Acoustic Radiation Generated By Phase Slippage in Superfluid ^4He

Below 2.2 K, ^4He is a superfluid that exhibits nearly dissipationless flow for velocities less than a well defined critical velocity. For flow through submicron apertures, the superfluid begins to dissipate energy at the critical velocity by the nucleation and motion of individual quantized vortex lines across the aperture. In the simplest case, the application of a constant pressure drop (Δ P) across the aperture causes vortices to cross the aperture at an average Josephson frequency given by f_j=m_4Δ P/ρ h. Without thermal fluctuations, the periodic vortex crossing would cause the superfluid velocity in the aperture to oscillate and emit acoustic radiation at f_j. Thermal fluctuations destroy the periodicity and spread the acoustic power into a large frequency band. The spectrum of this radiation has been measured using a 1/4 wave resonant detector and a novel cryogenic microphone. The spectrum is interpreted in terms of a shot noise-like vortex nucleation process.