HIGH-TEMPERATURE SUPERCONDUCTIVITY OF DIAMAGNETIC HEAVY ELECTRONS - CROSSOVER BETWEEN COOPER PAIRING AND BOSE CONDENSATION

A negative-U Hubbard-type model with nearest-neighbor weak repulsion is studied in the 1/d expansion, with d being the spatial dimensionality. The leading-order effects in 1/d are "Kondo"-effect-type local charge fluctuations and mean-field-approximation-type charge-density waves and superconductivity. A "local" Kondo temperature T(K) is defined to show an energy scale of the charge fluctuations, which are responsible for the formation of diamagnetic heavy-electron systems. The bandwidth of the heavy electrons is about 4kB T(K). Superconductivity occurs when the system is away from half filling. Its critical temperature T(c) is highest in the crossover regime as a function of \U\, where \U\ is as large as the bare bandwidth of electrons. The superconductivity in such a regime is of a crossover type between Cooper pairing and Bose-Einstein condensation. On the basis of Varma's proposal that a negative U results from nonlinear screening [Phys. Rev. Lett. 61, 2713 (1988)], it is argued that superconducting BaxK1-xBiO3 with T(c) almost-equal-to 30 K are diamagnetic heavy-electron systems with T(K) almost-equal-to (2-5) X 10(3) K.