Correlations among c/b, Tc, and Madelung potentials in the system of RBa2Cu3Ox superconductors

We have observed a correlation between the lattice-constant ratio c/b and the superconducting transition temperature Tc in R-Be-Cu-O superconductors (where R is a rare-earth element). Madelung potentials and the bond length of Cu(1)-O(3) are calculated and correlated with the distribution of Tc over the various RBa2Cu3O7 superconductors. Further consideration of YBa2Cu3Ox with variable x of suitable oxygen content leads to an explanation of the correlation of decreasing c/b ratio with increasing Tc. A Coulomb-type ionic mechanism is therefore suggested so that shortening of the c axis and increased chain length provides (1) a channel of charge-transfer excitation from chain to Cu-O layers and (2) a long-range Coulomb interaction between the two Cu-O layers, both of which result in stronger coupling between the two Cu-O layers and higher Tc.. AE