Quasiparticle Beam Experiments in Superfluid 3He

A. M. Guénault ； G. R. Pickett

《Chinese Journal of Physics》 26卷1S期 (1988/02) Pp. 71-79

並列摘要

In superfluid 3He-B at very low temperature the normal fluid density becomes very low and the mean free paths for scattering of the unpaired quasiparticles become very long. At the lowest temperatures which we can reach (about 100 μK at P=0 this mean free path is of the order of kilometers). As a consequence the quasiparticles behave entirely ballistically. This makes possible a large number of simple but interesting experiments.A solid object can move through the liquid at low velocities without dissipation, but at higher velocities produces quasiparticles by pairbreaking of the condensate, which can be seen experimentally as a sudden onset of damping at a critical velocity. A wire resonator driven above this critical velocity thus acts as a quasiparticle source. The ballistic propagation of the quasiparticles produced can be detected on a distant wire used as a quasiparticle-wind detector. The mechanical force on the detector wire leads to a small movement of the wire which can be measured with a SQUID.Our detection of this quasiparticle wind means that we can make direct quasiparticle beam experiments with what is essentially a quasiparticle spectrometer. Using such a device we can measure directly such straightforward quantities as the group velocity of the quasiparticles, and also the mean free path as a function of temperature. However, this arrangement of source and detector opens up a whole range of possible scattering experiments, for example, direct observation of Andreev reflection from various potential gradients. These phenomena have many analogies with the behaviour of quasiparticles in superconductors but here the fermion ”gas” is not embedded in a rigid lattice making a much wider range of propagation experiments possible.