1. K. R. Atkins and I. Rudnick, inProgress in Low Temperature Physics, Vol. 6, C. J. Gorter, ed. (North-Holland, Amsterdam, 1970).
2. D. J. Bergman, inPhysical Acoustics, Vol. XI, W. P. Mason and N. Thurston, eds. (Academic Press, New York, 1975), Chapter 1.
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4. T. G. Wang, Ph.D. Thesis, University of California, Los Angeles (1971), unpublished.
5. C. W. F. Everitt, K. R. Atkins, and A. Denenstein,Phys. Rev. Lett. 8, 161 (1962);Phys. Rev. 136, A1494 (1964).