Observation of 'third sound' in superfluid ³He

Waves on the surface of a fluid provide a powerful tool for studying the fluid itself and the surrounding physical environment. For example, the wave speed is determined by the force per unit mass at the surface, and by the depth of the fluid: the decreasing speed of ocean waves as they approach the shore reveals the changing depth of the sea and the strength of gravity. Other examples include propagating waves in neutron-star oceans and on the surface of levitating liquid drops. Although gravity is a common restoring force, others exist, including the electrostatic force which causes a thin liquid film to adhere to a solid. Usually surface waves cannot occur on such thin films because viscosity inhibits their motion. However, in the special case of thin films of superfluid ⁴He, surface waves do exist and are called 'third sound'. Here we report the detection of similar surface waves in thin films of superfluid ³He. We describe studies of the speed of these waves, the properties of the surface force, and the film's superfluid density.