ENHANCEMENT EFFECT DUE TO HOP-ON AND HOP-OFF INTERACTION BETWEEN COOPER-PAIR AND BIPOLARON ON THE CHARACTERISTICS OF THE HIGH-T-C CERAMIC SUPERCONDUCTOR

Enhancement in the critical temperature of high-T-c ceramic supercondu ctors has been studied based on the bipolaronic theory which involves the hop-on and hop-off interaction between the Cooper pair and the bip olaron. As a consequence, the BCS energy gap is broadened to a certain extent which is determined by the interaction between the Cooper pair and the bipolaron. The effective superconducting energy gap for the c eramics consists of two parts: the traditional BCS energy gap of the c eramic and the gap broadening contributed by the hop-on and hop-off pr ocess. The critical temperature has been formulated from the effective BCS energy gap, and expressed in terms of the electronegativity diffe rence Delta(chi). Results of our calculation on the critical temperatu re for MBCO-type, T(n)BCCO, and HBCCO match with the experimental data excellently. For a certain series of ceramic superconductors, the var iation of T-c against Delta(chi) traces out a curve having a high enou gh maximum value, in general.