Abstract:

Below 2.2 K, [sup 4]He 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 creation and motion of individual quantized vortices across the aperture. Using a recently developed pressure control technique, a constant pressure drop, (Delta)P, is applied across the aperture. This causes the vortices to cross the aperture at a Josephson frequency given by f[inf j]=(m[inf 4](Delta)P)/((rho)h) where m[inf 4] is the mass of a helium atom and h is Plank's constant. The periodic vortex crossing causes the superfluid velocity in the aperture to oscillate and emit acoustic radiation at f[inf j]. This sound source will radiate a total power of 10[sup -21] W into freespace. Radiation from the vortex motion has been detected using a 1/4 wave resonant detector and a novel cryogenic microphone. This phenomena could lead to a frequency based pressure standard where the frequency is determined by quantum mechanics. [Work partially supported by the Office of Naval Research and NASA.]