1. G. J. van Gurp,Phys. Rev. 166, 436 (1968).
2. C. Heiden, inLow Temperature Physics?LT 13 (Plenum, New York, 1975), Vol. III, p. 75; C. Heiden and D. Kohake,Phys. Stat. Sol. (b)64, K83 (1974); C. Heiden,Solid State Commun. 18, 253 (1976).
3. S. W. Shen and A. van der Zeil,Physica 64, 587 (1973); H. M. Choe and A. van der Zeil,Physica 81B, 237 (1976).
4. P. Jarvis and J. G. Park,J. Phys. F 5, 1573 (1975).
5. J. D. Thompson, Ph.D. Thesis, University of Cincinnati (1975).