Autopsy of superflow collapses and multiple phase slips

Although phase slippage across an orifice in its purest form takes place by individual phase increments by 2π, there are many instances of flow in which the energy dissipation occurs by lumps involving much larger phase difference changes. Such events, the observation of which has been reported first by Sabo and Zimmermann, reduce the flow through the micro-aperture of two-hole resonators to residual values which can be close to zero. We have studied experimentally their detailed features and report the existence of two rather distinct manifestations of superflow collapses as well as the strong effect of hydrostatic pressure on their occurrence. Possible mechanisms are reviewed to explain a phenomenon that involves many vortex lines which evolve in time in such a way as to change the phase difference across the micro-aperture by 2nπ.