QUANTUM NUCLEATION OF VORTICES IN THE FLOW OF SUPERFLUID HE-4 THROUGH AN ORIFICE

Flow measurements in ultrapure He-4 through a micron-size orifice at millikelvin temperatures show, for the first time, the transition from thermal to quantum nucleation of nanometer-size vortices below a crossover temperature of 0.147 K. These observations establish the close relationship between this type of critical flow and negative-ion motion in superfluid 4He and strongly suggest that the underlying mechanism, the nucleation of vortex half rings, is identical.