Sound-Modulated Flow of Superfluid Helium through a Small Orifice: An Attempt to Observe the ac Josephson Effect

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Abstract
Two chambers filled with superfluid helium to different levels were connected by a small orifice (10-15 μm). The flow of the liquid through the orifice was modulated by a sound field. With one experimental apparatus only a continuous flow was observed. With a modified apparatus it was possible to obtain stable states with no net flow at finite level differences between the baths. The spacing between these states is inversely proportional to the sound frequency. It depends on the total height of the liquid in one of the baths and not on the level difference between them. We conclude that the stable states of zero flow result from ultrasonic standing waves of the superfluid in one of the chambers. A phase coupling or Josephson effect can be excluded as an explanation for these states.