THE NUCLEATION OF VORTICES IN SUPERFLUID HE-4 - ANSWERS AND QUESTIONS

The velocity threshold for phase slips, upsilon(c), has been measured in ultra-pure He-4 and ultra-dilute He-3-He-4 mixtures, down to a temperature of 15 mK. These experiments have revealed a crossover from a temperature-dependent regime above approximately 150 mK to a plateau of upsilon(c) below in ultra-pure He-4. Concentrations of He-3 impurities as low as a few parts in 10(-9) greatly affect the plateau regime, causing upsilon(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 approximately 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.