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

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In order to explore the reason why the single-layered cuprates, La₂₋ₓ(Sr/Ba)ₓCuO₄ (Tc≃ 40K) and HgBa₂CuO₄₊δ (Tc≃ 90K), have such a significant difference in Tc, we study a two-orbital model that incorporates the dz₂ orbital on top of the dₓ₂₋y₂ orbital. It is found, with the fluctuation exchange approximation, that the dz₂ 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.