No abstract

We perform a thorough analysis of the experimental dynamic structure function measured by inelastic neutron scattering for a low-temperature (Tϭ0.65 K͒ four-layer liquid 4 He film. The results are interpreted in light of recent theoretical calculations of the ͑nonvortex͒ excitations in thin liquid Bose films. The experimental system consists of four outer liquid layers, adsorbed to two solid inner 4 He layers, which are themselves adsorbed to a graphite substrate. Relatively intense surface ͑ripplon͒ and bulklike modes are observed. The analysis of the experimental data gives strong evidence for still other modes and supports the long-standing theoretical predictions of layerlike modes ͑layer phonons͒ associated with excitations propagating primarily within the liquid layers comprising the film. The results of the analysis are consistent with the occurrence of level crossings between modes, and the existence of a layer modes for which the theory predicts will propagate in the vicinity of the solid-liquid interface. The theory and experiment agree on the detailed nature of the ripplon; its dispersion at low momenta, its fall off in intensity at intermediate momenta, and the level crossings at high momentum. Similar to experiment, the theory yields an intense mode in the maxon-roton region which is intrepreted as the formation of the bulklike excitation.

Molecular-dynamics calculations have been used to explore and characterize the many-particle potential underlying the motion of particles in the monatomic liquid state. The potential used accurately represents metallic sodium at the density of the liquid at melt. It is found that the potential surface is composed of a large number of stable nearly harmonic valleys, and that these can be classified as random, symmetric, or crystalline. The random valleys cover by far the major portion of configuration space; they are macroscopically uniform, i.e., they all have the same structural potential and vibrational spectrum; and they all have microscopically irregular anharmonicity. The symmetric valleys lie at potential energies below the random valleys, but above the bcc crystalline valley. The symmetric valleys are not macroscopically uniform, but show scatter in their structural potentials and their eigenvalue spectra, and the symmetric valleys also have microscopically irregular anharmonicity. The equilibrium states of our system, from zero temperature up to and including the liquid states, fall into three groups, random, symmetric, and crystalline, according to which class of potential valley is mainly visited in the system motion. The random states are well separated from the symmetric and crystalline states, on the graph of mean potential energy versus temperature. The random states lie on a single line over the entire temperature range, and they include the liquid states, demonstrating that the random valleys dominate the statistical mechanics of the liquid. The present results provide detailed confirmation of the liquid-dynamics Hamiltonian previously used in equilibrium and nonequilibrium calculations. Further, the liquid-dynamics prediction of near equality of the log moment of the vibrational spectra, for the liquid and crystal at the same density, is verified here for the example of sodium.