1. J. H. Vignos and H. A. Fairbank, Phys. Rev. 147, 185 (1966).
2. R. A. D. Hewko, Ph.D. Thesis, Univ. of Alberta, Edmonton (1973).
3. I. Iwasa, Ph.D. Thesis, Technische Univ. M�nchen (1974).
4. Y. Hiki and F. Tsuruoka, Phys. Lett. 56A, 484 (1976).
5. R. Truell, C. Elbaum, and B. B. Chick, Ultrasonic Methods in Solid State Physics (Academic Press, 1969).