Unique dxy superconducting state in the cuprate Ba2CuO3.25

Recent discovery of superconductivity at a transition temperature of $73\phantom{\rule{0.222222em}{0ex}}\mathrm{K}$ in the doped layered compound ${\text{Ba}}₂{\mathrm{CuO}}_3+x$ for $x\ensuremath{\sim}0.2$ has generated a lot of interest. Experiments in this alternately stacked oxygen octahedral and chain layered structure reveal that a compression of the octahedra causes the Cu-${d}_{{z}²}$ orbital to lie above the Cu-${d}_{{x}²\ensuremath{-}{y}²}$ orbital unlike in the well-known cuprate superconducting materials. Our first-principles calculations and low-energy Hamiltonian studies on the $x$ = 0.25 system reveal that this energy ordering results in formation of ${d}_{{z}²}$-dominated electron pockets. The strong nesting in the Fermi pockets leads to an AFM spin fluctuation mediated ${d}_xy$-wave superconducting state dominated by pairing among the ${d}_{{z}²}$ orbitals. This is in contrast to the cuprate superconductors (e.g., YBCO) where both electron and hole pockets exist and the superconducting state with ${B}_1g$ symmetry are formed by the ${d}_{{x}²\ensuremath{-}{y}²}$ orbital electrons. Unlike the earlier reports we find the interlayer hybridization has an important contribution to the low-energy band structure and formation of the unconventional superconducting state.