Current situation in the superfluid He-3 collective mode study: experimentand theory

For a long time the study of the collective modes (CMs) has been an essenti al part of the experiments in superfluid He-3. But at present such kinds of experiments in superfluid He-3 are practically stopped while many very int eresting features of the CM spectrum still remain unexplored by experiment. There are a few causes for this. One is the discovery of heavy fermion and high-temperature superconductors, the investigation of which drew away man y scientists. From the other side most of the basic experiments with CMs ha ve been done and the next step is to study the fine structure like the spli tting of the CM spectra by electric and magnetic fields, and by dispersion, superflow, rotation and other perturbations. This step requires much longe r time to prepare the experiments as well as much more skills and luck. We analyze the difficulties that arise in experiments and discuss some old and new effects in CMs in superfluid 3He which should be studied experimentall y. The purpose of this paper is to call experimentalists to renew the ultra sound experiments in superfluid He-3 by attracting their attention to new p ossibilities in this area. (C) 2000 Published by Elsevier Science B.V. All rights reserved.