UNIVERSITY OF CALIFORNIA AT BERKELEY

PACKARD GROUP

Third Sound in Superfluid ³He  

RESEARCH TEAM

   GRADUATE STUDENTS:

• Joan A. Hoffman
• Ray W. Simmonds

   POST-DOC RESEARCHERS:

• Konstantin Penanen

   PRINCIPAL  INVESTIGATORS:

• Prof. Richard Packard
• Prof. J.C. (Seamus) Davis

   PAST RESEARCHERS:

• Andrew Schechter

RESEARCH SUMMARY

   What is Third Sound?
   Unconventional Superfluidity in ³He
   Experimental Design
   Results: Third Sound Spectra
   Results: New Information - c₃ & r_s
   Future Experiments

RELEVANT PUBLICATIONS

• Observation of 'third-sound' in superfluid ³He - A. M. R. Schechter, R. W. Simmonds, R. E. Packard, and J. C. Davis, Nature 396, 554-557, (1998).

• Capacitive Generation and Detection of Third Sound Resonances in Saturated Superfluid ⁴He Films, , A. M. R. Schechter, R. W. Simmonds, and J. C. Davis, J. Low Temp. Phys. 110, 603-608, (1998).

QUICK OVERVIEW

Third sound is a surface wave on a superfluid film, which occurs despite the fact that the film is so thin that viscosity prevents all normal motion. It has been known and used for decades as a tool for studying superfluid ⁴He, and it has just recently been observed for the first time in superfluid 3He films by this research group.

³He  is a remarkable material, a neutral, Fermi liquid which becomes superfluid at low temperatures by a BCS pairing mechanism. The details of this transition, and the behavior of the superfluid state, are related to the superconductivity now being studied in unconventional, high-temperature superconductors. Studies of ³He  promise to shed light on this important topic.

We designed an experimental cell to generate and detect third sound waves in ³He films at temperatures near 300 mK.

We obtained spectra showing resonances in the surface motion of the film, as a function of frequency, which behave as expected for third sound waves. This is the first evidence for the existence of third sound waves in superfluid ³He films.  From these spectra, we were able to obtain new physical information; in particular the speed of third sound and the average superfluid density of the films, for a range of temperatures and film thicknesses.

Finally, there are a variety of experiments which we are planning to carry out in the near future, which the existence of third sound as a probe should make possible for the first time.
    

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