Material-parameter Dependence of Superconductivity in High-temperature Cuprates

We show that there is an interesting correlation between material parameters and critical temperature Tc in cuprate high temperature superconductors. Our analysis is based on the d-p model, that is, the three-band Hubbard model including d and p orbitals explicitly. This model contains many parameters; the transfer integrals tdp and tpp, the energy levels ɛp and ɛd, and the Coulomb interaction parameters Ud and Up. Our main results are the following: (a) Tc increases as ɛp−ɛd is increased for Up = 0, (2) Tc is lowered with increase of Up when ɛp−ɛd > 0, (3) Tc is increased with increase of Up when ɛp−ɛd < 0, (4) Tc has a minimum at near ɛp−ɛd = 0 as a function of ɛp−ɛd when Ud and Up are comparable, (5) Ud induces dx2-y2 pairing while Up induces dxy pairing, (6) Tc has a peak as a function of tpp. The results imply that Tc will increase if we can suppress Up. The role of Up is consistent with the experimental tendency that Tc increases as the relative ratio of the hole density at oxygen site to that at copper site is increased, which means that when Up increases, the number of p holes is decreased and Tc is also decreased.