1. E. Van Sprousen, H. J. Verbeek, R. De Bruyn Ouboter, K. W. Taconis, and H. Van Beelen,Physica 61, 129 (1972).
2. R. P. Henkel, G. Kuckich, and J. D. Reppy, inProc. 11th Int. Conf. Low Temp. Phys., St. Andrews, Scotland, 1968.
3. J. D. Reppy,Phys. Rev. Letters 14, 733 (1965); J. B. Mehl and W. Zimmermann, Jr.,Phys. Rev. Letters 14, 815 (1965).
4. T. G. Wang and I. Rudnick, inLow Temperature Physics LT 13 (Proc. 13th Int. Conf. Low Temp. Phys., 1972) (Plenum, New York, 1973).
5. K. R. Atkins and I. Rudnick,Progress in Low Temperature Physics, Vol. 6, C. J. Gorter, ed. (North-Holland, Amsterdam, 1970), Chapter 2.