1. B. Goss Levi: Phys. Today, 46, 17 (1993).
2. For a theoretical account and related bibliography see for instance: D. J. Scalapino: to be published in Phys. Rep.: V. Z. Kresin and S. A. Wolf: J. Supercond., 7, 865 (1994).
3. R. B. Laughlin: Common Trends in Condensed Matter and High Energy Physics; 5–12 September, 1993, Chia (Cagliari), unpublished;
4. Density of states according to Tachiki, Nakahara and Teshima, to be published in J. Magn & Magn. Mater.
5. V. B. Geshkenbein, A. I. Larkin and A. Barone: Phys. Rev. B, 36, 235 (1987).