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Within this context, the main idea of this Special issue was to bring together experimental, theoretical and simulation groups in order to present the recent developments in experimental techniques, new possibilities of experimental studies, different methods of analysing the experimental data, recently elaborated theoretical approaches and modern simulation techniques - all focused on collective processes in liquids. Fifteen research groups from Italy, Germany, France, Japan, Spain, Great Britain, United States, Korea and Ukraine report in this volume the current studies and small reviews of their experimental/theoretical methodology.

Guest editors: T.Bryk & T.Scopigno

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Title: Inelastic neutron scattering applied to the investigation of collective excitations in topologically disordered matter
Author(s):

J.-B.Suck (Institute of Physics, University of Technology Chemnitz, D-09107 Chemnitz, Germany) ,

Inelastic neutron scattering techniques are introduced here as one of the most important experimental techniques in the investigation of collective excitations in fluids (liquids and compressed gases) and amorphous solids. The correlation functions involved, the spectra of which are determined in inelastic neutron scattering experiments, the dispersion relations of the collective excitations and how they are obtained from the measured spectra and finally two of the most often used instrumental techniques are briefly discussed.

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Title: BRISP, a new small-angle time-of-flight neutron spectrometer to study collective dynamics in disordered matter
Author(s):

	A.Orecchini (Department of Physics, University of Perugia, I-06123 Perugia, Italy ) ,
	W.-C.Pilgrim (Physical Chemistry, Philipps-University of Marburg, D-35032 Marburg, Germany ) ,
	C.Petrillo (Department of Physics, University of Perugia, I-06123 Perugia, Italy ) ,
	J.-B.Suck (Institute of Physics, Technical University of Chemnitz, D-09126 Chemnitz, Germany ) ,
	F.Sacchetti (Department of Physics, University of Perugia, I-06123 Perugia, Italy ) ,

An outline is given of the new neutron Brillouin Spectrometer BRISP which was installed during the last years and recently became fully operational at the Institute Laue-Langevin in Grenoble (France). The main technical features are described and some of the first experimental results are presented, demonstrating the present capabilities of this new-type inelastic small-angle spectrometer. Future upgrades are foreseen and expected to further improve the instrument performances.

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Title: The history of the "fast sound" in liquid water
Author(s):

	G.Ruocco (Dipartimento di Fisica, Universita' di Roma "La Sapienza", Piazza Aldo Moro 2, I-00185 Roma, Italy ) ,
	F.Sette (European Synchrotron Radiation Facility, B.P. 220 F-38043 Grenoble, Cedex France) ,

This paper aims to discuss the present understanding of the high frequency dynamics in liquid water, with particular attention to a specific phenomenon - the so-called fast sound - since its first appearance in the literature up to its most recent explanation. A particular role in this history is played by the inelastic x-ray scattering (IXS) technique, which - with its introduction in the middle '90- allowed to face a large class of problems related to the high frequency dynamics in disordered materials, such as glass and liquids. The results concerning the fast sound in water obtained using the IXS technique are here compared with the inelastic neutron scattering (INS) and molecular dynamics simulation works. The IXS work has allowed us to demonstrate experimentally the existence of two branches of collective modes in liquid water: one linearly dispersing with the momentum (apparent sound velocity of ≈3200 m/s, the "fast sound") and the other at almost constant energy (5..7 meV). It has been possible to show that the dispersing branch originates from the viscoelastic bend up of the ordinary sound branch. The study of this sound velocity dispersion, marking a transition from the ordinary sound, c_o to the "fast sound", c_∞, as a function of temperature, has made it possible to relate the origin of this phenomenon to a structural relaxation process, which presents many analogies to those observed in glass-forming systems. The possibility to estimate from the IXS data the value of the relaxation time, τ, as a function of temperature leads to relating the relaxation process to the structural re-arrangements induced by the making and breaking of hydrogen bonds. In this framework, it is then possible to recognize an hydrodynamical "normal" regime, i. e. when one considers density fluctuations whose period of oscillation is on a timescale long with respect to τ, and a solid-like regime in the opposite limit. In the latter regime, the density fluctuations feel the liquid as frozen and the sound velocity is much higher: this is "fast sound" whose value is equivalent to the sound velocity found in crystalline ice.

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Title: Water dynamics at the nanoscale
Author(s):

	C.Masciovecchio (Sincrotrone Trieste, S.S. 14 km 163,5 in AREA Science Park 34012 Basovizza, Trieste, Italy) ,
	F.Bencivenga (Sincrotrone Trieste, S.S. 14 km 163,5 in AREA Science Park 34012 Basovizza, Trieste, Italy) ,
	A.Gessini (Sincrotrone Trieste, S.S. 14 km 163,5 in AREA Science Park 34012 Basovizza, Trieste, Italy) ,

The recent construction of an Inelastic UltraViolet Scattering (IUVS) beamline at the ELETTRA Synchrotron Light Laboratory opens new possibilities for studying the density fluctuation spectrum, S(Q,E), of disordered systems in the mesoscopic momentum (Q) and energy (E) transfer region not accessible by other spectroscopic techniques. As an example of possible application of IUVS technique we will discuss the new insights provided in the case of water dynamics. From the analysis of IUVS spectra we were able to measure the temperature and pressure dependencies of structural relaxation time (τ) in water. In the case of room-pressure water the values of τ, as derived by IUVS, are fairly consistent with previous determinations and, most important, its temperature dependence agrees with Mode Coupling Theory (MCT) predictions. Moreover we found that τ decreases with increasing pressure at fixed temperature. The observed trend demonstrates that the structural relaxation phenomenology is strongly affected by the applied pressure. However, further investigations are necessary in order to clarify the physical meaning of these preliminary experimental results.

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Title: Nanometer-scale dynamics of high-temperature levitated liquids
Author(s):

	D.L.Price (Centre de Recherche sur les Matériaux á Haute Température, 1d avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France ) ,
	L.Hennet (Centre de Recherche sur les Matériaux á Haute Température, 1d avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France ) ,
	I.Pozdnyakova (Centre de Recherche sur les Matériaux á Haute Température, 1d avenue de la Recherche Scientifique, 45071 Orléans cedex 2, France ) ,
	M.-L.Saboungi (Centre de Recherche sur la Mati\`ere Divisée, 1b rue de la Férollerie, 45071 Orléans cedex 2, France) ,

We review recent measurements of the microscopic dynamics of high-temperature liquids made with inelastic x-ray scattering and aerodynamic levitation of the samples. Phenomenological and theoretical interpretations are also discussed.

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Title: Inelastic X-ray scattering experiments at extreme conditions: high temperatures and high pressures
Author(s):

S.Hosokawa (Center for Materials Science using Third-Generation Synchrotron Radiation Facilities, Hiroshima Institute of Technology, 2-1-1 Miyake, Saeki-ku, Hiroshima 731-5193, Japan; Institut für Physikalische-, Kern-, und Makromolekulare Chemie, Philipps Universität Marburg, Hans-Meerwein Str., 35032 Marburg, Germany) ,

In this article, we review the present status of experimental techniques under extreme conditions of high temperature and high pressure used for inelastic X-ray scattering (IXS) experiments of liquid metals, semiconductors, molten salts, molecular liquids, and supercritical water and methanol. For high temperature experiments, some types of single-crystal sapphire cells were designed depending on the temperature of interest and the sample thickness for the X-ray transmission. Single-crystal diamond X-ray windows attached to the externally heated high-pressure vessel were used for the IXS experiment of supercritical water and methanol. Some typical experimental results are also given, and the perspective of IXS technique under extreme conditions is discussed.

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Title: Experimental techniques of high-resolution inelastic X-ray scattering measurements for supercritical metallic fluids at high temperature and high pressure using synchrotron radiation at SPring-8
Author(s):

	M.Inui (Graduate School of Integrated Arts and Sciences, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8521, Japan ) ,
	D.Ishikawa (SPring-8/RIKEN 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan ) ,
	K.Matsuda (Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan ) ,
	K.Tamura (Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan ) ,
	A.Q.R.Baron (SPring-8/RIKEN 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan; SPring-8/JASRI 1-1-1 Kouto, Sayo-cho, Sayo-gun, Hyogo 679-5198, Japan ) ,

We have conducted high-resolution inelastic X-ray scattering (IXS) measurements of supercritical metallic fluids at high temperature and high pressure using synchrotron radiation at SPring-8 in Japan. We developed a high-pressure vessel for IXS measurements pressurized by He gas up to 196 MPa and installed the high-pressure gas apparatus in the experimental hutch. After the installation IXS experiments were carried out for supercritical fluid Hg to study atomic dynamics in the metal-nonmetal transition near the liquid-vapor critical point. Next the apparatus is used to study atomic dynamics in the semiconductor-metal transition in supercritical fluid Se. In this article we report the experimental technique at high temperature and high pressure in detail, and show recent results using the technique.

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Title: Multiple time scales in the microscopic dynamics of simple and complex liquids as studied by radiation scattering
Author(s):

	F.J.Bermejo (Instituto de Estructura de la Materia, C.S.I.C. and Unidad Asociada de Investigación en Física Aplicable, Dept. Electricidad y Electónica, Facultad de Ciencia y Tecnología, Universidad del País Vasco/ Euskal Herriko Unibertsitatea, P.O.Box 644, E-48080 Bilbao, Spain ) ,
	F.Fernandez-Alonso (ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire OX11 0QX, United Kingdom; Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, United Kingdom) ,
	C.Cabrillo (Instituto de Estructura de la Materia, C.S.I.C. and Unidad Asociada de Investigación en Física Aplicable, Dept. Electricidad y Electónica, Facultad de Ciencia y Tecnología, Universidad del País Vasco/ Euskal Herriko Unibertsitatea, P.O.Box 644, E-48080 Bilbao, Spain ) ,

Experimental investigations into the time-scales spanned by the microscopic dynamics of simple (metallic) and molecular liquids, as explored by neutron scattering and muon spin rotation experiments are reviewed.

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Title: Collective dynamics in noble-gas and other very simple classical fluids
Author(s):

	U.Bafile (Istituto dei Sistemi Complessi, Consiglio Nazionale delle Ricerche, via Madonna del Piano 10, I-50019 Sesto Fiorentino, Italy ) ,
	F.Barocchi (Dipartimento di Fisica, Universitá di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy; CNR-INFM CRS-Soft c/o Dipartimento di Fisica, Universitá di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy ) ,
	E.Guarini (Dipartimento di Fisica, Universitá di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy; CNR-INFM CRS-Soft c/o Dipartimento di Fisica, Universitá di Firenze, via G. Sansone 1, I-50019 Sesto Fiorentino, Italy ) ,

Rare gases and their liquids are the simplest systems to study for accurate investigations of the collective dynamics of fluid matter. Much work has been done using different spectroscopic techniques, molecular-dynamics simulations, and theoretical developments, in order to gain insight into the microscopic processes involved, in particular, in the propagation of acoustic excitations in gases and liquids. Here we briefly review the interpretation schemes currently applied to the characterization of such excitations, and recall a few results obtained from the analysis of rare-gas fluids and other very simple systems.

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Title: Ion density fluctuations in liquid metals: the strongly interacting ion-electron plasma
Author(s):

	L.E.Bove (Department Physique des Milieux Denses CNRS-IMPMC, Universit\`e Paris 6, F-75015 Paris, France ) ,
	C.Petrillo (Dipartimento di Fisica, Universitá di Perugia, I-06123 Perugia, Italy and CNR-INFM CRS-Soft, Roma) ,
	F.Sacchetti (Dipartimento di Fisica, Universitá di Perugia, I-06123 Perugia, Italy and CNR-INFM CRS-Soft, Roma) ,

An unified description of liquid metals dynamics based on an interacting two-component model for the ion-electron plasma is tempted. The propagation velocity of the collective modes in alkali and polyvalent metals, derived from inelastic neutron and x-rays scattering experiments, is compared with the estimates obtained by different theoretical approximations for the strongly-interacting plasma. Using accurate results for the electron gas correlation energy, and taking into account the ion finite size effects, a good overall description of a large set of liquid metals is obtained. The observed trend for the damping of the collective modes in liquid metals is also discussed within the framework of the two-component description.

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Title: Collective dynamics of simple liquids: A mode-coupling description
Author(s):

	W.Schirmacher (Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA; Physik-Department E13, Technische Universität München, James-Franck-Strasse 1, D-85747 Garching, Germany ) ,
	H.Sinn (Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA; Hasylab/DESY, Notkestrasse 85, D-22607 Hamburg, Germany) ,

We use the mode-coupling theory (MCT), which has been highly successful in accounting for the anomalous relaxation behaviour near the liquid-to-glass transition, for describing the dynamics of simple (i.e. monatomic) liquids away from the glass formation regime. We find that the dynamical structure factor predicted by MCT compares well to experimental findings and results of computer simulations. The memory function exhibits a two-step decay as found frequently in experimental and simulation data. The long-time relaxation regime, in which the relaxation rate strongly depends on the density and is identified as the α relaxation. At high density this process leads the glass instability. The short-time relaxation rate is fairly independent of density.

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Title: Structural relaxation in pure liquids: Analysis of wavenumber dependence within the approach of generalized collective modes
Author(s):

	T.Bryk (Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine, 1 Svientsitskii St., 79011 Lviv, Ukraine; Institute of Applied Mathematics and Fundamental Sciences Lviv Polytechnic National University, 12 Bandery Str., 79013 Lviv, Ukraine ) ,
	I.Mryglod (Institute for Condensed Matter Physics, National Academy of Sciences of Ukraine, 1 Svientsitskii St., 79011 Lviv, Ukraine; Institute of Applied Mathematics and Fundamental Sciences Lviv Polytechnic National University, 12 Bandery Str., 79013 Lviv, Ukraine ) ,

Wavenumber dependence of structural relaxation in liquids is studied by the method of generalized collective modes (GCM). A new perturbation approach within the GCM method is proposed and applied in the long-wavelenth limit in order to obtain analytical expressions for the wavenumber-dependent structural relaxation and sound dispersion within a simplified three-variable dynamical model. Analytical results are compared with numerical study of generalized modes within a more general five-variable dynamical model, which accounts for thermal processes in liquids. Numerical results are presented for four thermodynamic points of Lennard-Jones fluid at the reduced temperature T^*=1.71. We discuss the features of non-hydrodynamic process of structural relaxation in different regions of wavenumbers.

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Title: Microscopic dynamics in liquid binary alloys: orbital-free \textsl{ab-initio} molecular dynamics studies
Author(s):

	D.J.González (Departamento de Física Teórica, Universidad de Valladolid, 47011 Valladolid, Spain) ,
	L.E.González (Departamento de Física Teórica, Universidad de Valladolid, 47011 Valladolid, Spain) ,

We report an ab-initio molecular dynamics study on the collective excitations in several s-p bonded liquid binary alloys. Results are reported for the Li-Na, Li-Mg, K-Cs and Li-Ba liquid alloys at different concentrations, which display mass ratios ranging from ≈3 for Li-Na to ≈20 for Li-Ba, and varying ordering tendencies, ranging from strong homocoordinating in Li-Na to mildly heterocoordinating for one concentration of Li-Ba. The study has been carried out using the orbital free {\em ab-initio} molecular dynamics method, combined with local ionic pseudopotentials constructed within the same framework. We analyze the dependence of the collective modes on the concentration and the mass ratio of the alloy, finding a common behaviour for all systems notwithstanding the different ordering tendencies.

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Title: Atomic dynamics of alumina melt: A molecular dynamics simulation study
Author(s):

	S.Jahn (GeoForschungsZentrum Potsdam, Department 4, Telegrafenberg, 14473 Potsdam, Germany) ,
	P.A.Madden (Chemistry Department, University of Edinburgh, Edinburgh EH9 3JJ, U.K. ) ,

The atomic dynamics of Al₂O₃ melt are studied by molecular dynamics simulation. The particle interactions are described by an advanced ionic interaction model that includes polarization effects and ionic shape deformations. The model has been shown to reproduce accurately the static structure factors S(Q) from neutron and x-ray diffraction and the dynamic structure factor S(Q,ω) from inelastic x-ray scattering. Analysis of the partial dynamic structure factors shows inelastic features in the spectra up to momentum transfers, Q, close to the principal peaks of partial static structure factors. The broadening of the Brillouin line widths is discussed in terms of a frequency dependent viscosity η(ω).

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Title: An attempt toward the generalized Langevin dynamics simulation
Author(s):

	B.Kim (Department of Physics, Changwon National University, Changwon 641-773, Korea; Department of Theoretical Studies, Institute for Molecular Science, Okazaki 444-8585, Japan) ,
	S.-H.Chong (Department of Theoretical Studies, Institute for Molecular Science, Okazaki 444-8585, Japan) ,
	R.Ishizuka (Department of Theoretical Studies, Institute for Molecular Science, Okazaki 444-8585, Japan) ,
	F.Hirata (Department of Theoretical Studies, Institute for Molecular Science, Okazaki 444-8585, Japan) ,

An attempt to generalize the Langevin dynamics simulation method is presented based on the generalized Langevin theory of liquids, in which the dynamics of both solute and solvent is treated by the generalized Langevin equations, but the integration of the equation of motion of solute is made in the manner similar to the ordinary molecular dynamics simulation with discretized time steps along a trajectory. A preliminary result is derived based on an assumption of the uniform solvent density. The result is regarded to be a microscopic generalization of the phenomenological Langevin theory for the harmonic oscillator immersed in a continuum solvent developed by Wang and Uhlenbeck.