BOND-CHARGE REPULSION AND HOLE SUPERCONDUCTIVITY IN THE ATOMIC REPRESENTATION OF THE CUO2 PLANE

The pairing theory of superconductivity is applied to tightly bound electrons in narrow energy bands that interact via short-range Coulomb interactions. We depart from the model Hamiltonian proposed by Hirsch that emphasizes the role of the bond-charge repulsion for the attractive interaction between holes in the almost-filled ppi band of the planar O2- anion lattice of the CuO2 layer. Using the method of Appel and Kohn we cast the pairing theory for the ppi electrons into the atomic representation and construct the set of vertex equations on the two-dimensional square lattice that determine the transition temperature T(c). A parameter study of T(c), is presented that gives a strong increase of T(c) with increasing value of the nearest-neighbor bond-charge parameter DELTAt, confirming the results of the theory of Hirsch and Marsiglio. We extend our parameter study to incorporate also both the next-near-neighbor bond-charge repulsion DELTAt3 and the Coulomb interaction V2. The trends of the previous T(c) results remain unchanged: The adverse effect of V2 is compensated for by the increase of T(c) with DELTAt3. Analytical evaluation of the unscreened bond-charge matrix elements yields the quantitative values: DELTAt = 0.806 eV and DELTAt3 = 0.13 eV, resulting in an attractive pairing interaction. The effect of dielectric screening on DELTAt and DELTAt3 is not known at this time and, therefore, it remains an open question whether the bond-charge repulsions by themselves lead to an attraction between holes or mitigate the Coulomb repulsions to the extent that an attractive bosonic exchange can result in high-T(c) superconductivity.