1. C. Taliani et al., in Electronic Properties of High-TcSuperconductors and Related Compounds, eds. H. Kuzmany, M. Mehring, and J. Fink (Springer 1990), p. 280.
2. Xiang-Xin Bi and P. Ecklund, Phys. Rev. Lett. 70, 2625 (1993).
3. P. Calvani et al., Phys. Rev. B 53, 2756 (1996).
4. B. K. Chakraverty, J. Ranninger, and D. Feinberg, Phys. Rev. Lett. 81, issue 1 (6 July 1998).
5. E. V. L. de Mello and J. Ranninger, preprint (1998).