SIGNIFICANT EVENTS - SCIENCE EVENTS
06/01/01
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
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.
j is
an integer multiple of 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 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.
