1. See Stewart, G.R.: Rev. Mod. Phys.56, 755 (1984), and references therein
2. See Lee, P.A., Rice, T.M., Serene, J.W., Sham, L.J., Wilkins, J.W.: Comm. Solid State Phys. (to be published) and references therein
3. Ott, H.R., Rudigier, H., Rice, T.M., Ueda, K., Fisk, Z., Smith, J.L.: Phys. Rev. Lett.50, 159 (1983)
4. MacLaughlin, D.E., Cheng Tien, Clark, W.G., Lan, M.D., Fisk, Z., Smith, J.L., Ott, H.R.: Phys. Rev. Lett.53, 1833 (1984)
5. Bishop, D.J., Varma, C.M., Batlogg, B., Bucher, E.: Phys. Rev. Lett.53, 1009 (1984)