Ion Motion and Vortex-Ring Formation in Pure Liquid He4 and He3-He4 Solutions Between 0.05 and 0.5 K

Abstract

Measurements of the behavior of charge carriers produced by $\alpha$ particles in pure liquid ${\mathrm{He}}^4$ and in ${\mathrm{He}}^3$-${\mathrm{He}}^4$ solutions in the temperature range 0.05-0.5 K are reported. Attempts to study very small vortex rings with less than 0.1-eV energy were not successful due to stray electrostatic fields. The origin of these fields was investigated in detail. Above 0.36 K negative ions under pressure in pure ${\mathrm{He}}^4$ can be accelerated to the Landau velocity for roton production, but it was found that they produce only vortex rings at lower temperatures. The mobilities of both positive and negative ions in ${\mathrm{He}}^3$-${\mathrm{He}}^4$ solutions show temperature dependences in disagreement with all previously proposed theories; however, the negative-ion mobility increases sharply below $T=0.1\mathrm{}$ K in a fashion similar to the predicted phonon-limited mobility. The scattering per ${\mathrm{He}}^3$ atom is not a simple function of ${\mathrm{He}}^3$ concentration and does not show any appreciable change as the solution varies from a dilute to a mildly degenerate Fermi system. At 0.2 K the pressure dependence of the negative-ion mobility is consistent with the change of bubble radius derived from other experiments, but at 0.08 K it is different.