Superconductivity proximate to antiferromagnetism in copper oxide monolayer grown on Bi₂Sr₂CaCu₂O_8+δ

Nodeless superconducting (SC) gap was reported in a recent scanning tunneling spectroscopy experiment in the copper oxide monolayer grown on the Bi₂Sr₂CaCu₂O_8+δ (Bi2212) substrate [Y. Zhong et al, Sci. Bull. 61, 1239 (2016)], which is in stark contrast to the nodal d-wave pairing gap in the bulk cuprates. Motivated by this experiment, we first show with first-principles calculations that the tetragonal CuO (T-CuO) monolayer on the Bi2212 substrate is more stable than the commonly postulated CuO₂ structure. The T-CuO monolayer is composed of two CuO₂ layers sharing the same O atoms. The band structure is obtained by first-principles calculations, and its strong electron correlation is treated with the renormalized mean field theory. We argue that one CuO₂ sublattice is hole-doped while the other sublattice remains half-filled and may have antiferromagnetic (AF) order. The doped Cu sublattice can show d-wave SC; however, its proximity to the AF Cu sublattice induces a spin-dependent hopping, which split the Fermi surface and may lead to a full SC gap. Therefore, the nodeless SC gap observed in the experiment could be accounted for by the d-wave SC proximity to an AF order, thus it is extrinsic rather than intrinsic to the CuO₂ layers.