Reflections on Excitations in Superfluid Helium

Abstract

A brief review of the excitations in liquid He⁴ is presented, with an emphasis on the interactions which were found to give two-roton resonances. The implications of this discovery were particularly relevant to the analysis of the Raman spectrum and neutron scattering experimental data. Later studies also demonstrated the inadequacy of the Born Approximation as a result of the resonant structure, and hence extensive theoretical progress was achieved by various groups in understanding the transport properties associated with phonons and rotons. Attempts to derive a truly microscopic description of the roton coupling encountered difficulties which were similar to those afflicting the meson scattering problem in nuclear physics. Nevertheless, semiphenomelogical inputs for the structure factor led to successful descriptions of the lowest order interactions, although the details of the resonances remain beyond the reach of current theory. Recent developments in the study of roton interactions as a function of pressure suggest directions for future studies.