The nucleation of vortices in superfluid4He: Answers and questions

The velocity threshold for phase slips, v_c, has been measured in ultra-pure⁴He and ultra-dilute³He-⁴He mixtures, down to a temperature of 15 mK. These experiments have revealed a crossover from a temperature-dependent régime above 150 mK to a plateau of v_c below in ultra-pure⁴He. Concentrations of³He impurities as low as a few parts in 10^–9 greatly affect the plateau régime, causing v_c to decrease markedly at low temperature. These observations are interpreted in the framework of the nucleation, either thermally activated or by quantum tunnelling, of vortices in the approximate shape of half-rings. These vortices form on wall asperities at local velocities u_s estimated to be 22 m/s in these experiments. The half-ring model is shown to yield a critical velocity of the same magnitude but leaves many basic questions unanswered.