ac Josephson Effect in Superfluid Helium

Two superfluid He baths were linked to each other through a small orifice. A quartz transducer produced a sound wave in the vicinity of the orifice, which served to couple the rate of vortex formation to the sound frequency. When the transducer was on, arrest of the flow at certain head differences was observed. The spacing between most of these steps in the flow curves was found to be in integral multiples of the basic unit $z_0=\frac{h\nu }{\mathrm{mg}}$, although in a few instances it was $\left(\frac{n_2}{n_1}\right)z_0$. A model for the phase slippage due to the motion of quantized vortex rings, which can account for the observed effect, is discussed. The stability of the liquid level at one of these "steps" appears to be indefinite: Once arrested, the liquid remains arrested until disturbed by fluctuations. These observations provide convincing confirmation of the ac Josephson Effect in liquid He II.