Be. Clements et al., EXCITATIONS IN A THIN LIQUID-HE-4 FILM FROM INELASTIC NEUTRON-SCATTERING, Physical review. B, Condensed matter, 53(18), 1996, pp. 12242-12252
We perform a thorough analysis of the experimental dynamic structure f
unction measured by inelastic neutron scattering for a low-temperature
(T=0.65 K) four-layer liquid He-4 film. The results are interpreted i
n light of recent theoretical calculations of the (nonvortex) excitati
ons in thin liquid Bose films. The experimental system consists of fou
r outer liquid layers, adsorbed to two solid inner He-4 layers, which
are themselves adsorbed to a graphite substrate. Relatively intense su
rface (ripplon) and bulklike modes are observed. The analysis of the e
xperimental data gives strong evidence for still other modes and suppo
rts the long-standing theoretical predictions of layerlike modes (laye
r phonons) associated with excitations propagating primarily within th
e liquid layers comprising the film. The results of the analysis are c
onsistent with the occurrence of level crossings between modes, and th
e existence of a layer modes for which the theory predicts will propag
ate in the vicinity of the solid-liquid interface. The theory and expe
riment agree on the detailed nature of the ripplon; its dispersion at
low momenta, its fall off in intensity at intermediate momenta, and th
e level crossings at high momentum. Similar to experiment, the theory
yields an intense mode in the maxon-roton region which is interpreted
as the formation of the bulklike excitation.