DYNAMICS OF BOSON QUANTUM FILMS

We employ a quantitative microscopic theory of nonuniform quantum liqu ids to explore the excitations in thin films of He-4 adsorbed onto a s ubstrate. These liquid films studied undergo a series of structural ph ase transitions coinciding with the completion of individual atomic la yers. A generalized Feynman ansatz is used for the wave function of th e excited states; multiphonon effects are included by generalizing the Feynman theory to allow for time-dependent pair correlations. We stud y the dispersion relation, excitation mechanisms, transition densities , and particle currents, as a function of the surface coverage, includ ing coverages near the phase transitions. Because of the film's layere d growth, the sound velocity exhibits a series of minima and maxima. A pronounced long-wavelength softening of the lowest-energy mode is obs erved near the transitions. In the monolayer, the nature of the excita tions undergoes a noticeable change at the coverage where the velocity of sound starts to decrease. This is a crossover from ''essentially t wo-dimensional'' to ''essentially three-dimensional'' behavior. At lon g wavelengths, below and above the crossover coverage, the lowest-ener gy excitation is a longitudinal phonon (propagating within the monolay er) and a surface excitation, respectively. At shorter wavelengths, a layer-phonon propagating within the liquid layers, level crosses with a surface excitation to become the lowest-energy mode. For double- and higher-layer films the excitations are complicated by multiple (layer phonon with layer phonon and layer phonon with surface excitation) le vel crossings. At higher coverages, a mode is identifiable that will e volve into the bulk phonon-maxon roton. Our results agree qualitativel y with the available spectra obtained by neutron-scattering experiment s.