1. G. Bednorz andK. A. Muller:Z. Phys. B,64, 18 (1986);M. K. Wu, J. R. Ashburn, C. J. Torng, P. H. Khor, R. L. Meng, L. Gao, Z. J. Huang, Y. Q. Wang andC. W. Chu:Phys. Rev. Lett.,58, 908 (1987);Z. X. Zhao, L. Q. Chen, Q. S. Yang, Y. Z. Huang, C. G. Cui, R. M. Tang, G. R. Liu, L. Chen, L. Z. Wang, S. Q. Guo, S. L. Li andJ. Q. BI:Kexue Tongbao,6, 412 (1987).
2. Given the large number of theoretical papers dealing with several possible new mechanisms for high-T c superconductivity, we do not cite any and refer the reader to the current abundant literature on the subject.
3. M. P. Fontana andD. Cassi: inWeak Superconductivity, edited byA. Barone andA. Larkin (World Scientific Press, Singapore, 1987), p. 384.
4. For a general introduction to fractals, see,e.g.,B. B. Mandelbrot:The Fractal Geometry of Nature (Freeman, S. Francisco, Cal., 1982); for dynamics on fractals, seeR. Orbach:J. Stat. Phys.,36, 375 (1984);R. Rammal:J. Stat. Phys.,36, 547 (1984).
5. H. Buttner andA. Blumen:Nature,329, 700 (1987).