tackled, addressing the purely optical, nonmagnetic artifacts on the time resolved two-magnon stimulated Raman spectrum of an antiferromagnet, rationalizing the recent observation on the exchange energy modification upon photo-excitation. It is demonstrated how the genuine dynamics of the magnetic eigenmode can be disentangled from the nonlinear optical effects, generated by cross phase modulation, on the femtosecond timescale. The introduced approach can be extended for the investigation of l100nfs dynamic processes by means of coherent Ramannscattering.

Femtosecond stimulated Raman scattering is a time-resolved vibrational spectroscopic technique able to access sub-picosecond dynamics and providing accurate structural information. Thanks to an appropriate combination of three laser pulses, triggering vibrational coherences delayed with respect to the photoinduced event of interest, it is capable of uncompromised temporal precision (down to 50 fs) and spectral resolution (a few wavenumbers), better than spontaneous Raman. Reaching such extreme time scales requires significant temporal overlap of pulses, which gives rise to undesired nonlinear artifacts, often hampering an immediate interpretation of the Raman spectra. Building on a perturbative expansion of the density matrix, we identify the origin of such artifacts in cross phase modulation effects and show how they can be theoretically evaluated and factored out from the signals. We experimentally benchmark the theoretical predictions in a nonreactive model system, namely cyclohexane.

A proposal for building a new Free Electron Laser facility at the Laboratori Nazionali di Frascati, EuPRAXIA@SPARC_LAB, is at present under consideration. This FEL facility will exploit plasma acceleration to produce ultra-bright photon pulses with durations of few femtoseconds down to the wavelengths between 2 and 4 nm, in the so called “water window”. The main class of experiments to be performed will include coherent diffraction imaging, soft X-ray absorption spectroscopy, Raman and photofragmentation measurements. In this article we present the updates on the photon beamlines design for the facility.

In a previous letter and other papers, we have interpreted the supercritical region of fluids as being parted into a gas-like and a liquid-like regimes. The boundary between these two distinct regions of the P-T phase diagram of substances is represented by the Widom line, extending the subcritical liquid-gas 1st order transition line above the critical point. Out authors have been commenting on our interpretation, claiming that the two regions are parted by the so called Frenkel line, which does not reconnect to the subcritical liquid-gas phase boundary. In this work, we show on a solid theoretical basis that the Frenkel line is incorrect and that indeed the Widom line is the relevant boundary between to two supercritical regimes.

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

Spontaneous Raman spectroscopy is a formidable tool to probe molecular vibrations. Under electronic resonance conditions, the cross section can be selectively enhanced enabling structural sensitivity to specific chromophores and reaction centers. The addition of an ultrashort, broadband femtosecond pulse to the excitation field allows for coherent stimulation of diverse molecular vibrations. Within such a scheme, vibrational spectra are engraved onto a highly directional field and can be heterodyne detected overwhelming fluorescence and other incoherent signals. At variance with spontaneous resonance Raman, however, interpreting the spectral information is not straightforward, due to the manifold of field interactions concurring to the third order nonlinear response. Taking as an example vibrational spectra of heme proteins excited in the Soret band, we introduce a general approach to extract the stimulated Raman excitation profiles from complex spectral lineshapes. Specifically, by a quantum treatment of the matter through density matrix description of the third order nonlinear polarization, we identify the contributions which generate the Raman bands, by taking into account for the cross section of each process.

Femtosecond stimulated Raman experiments on the antiferromagnetic system KNiF3 are implemented to understand how the exchange interaction — a crucial interaction that rules magnetic phenomena — is influenced by ultrafast optical excitation. Manipulating the macroscopic phases of solids using ultrashort light pulses has resulted in spectacular phenomena, including metal–insulator transitions1,2,3, superconductivity4 and subpicosecond modification of magnetic order5. The development of this research area strongly depends on the understanding and optical control of fundamental interactions in condensed matter, in particular the exchange interaction. However, disentangling the timescales relevant for the contributions of the exchange interaction and spin dynamics to the exchange energy, Eex, is a challenge. Here, we introduce femtosecond stimulated Raman scattering to unravel the ultrafast photo-induced dynamics of magnetic excitations at the edge of the Brillouin zone. We find that femtosecond laser excitation of the antiferromagnet KNiF3 triggers a spectral shift of the two-magnon line, the energy of which is proportional to Eex. By unravelling the photo-induced modification of the two-magnon line frequency from a dominating nonlinear optical effect, we find that Eex is increased by the electromagnetic stimulus.

The theory of elasticity with spatially fluctuating elastic constants (heterogeneous-elasticity theory) is reviewed. It is shown that the vibrational anomalies associated with the boson peak can be qualitatively and quantitatively explained in terms of this theory. Two versions of a mean-field theory for solving the stochastic equation of motion are presented: the coherent-potential approximation (CPA) and the self-consistent Born approximation (SCBA). It is shown that the latter is included in the former in the Gaussian and weak-disorder limit. We are able to discuss and explain cases in which the change of the vibrational spectrum by varying an external parameter can be accounted for by changing the Debye frequency (elastic transformation) and cases in which this is not possible. In the latter case a change in the distribution of the elastic moduli has occurred.

Glasses are out-of-equilibrium systems aging under the crystallization threat. During ordinary glass formation, the atomic diffusion slows down rendering its experimental investigation impractically long, to the extent that a timescale divergence is taken for granted by many. We circumvent here these limitations, taking advantage of a wide family of glasses rapidly obtained by physical vapor deposition directly into the solid state, endowed with different "ages" rivaling those reached by standard cooling and waiting for millennia. Isothermally probing the mechanical response of each of these glasses, we infer a correspondence with viscosity along the equilibrium line, up to exapoise values. We find a dependence of the elastic modulus on the glass age, which, traced back to temperature steepness index of the viscosity, tears down one of the cornerstones of several glass transition theories: the dynamical divergence. Critically, our results suggest that the conventional wisdom picture of a glass ceasing to flow at finite temperature could be wrong., Comment: 4 figures and 1 supplementary figure

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 experimental results relevant for the understanding of the microscopic dynamics in liquid metals are reviewed, with special regards to the ones achieved in the last two decades. Inelastic Neutron Scattering played a major role since the development of neutron facilities in the sixties. The last ten years, however, saw the development of third generation radiation sources, which opened the possibility of performing Inelastic Scattering with X rays, thus disclosing previously unaccessible energy-momentum regions. The purely coherent response of X rays, moreover, combined with the mixed coherent/incoherent response typical of neutron scattering, provides enormous potentialities to disentangle aspects related to the collectivity of motion from the single particle dynamics. If the last twenty years saw major experimental developments, on the theoretical side fresh ideas came up to the side of the most traditional and established theories. Beside the raw experimental results, therefore, we review models and theoretical approaches for the description of microscopic dynamics over different length-scales, from the hydrodynamic region down to the single particle regime, walking the perilous and sometimes uncharted path of the generalized hydrodynamics extension. Approaches peculiar of conductive systems, based on the ionic plasma theory, are also considered, as well as kinetic and mode coupling theory applied to hard sphere systems, which turn out to mimic with remarkable detail the atomic dynamics of liquid metals. Finally, cutting edges issues and open problems, such as the ultimate origin of the anomalous acoustic dispersion or the relevance of transport properties of a conductive systems in ruling the ionic dynamic structure factor are discussed., 53 pages, 41 figures, to appear in "The Review of Modern Physics". Tentatively scheduled for July issue

Despite that the thermodynamic distinction between a liquid and the corresponding gas ceases to exist at the critical point, it has been recently shown that reminiscence of gaslike and liquidlike behavior can be identified in the supercritical fluid region, encoded in the behavior of hypersonic waves dispersion. By using a combination of molecular dynamics simulations and calculations within the approach of generalized collective modes, we provide an accurate determination of the dispersion of longitudinal and transverse collective excitations in soft-sphere fluids. Specifically, we address the decreasing rigidity upon density reduction along an isothermal line, showing that the positive sound dispersion, an excess of sound velocity over the hydrodynamic limit typical for dense liquids, displays a nonmonotonic density dependence strictly correlated to that of thermal diffusivity and kinematic viscosity. This allows rationalizing recent observation parting the supercritical state based on the Widom line, i.e., the extension of the coexistence line. Remarkably, we show here that the extremals of transport properties such as thermal diffusivity and kinematic viscosity provide a robust definition for the boundary between liquidlike and gaslike regions, even in those systems without a liquid-gas binodal line. Finally, we discuss these findings in comparison with recent results for Lennard-Jones model fluid and with the notion of the "rigid-nonrigid" fluid separation lines.

This paper describes the scientific aims and potentials as well as the preliminary technical design of RUDE, an innovative tool for multi-disciplinary investigations in a wide field of scientific, technological and industrial applications. IRIDE will be a high intensity "particles factory", based on a combination of high duty cycle radio-frequency superconducting electron linacs and of high energy lasers. Conceived to provide unique research possibilities for particle physics, for condensed matter physics, chemistry and material science, for structural biology and industrial applications, IRIDE will open completely new research possibilities and advance our knowledge in many branches of science and technology. [RIDE is also supposed to be realized in subsequent stages of development depending on the assigned priorities. (C) 2013 Elsevier B.V. All rights reserved.

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)].

The lack of long-range structural order in amorphous solids induces well known thermodynamic anomalies, which are the manifestation of distinct peculiarities in the vibrational spectrum. Although the impact of such anomalies vanishes in the long wavelength, elastic continuum limit, it dominates at length scales comparable to interatomic distances, implying an intermediate transition regime still poorly understood. Here we report a study of such mesoscopic domains by means of a broadband version of picosecond photo-acoustics, developed to coherently generate and detect hypersonic sound waves in the sub-THz region with unprecedented sampling efficiency. We identify a temperature-dependent fractal v3/2 frequency behaviour of the sound attenuation, pointing to the presence of marginally stable regions and a transition between the two above mentioned limits. The essential features of this behaviour are captured by a theoretical approach based on random spatial variation of the shear modulus, including anharmonic interactions., Characterizing the acoustic dynamics of glasses at mesoscopic wavelengths is challenging. Here, Ferrante et al. use the broadband picosecond acoustics technique to study attenuation at such length scales, where they find temperature-dependent, fractal frequency behaviour.

Editors: T. Scopigno,1 S. De Panfilis1,2, and A. Di Cicco,3,4 1 Dipartimento di Fisica, Universita di Roma “La Sapienza”, I-00185 Roma (Italy). 2 Centro Studi e Ricerche e Museo Storico della Fisica E. Fermi, I-00184 Roma (Italy). 3 CNISM, Sezione di Fisica, Scuola di Scienze e Tecnologie, Universita di Camerino, I-62032 Camerino (MC), Italy (permanent address). 4Institut de Mineralogie et de Physique des Milieux Condenses, UMR 7590, CNRS, Universites Paris 6 et Paris 7, Paris, France.

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.

The behaviour of high-frequency acoustic excitations in disordered materials is one of the most provocative and less understood aspects of glass science. The frequency dependence of sound damping, in particular, is the result of the interplay of several physical mechanisms crucial for many theoretical models. The emerging picture suggests the existence of a frequency crossover in sound attenuation identifying a transition from a macroscopic continuum-like behavior, due to anharmonic damping effects, to a microscopic regime in which the dynamics is dominated by topological disorder, characteristic of the amorphous phase. The very nature of this hypothetic crossover, its frequency position and its relation to other anomalies of the amorphous state is highly debated.

4 páginas, 3 figuras.-- PACS number(s): 64.70.kj, 61.05.C-, 63.50.-x, 64.70.Q-.-- et al., The connection between viscous flow and vibrational properties in glass-forming materials is scrutinized examining the fragility of a wide set of liquids and the nonergodicity factor of the corresponding glasses. Building on the same line of reasoning which allows us to extend the connection between viscosity and thermodynamics in complex systems, we show here how the two quantities are strongly correlated once the effect of those secondary relaxation processes due to internal degrees of freedom is correctly accounted for. This result provides a missing thermodynamic rationale for the recently debated universality of the correlation between fast and slow degrees of freedom., T.S. acknowledges support from the European Research Council under the European Community Seventh Framework Program (FP7/2007-2013)/ERC IDEAS Grant No. 207916.

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.

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"

We investigate a $d$-dimensional model ($d$ = 2,3) for sound waves in a disordered environment, in which the local fluctuations of the elastic modulus are spatially correlated with a certain correlation length. The model is solved analytically by means of a field-theoretical effective-medium theory (self-consistent Born approximation) and numerically on a square lattice. As in the uncorrelated case the theory predicts an enhancement of the density of states over Debye's $\omega^{d-1}$ law (``boson peak'') as a result of disorder. This anomay becomes reinforced for increasing correlation length $\xi$. The theory predicts that $\xi$ times the width of the Brillouin line should be a universal function of $\xi$ times the wavenumber. Such a scaling is found in the 2d simulation data, so that they can be represented in a universal plot. In the low-wavenumber regime, where the lattice structure is irrelevant there is excellent agreement between the simulation at small disorder. At larger disorder the continuum theory deviates from the lattice simulation data. It is argued that this is due to an instability of the model with stronger disorder., Comment: 5 pages, 3 figures, to be published in physica status solidi (c) March 2008

Developing a novel experimental technique, we applied photon correlation spectroscopy using infrared radiation in liquid sulfur around ${T}_{\ensuremath{\lambda}}$, i.e., in the temperature range where an abrupt increase in viscosity by 4 orders of magnitude is observed upon heating within few degrees. This allowed us---overcoming photoinduced and absorption effects at visible wavelengths---to reveal a chain relaxation process with characteristic time in the millisecond range. These results do rehabilitate the validity of the Maxwell relation in sulfur from an apparent failure, allowing rationalizing of the mechanical and thermodynamic behavior of this system within a viscoelastic scenario.

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 ($k$) dependence of the Brillouin peak position ($\Omega_k)$ and width ($\Gamma_k$), as well as the density of vibrational states ($g(\omega)$), in disordered systems. As a result, we give a firm theoretical ground to the ubiquitous $k^2$ dependence of $\Gamma_k$ observed in glasses. Moreover, we derive a quantitative relation between the excess of the density of states (the boson peak) and $\Gamma_k$, 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 PRL

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 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

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 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)

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 $Q$ 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., 7 pages, 7 .eps figs. To appear in Phys. Rev. E

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 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., LaTex format, 11 pages, 4 EncapsulatedPostScript figures

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.

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

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 to relate to each others: 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., 11 pages, 3 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

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

in the 100 GHz range: A broadband picosecond acoustic approach’” †Appl. Phys. Lett. 98, 246101 „2011...‡ E. Pontecorvo, C. Ferrante, M. Ferretti, M. Ortolani, D. Polli, G. Ruocco, G. Cerullo, and T. Scopigno Dipartimento di Fisica, Universita’ Roma “Sapienza,” P.le Aldo Moro 2, 00185 Roma, Italy IFN–CNR, Via Cineto Romano 42, 00156 Rome, Italy IFN–CNR and Dipartimento di Fisica, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy