Positive Ion Critical Velocity for Nucleation of Quantized Vortices in Pure 4He

The critical velocity for the vortex nucleation of positive ions is determined experimentally in isotopically purified 4He at temperatures as low as 50 mK. Systematic IV characteristic measurements for a two-dimensional positive ion pool at a depth of 37.6 nm from the surface are carried out with extremely fine control of driving electric fields. The critical velocity of similar to 32 ms(-1) at 500 mK decreases with decrease in temperature and approaches a temperature-independent value of similar to 18 ms(-1) below 200 mK. The decrease in critical velocity corresponds to the increase in nucleation rate. The temperature dependence of critical velocity is qualitatively attributed to the "superohmic" macroscopic quantum tunneling of the Caldeira-Leggett theory. The reduction in tunneling rate with increase in temperature is evidence for quantum friction.