1. A. A. Abrikosov, Zh. Eksp. Teor. Fiz. 32, 1442 (1957); [Sov. Phys.-JETP 5, 1174 (1957)].
2. P. W. Anderson and Y. B. Kim, Rev. Mod. Phys. 36, 39 (1964).
3. H. London and D. G. Walmsley, in Proceedings 11th Int. Conf. Low Temp. Phys. (St. Andrews, 1968), p. 879.
4. K. Yasukochi, T. Ogasawara, Y. Kubota, and K. Maryuama, in Proceedings 12th Int. Conf. Low Temp. Phys. (Academic Press of Japan, Kyoto, Japan, 1970), p. 178.
5. D. G. Walmsley, J. Phys. F 2, 510 (1972).