The Ballistics of Quasiparticles in 3He-B at Very Low Temperatures

Abstract

At temperatures below about Tc/4, the mean free paths of quasiparticles in superfluid 3He-B become extremely long. In consequence the quasiparticles behave entirely ballistically, making possible a number of simple but important experiments:(i) The damping of a vibrating wire measures the momentum density of the quasiparticles which is dominated by the exp (-Δ/kT) gap Boltzmann factor. The damping may be used as a direct 3He thermometer to measure a number of properties including, for example, the effective thermal conductance across a liquid to solid interface. This conductance also reflects the gap Boltzmann factor.(ii) A solid object can move through the liquid at low velocities without dissipation, but above a critical velocity produces quasiparticles by pair-breaking of the condensate seen as a sudden onset of damping.(iii) A supercritically-driven wire resonator thus acts as a quasiparticle source. The ballistic propagation of the quasiparticles can be detected on a distant (∼ mm) 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 observation of this quasiparticle wind yields direct information about the mean free paths and velocities of the excitations and also on the mechanism of the pair-breaking process. These quasiparticle beam methods open up a wide range of possible scattering experiments.