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,Physical Acoustics, Vol. XI, W. P. Mason and N. Thurston, eds. (Academic Press, New York, 1975), Chapter 1.
3. H. J. Verbeek, Ph. D. Thesis, University of Leiden, Leiden, The Netherlands (1979), unpublished; see also K. Telschow, J. Rudnick, and T. G. Wang,J. Low Temp. Phys. 18, 43 (1975).
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 D. Thesis, University of California, Los Angeles (1971), unpublished.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).