1. Stiles, P. J., Burstein, E., and Lagenberg, D. N., Phys. Rev. Letters, 6, 667 (1961).
2. Bruns, H., and Lautz, G., Abhandl. braunschweig wiss. Ges., 6, 47 (1954).
3. Berlincourt, T. G., Proc. Seventh Intern. Conf. on Low Temperature Physics, edit. by Graham, G. M., and Hollis Hallett, A. C., 231 (University of Toronto Press, Toronto, 1960).
4. Thorsen, A. C., and Berlincourt, T. G., Phys. Rev. Letters, 6, 617 (1961).
5. Schoenberg, D., Progress in Low Temperature Physics, edit. by Gorter, C. J., 2, 226 (Interscience Pub., Inc., New York, 1957).