Quantum liquids.

This article is dedicated to Roger A Cowley and his seminal contributions to our understanding of quantum liquids, both liquidHe andHe. Roger Cowley's neutron scattering measurements of the collective and independent particle response of liquidHe were made at Chalk River Laboratories in 1965-74 chiefly with A D B (Dave) Woods. They measured the phonon-roton (P-R) mode energy, intensity and width with new precision. Particularly, they extended the measurements to higher wave vector and identified both collective and single particle response regimes. They showed that the P-R mode terminated at a finite energy as predicted by Pitaeskii rather than continuing as predicted by Feynman and Feynman and Cohen. They determined both the single P-R mode and multimode contributions to the dynamics. They made direct comparison with theory which Roger understood well. They observed the Bose-Einstein condensate (BEC) fraction in liquidHe for the first time. This appears to be the first ever observation of BEC in any Bose gas or liquid. Roger Cowley's pioneering measurements of the density excitations of liquidHe were made at the Institut Laue Langevin (ILL) in the period 1973-80. Roger, Reinhard Scherm, W G (Bill) Stirling and collaborators showed for the first time that the density response of this highly neutron absorbing liquid could indeed be observed with neutrons. They documented with others the dynamic response as a function of temperature and pressure stimulating extensive theoretical and experimental interest that continues today.