A high resolution (1.5 meV) inelastic neutron scattering experiment was carried out, aiming at an accurate investigation of the high frequency and low momentum dynamic response in heavy water. The experimental data confirm the existence of a dispersionless mode, besides the ordinary longitudinal collective dynamics. A simplified model, based on the interaction of two vibrational branches, is proposed to interpret the observed features of the dynamic spectra. The validity extent of this scheme is proved by applying it to room temperature neutron and x-ray data, to temperature and pressure dependent x-ray data, and to room temperature neutron data of vibrational density of states. The overall successfull results provided by this model, in conjunction with the combined analysis of the x-ray and neutron data on collective dynamics, enable a deeper insight into the complex mechanisms of the water dynamics and provide a simple phenomenological explanation for the transition from ordinary to fast sound.

We have determined for the first time the phonon density of states of a nonequilibrium fee Alo.94Sio.06 solid solution quenched under high pressure by means of inelastic neutron scattering and lowtemperature specific heat. When compared with aluminum, a large softening of the transverse acoustic modes is observed similar to that found in amorphous metals. This shows that a phonon softening can be found in nonequilibrium crystalline alloys. The nonequilibrium state is due to the metallic state of silicon atoms trapped in the fee lattice during quenching.PACS numbers: 63.20. Dj, 61.12.Ex, 62.50.+p, Recent experimental results on rapidly quenched Al:Si alloys ] strongly suggest that the dynamical properties of these crystalline alloys are strongly affected by rapid quenching from the melt. Aluminum and silicon are usually immiscible elements and to obtain an increase of the silicon solubility requires techniques like melt spinning or splat cooling. By the use of either rapid solidification 1 " 3 or quenching under high pressure, 4 ' 5 silicon atoms can be trapped in the fee aluminum matrix, with concentrations up to 18% for the latter preparation method. 5 Moreover, these alloys have been shown to be in a nonequilibrium state as a large irreversible exothermal effect [an enthalpy variation up to 4.1 kJ/mol (Ref. 1)] has been measured during the segregation of the solid solution. As no structural transition occurs in the fee aluminum matrix, this enthalpy difference has been attributed to the energy difference between the metallic state of silicon atoms in the fee lattice and their covalent state in the segregated clusters with diamond symmetry. The metallic state of silicon in the fee aluminum matrix is emphasized since the dissolution of a high concentration of silicon under high pressure requires a pressure of the same order of magnitude as the one used to reach the metallic state of bulk silicon 6 and since the atomic volume of the metallic silicon (15.5 A 3 for /?-Sn structure) is much closer to the aluminum atomic volume (16.6 A 3 ) than that of the covalent silicon (20 A 3 ). As indicated by a large increase of the superconducting transition temperature (up to 11 K for 18% of silicon in aluminum 5 ) and by a steeper slope of the temperature dependence of the resistivity at low temperature in the quenched state, the silicon atoms in the fee lattice induce an enhancement of the electron-phonon coupling. This effect could be accompanied by a lattice instability due to the nonequilibrium state and associated with a characteristic change of the phonon density of states. From a general point of view, techniques used to put metallic alloys in nonequilibrium states, like rapid solidification from the melt, thin-film deposition, and irradiation, 7 have been shown to induce large changes in the dynamical properties of metallic alloys. In amorphous metallic alloys, this effect is seen in the large enhancement of the low-frequency phonon density of states as compared to crystalline alloys. 8 " 10 Recently, measurements of...