SUPERFLUID HE-3 AT VERY-LOW TEMPERATURES - A VERY UNUSUAL EXCITATION GAS

The excitation gas in superfluid He-3 at low temperatures shows a numb er of remarkable dynamical properties arising from the unusual dispers ion curve. The existence of an energy gap leads to many of the observe d properties varying rapidly with temperature, since the excitation de nsity is dominated by the gap Boltzmann factor exp(-DELTA/kT). But als o, the fact that the minimum energy lies at finite momentum gives rise to Andreev scattering processes, in which the velocity of the excitat ion is reversed but the momentum left virtually unchanged. Since the d ispersion curve looks different to a moving observer, there is the pos sibility of the free production of quasiparticle-quasihole pairs at a Landau critical velocity. At low temperatures the mean free path becom es much larger than any experimental size. Using vibrating wire resona tors as universal probes, we can monitor the temperature, measure the Kapitza resistance, examine the nonlinear regime beyond the two-fluid model, observe the Landau velocity, create and detect thermal beams of excitation with black-body radiators, observe Andreev reflection dire ctly and probe A-phase textures (in which the gas is one-dimensional). Future possibilities are discussed.