Effect of ³He impurities on the nucleation of phase slips in superfluid ⁴He

The critical value of the dc current of superfluid ⁴He through a micro-aperture is found to decrease at low temperatures in the presence of ³He impurities. We measure the dependence of the critical current on the pressure difference across the micro-aperture. In the temperature region where the ³He affected decrease in the critical current is detected, we measure a large increase in critical current with pressure difference. The effect can be explained by the finite rate of arrival of ballistic ³He atoms to a nucleation site. The critical velocity increases at high pressure difference when the rate of phase slippage becomes comparable to the flux of ³He atoms.