STAIRCASE PATTERN IN A SUPERFLUID HE-II TORSIONAL-OSCILLATOR ANALOG OF THE RADIOFREQUENCY SQUID

We are developing a superfluid He-4 analog of the rf SQUID. A thin par tition with a micrometric orifice is placed inside a toroidal containe r filled with liquid helium; ac and de superflows through the orifice are induced by rotating the whole torus, which is the inertial member of a torsional oscillator. The torus oscillation amplitude vs drive cu rve shows a small but reproducible staircase pattern, which is detecta ble between 1.4 and 2.05 K, but is not modulated by a superimposed ste ady rotation. We discuss this behavior in terms of the occurrence of m ultiple phase slips.