Two-Orbital Model Explains the Higher Transition Temperature of the Single-Layer Hg-Cuprate Superconductor Compared to That of the La-Cuprate Superconductor

In order to explore the reason why the single-layered cuprates, La2-x(Sr/Ba)(x)CuO4 (T-c similar or equal to 40 K) and HgBa2CuO4+delta (T-c similar or equal to 90 K) have such a significant difference in T-c, we study a two- orbital model that incorporates the d(z)(2) orbital on top of the d(x2-y2) orbital. It is found, with the fluctuation exchange approximation, that the d(z)(2) orbital contribution to the Fermi surface, which is stronger in the La system, works against d-wave superconductivity, thereby dominating over the effect of the Fermi surface shape. The result resolves the long-standing contradiction between the theoretical results on Hubbard-type models and the experimental material dependence of T-c in the cuprates.