1. For a more in depth review and full references see: J.F. Annett, N.D. Goldenfeld and A.J. Leggett in ?Physical Properties of High Temperature Superconductors V?, D.M. Ginsberg (ED.), (World Scientific, Singapore 1996).
2. C. Panagopulous et al.,Phys. Rev. B 53, R2999 (1996); S-F Lee, D.C. Morgan, R.J. Ormeno, D. Broun et al., preprint.
3. S.M. Anlage et al.,J. Superconductivity 7, 453 (1994).
4. R.C. Dynes,Solid State Commun. 92, 53 (1994); J.R. Schrieffer,Solid State Commun. 92, 129 (1994); D. Pines,Physica C 235?240, 113 (1994); D.J. Scalapino,Phys. Rept. 250, 331 (1995).
5. I. Iguchi, in ?Coherence in High T c Superconductors?, Workshop Herzyla, Israel May 1?3 1995, (World Scientific, Singapore 1995).