Faraday instability of superfluid surface.

We observed that Faraday waves are parametrically generated on a free surface of superfluid 4He when a sample cell is vibrated vertically. Standing-wave patterns appear on the surface, and their frequencies are one-half the driving frequency. We observed clear threshold amplitudes of the vibration for the instability. The difference in the threshold between the superfluid and the normal fluid is explained by a wall damping.