QUASI-PARTICLE DYNAMICS IN SUPERFLUID HE-3

Excitations in many areas of condensed matter physics have dispersion relations which are very unlike those of simple newtonian objects. An unusual energy-momentum relation should lead to unconventional dynamics. Unfortunately most excitations are trapped in lattices or are otherwise inaccessible. One place where dynamic experiments are possible is in the superfluids, since here the excitations propagate freely in the superfluid condensate which we can regard as a mechanical vacuum. We can now cool superfluids, since here the excitations propagate freely in the superfluid condensate which we can regard as a mechanical vacuum. We can now cool superfluid He-3 to a temperature where the excitation mean free paths are long enough for dynamic experiments to be feasible, which implies temperatures of the order of 100-mu-K. Since the dispersion relation is tied to the rest frame of the condensate, it will be distorted by condensate motion, providing us with a way of influencing the dispersion curve. For example, a container of superfluid with a velocity gradient will have a range of excitation dispersion curves depending on the local velocity of the condensate. This means that the density of states available to the excitations will vary with position. We discuss the dynamical implications of this in terms of the behaviour of a vibrating wire resonator in the liquid, which shows unusual and often counter-intuitive behaviour. Among other things, for example, at a low enough temperature the force on a moving object in the excitation gas becomes independent of the velocity.