Quantized phase slippage in superfluid He4

We have studied dissipative phase-slip events in the oscillatory superflow of He4 through two different orifices of submicron size. The dissipation events are found to be caused by the phase difference across the orifice changing by 2n, where the slip integer n depends on the orifice geometry, flow direction, and temperature. The temperature dependence of the phase-slip critical velocity indicates that surface microstructure determines the magnitude of relevant energy barriers.