1. P.W. Anderson, Science235 (1987) 1196.
2. For early evidence of non-conventional superconductivity see, J. Annett, N. Goldenfeld and S.R. Renn inPhysical Properties of High Temperature Superconductors, Vol. II, ed. D. Ginsberg (World Scientific, Singapore, 1990) and references therein.
3. For a recent review of evidence for d-wave superconductivity in the high Tc oxides, see Physics Today, May 1993, p. 17.
4. H.J. Tao, A. Chang, F. Lu and E.L. Wolf, Phys. Rev.B45 (1992) 10622.
5. R.T. Collins, Z. Schlesinger, F. Holtzberg, C. Feild, U. Welp, G.W. Crabtree, J.Z. Liu, and Y. Fang Phys. Rev.B43 (1991) 8701.