Results at UC Berkeley Demonstrate Detailed Josephson Effects

Richard Packard's group at UC. Berkeley has studied the mass currents through a superfluid 3He Josephson weak link in the presence of an externally applied ac pressure modulation. In a paper accepted for publication in Physical Review Letters, they report that characteristic changes in the dc mass currents are observed whenever the superfluid Josephson frequency superfluid Josephson frequencyj is an integer multiple of the ac modulation frequency the ac modulation frequency 1. The measured dependences of these current changes on ac pressure amplitude are in excellent agreement with theory describing quantum phase dynamics of a superfluid Josephson weak-link. These results establish the superfluid analog of the superconducting Shapiro Effect.

The figure below shows a schematic diagram of their apparatus. It consists of an "outer" volume of superfluid, which houses a flat cylinder containing an "inner" volume of superfluid. Two flexible Kapton diaphragms form the top and bottom surfaces of this inner cylinder. Each diaphragm has a metalized surface and a corresponding electrode that allows the application of electrostatic pressures to it. The upper "soft" membrane is magnetically coupled to a superconducting dc-SQUID displacement transducer that registers its position. The silicon chip containing the micro-aperture array is glued to the center of the lower "stiff" membrane. The cell can be cooled well below the superfluid transition temperature in 3He using a nuclear demagnetization cryostat.

a schematic diagram of their apparatus

The striking agreement of the experiment with all aspects of the model confirms the deep analogy between superfluid Josephson flow dynamics and the well known electrical current dynamics of superconducting Josephson junctions. One significant aspect of this result is that it provides a stringent demonstration that a microaperture-array is fully describable as a single superfluid weak link following Josephson's equations, removing any doubts on this issue. These observations also open the way to potential applications of superfluid Josephson devices, including a superfluid quantum pressure standard and a superfluid dc-SQUID.