Excitations in Quantum Boson Fluids

Abstract

Excitations in the superfluid ⁴He have been an interesting subject of studies for decades.’ Yet, recently precise measurements of excitations in thin films²have created new interest in the possible excitation mechanisms in reduced dimensions. A thin ⁴He film adsorbed on strongly binding substrates like lithium, solid hydrogen, magnesium or solid helium on graphite forms a pronounced layered structure with one atomic layer thickness.^3,4 The fluid within the first layer can well be approximated with a two-dimensional fluid. The energy gap between the lowest two single particle states perpendicular to the surface is between 7 and 13 Kelvins on these substrates. That makes it possible to observe clearly the collective, two-dimensional phonon - roton mode within the layer⁵ With the increasing layer density that mode turns into the ripplonlike surface mode. In the case of very thick films the lowest energy mode at small k is the ripplon mode and the higher lying layer modes merge into the three-dimensional phonon-roton mode.