Vortex nucleation in superfluidHe4probed byHe3impurities

Abstract

The critical velocity for the onset of phase slips in the flow of superfluid ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ through a micro-orifice has been found to be strongly affected at temperatures below ∼0.1 K by minute quantities of ${}_{}{}^3{}_{}{}^{}\mathrm{He}$ impurities, in the few parts per billion range. It is shown that this effect yields essential information on the vortex nucleation mechanism and in particular the local superfluid velocity at which vortices are formed, which is 22 m/s in these experiments.