JPL Group Observes Josephson Effects in Liquid Helium

A group of JPL scientists report observation of Josephson junction effects in an experiment to study flow of superfluid helium through an array of sub-micron wide slit apertures near the lambda transition. This experiment was performed by Kalyani Sukhatme, Yury Mukharsky, Dave Pearson and Talso Chui. The data were obtained by applying an AC drive voltage to the diaphragm in a Helmholtz resonator cell. The investigators observed a staircase pattern in the measurements of the amplitude of diaphragm motion as a function of the applied drive. At the temperature approximately 60 microKelvins below the lambda phase transition they saw a critical phase difference less than 2*Pi. This reduced phase shift indicates that the array must be close to Josephson regime. The dependence of the resonant frequency on the amplitude also indicates the nonlinear current-phase characteristic typical of the Josephson effect. The critical current for the flow was also measured as a function of temperature.