Nodeless high-Tc superconductivity in highly-overdoped monolayer CuO₂

We study the electronic structure and superconductivity in CuO₂ monolayer grown recently on d-wave cuprate superconductor Bi₂Sr₂CaCu₂O_8+δ. Density functional theory calculations indicate significant charge transfer across the interface such that the CuO₂ monolayer is heavily overdoped into the hole-rich regime. We show that both the Cu d_x²−y² and d_3z²−r² orbitals become important and the Fermi surface contains one electron and one hole pocket associated with the two orbitals respectively. The liberated low-energy d_3z²−r² band and the hole FS pocket around M enable an analogy to the multiorbital Fe-pnictides superconductors. Constructing a minimal strongly correlated two-orbital model for the e_g complex, we show that the spin-orbital exchange interactions produce an intrinsic nodeless superconductor with extended s-wave pairing symmetry and a pairing energy gap comparable to the bulk d-wave gap, in agreement with recent experiments. The findings point to a direction of realizing new high-Tc superconductors over-extended doping regimes with liberated orbitals in ozone grown transition-metal-oxide heterostructures.