Faraday instability on a free surface of superfluid He-4

We observed that the Faraday waves were parametrically generated on a free surface of superfluid He-4. Our sample cell was supported by plate springs and had a permanent magnet on the top. It was oscillated by applying oscillating current to a superconducting coil. We prepared the sample cell made of polycarbonate so as to optically observe the superfluid surface. Standing wave patterns appeared on the surface and their frequencies were one half of a driving frequency. We observed clear threshold amplitude of the oscillation for the instability. Threshold is usually determined by the viscosity of fluid and thus there must be a dissipation mechanism in superfluid for the threshold.