Introduction

Abstract

This symposium was designed to report and to discuss recent research into excitations in quantum fluids such as phonons, rotons, maxons, plateaus, vortices, ripplons, and local modes in films, mainly in superfluid ⁴He (He II). Many new interesting experimental results were reported, which were obtained by various methods such as neutron scattering, Raman scattering, quantum evaporation, time-of-flight measurements of moving ions and mobility measurements of ions and electrons. This symposium emphasized the dynamic correlations in He II, which are characterized by correlation functions. The elementary excitations are defined as the poles of the single particle Green’s function G(k, ω). By neutron inelastic scattering, the dynamic structure factor S(k, ω) is measured, which is the two-body correlation function of density fluctuations. Raman scattering measures the four-body correlation function of density fluctuations H(k, k′, ω). Properties of these correlation functions and their mutual relations were discussed both theoretically and experimentally. Ab initio calculations of S(k, ω) were reviewed and a few models of temperature dependence of S(k, ω) were proposed. The coupling of G(k, ω) and S(k, ω) was pointed out as a key effect of Bose condensation on the dynamic properties of He II. Theories of the Raman scattering of He II were reviewed from various aspects. The process of scattering was clarified by the observation of the s-wave contribution. The fine structures in the Raman spectrum, confirmed in recent experiments, stimulated theoretical research into the relation between S(k, ω) and H(k, k′, ω) as well as discussion of the relation between density fluctuations and quasiparticle operators. The importance of not only roton-roton but also roton-maxon and maxon-maxon interactions was stressed. In the following, short summaries of the papers are given.