Recent discovery of superconductivity at a transition temperature of $73\mathrm{K}$ in the doped layered compound ${\text{Ba}}_2{\mathrm{CuO}}_{3+x}$ for $x\sim 0.2$ 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-$d_{x^2-y^2}$ 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 $d_{x^2-y^2}$ 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.

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• Chemical bonding
• Electronic structure
• Fermi surface
• First-principles calculations
• Multiband superconductivity
• Pairing mechanisms
• Spin fluctuations
• Spin-singlet pairing
• Superconducting gap
• Superconducting order parameter
• Superconducting phase transition
• Cuprates
• High-temperature superconductors
• Density functional theory
• Tight-binding model