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, La_2-x(Sr/Ba)ₓCuO₄ (Tc40 K) and HgBa₂CuO_4+δ (Tc90 K) have such a significant difference in Tc, we study a two-orbital model that incorporates the d_z² orbital on top of the d_x²-y² orbital. It is found, with the fluctuation exchange approximation, that the d_z² 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 Tc in the cuprates.