Excitations in the quantum liquid⁴He: A review

Abstract

Progress made in measuring and interpreting the elementary excitations of superfluid and normal liquid \4he in the past 25 years is reviewed. The goal is to bring up to date the data, calculations and our understanding of the excitations since the books and reviews of the early 1990s. Only bulk liquid \4he is considered. Reference to liquid \3he, mixtures, reduced dimensions (films and confined helium) is made where useful to enhance interpretation. The focus is on the excitations as measured by inelastic neutron scattering methods. The review covers the dynamic response of liquid \4he from the collective excitations at low energy and long wavelength (i.e. phonon-roton modes) to the single particle excitations at high energy from which the atomic momentum distribution and Bose-Einstein condensate fraction are determined. A goal is to show the interplay of these excitations with other spectacular properties such as superfluidity and the test of fundamental calculations of quantum liquids that is possible. The role of Bose-Einstein condensation in determining the nature of the \pr~ mode and particularly it's temperature dependence is emphasized. The similarity of normal liquid \4he with other quantum and classical liquids is discussed. &#13.

Patterned Supersolids in Dipolar Bose Systems

“…Eq. 6, known as Bijl-Feynman formula, provides a reasonable qualitative and semi-quantitative account of the elementary excitation spectrum in superfluid 4 He, in particular of the presence and position of the roton minimum [41]. The behavior of the excitation spectrum as a function of σ is the same across all values of density investigated in our simulations.…”

High-Frequency Sound in a Unitary Fermi Gas

“…Inspired by studies of the excitations in liquid helium, we fit our experimental spectra to a damped harmonic oscillator function [33][34][35]…”

Microscopic dynamics of superfluid He4 : A comprehensive study by inelastic neutron scattering

“…Using optimized incident neutron energies reveals the complete evolution of the system, characterized by several multiexcitation branches merging progressively at high wave vectors to form a broad but rather intense feature. Intensity in this region was observed in early studies 4,17 , but the data where either strongly truncated 24,25,29 , or measured with low resolution 22 . This feature finally becomes, after a strong oscillation, a less intense branch progressively approaching the free particle parabolic dispersion.…”

“…Finally, at high energies, the dynamic structure factor gradually approaches a quasi-free-particle behavior 22 described by the impulse approximation 4,17,31 .…”