Josephson oscillations in superfluid $^{4}$He

We will describe observations of superfluid oscillations between two samples of $^{4}$He joined by an array of submicron-sized apertures. The fluid oscillates at the Josephson frequency, $f_{j} = \Delta\mu/h$, where $h$ is Plank’s constant and $\Delta\mu$ is the full chemical potential difference, containing both temperature and pressure differences. The oscillations are observed at temperatures sufficiently below the superfluid transition temperature $T_{\lambda}$ that the current phase relation is linear, ie. not sine-like. Evidently the oscillations are the signature of coherent $2\pi$ phase slippage in the array. Work supported in part by grants from the NSF and NASA.