Electronic structure and two-band superconductivity in unconventional high- Tc cuprates Ba2 CuO3+δ

The recently discovered cuprate superconductor ${\mathrm{Ba}}₂\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$ exhibits a high ${T}_c\ensuremath{\simeq}73\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ at $\ensuremath{\delta}\ensuremath{\simeq}0.2$. The polycrystal grown under high pressure has a structure similar to ${\mathrm{La}}₂\mathrm{Cu}{\mathrm{O}}₄$ but with dramatically different lattice parameters due to the $\mathrm{Cu}{\mathrm{O}}₆$ octahedron compression. The crystal field in the compressed ${\mathrm{Ba}}₂\mathrm{Cu}{\mathrm{O}}₄$ leads to an inverted Cu $3d\phantom{\rule{4pt}{0ex}}{e}_g$ complex with the ${d}_{{x}²\ensuremath{-}{y}²}$ orbital sitting below the ${d}_{3{z}²\ensuremath{-}{r}²}$ and an electronic structure highly unusual compared to the conventional cuprates. We construct a two-orbital Hubbard model for the Cu ${d}⁹$ state at hole doping $x=2\ensuremath{\delta}$ and study the orbital-dependent strong correlation and superconductivity. For the undoped case at $x=0$, we found that strong correlation drives an orbital-polarized Mott-insulating state with the spin-$1/2$ moment of the localized ${d}_{3{z}²\ensuremath{-}{r}²}$ orbital. In contrast to the single-band cuprates where superconductivity is suppressed in the overdoped regime, hole doping the two-orbital Mott insulator leads to orbital-dependent correlations and the robust spin and orbital exchange interactions produce a high-${T}_c$ antiphase $d$-wave superconductor even in the heavily doped regime at $x=0.4$. We conjecture that ${\mathrm{Ba}}₂\mathrm{Cu}{\mathrm{O}}_{3+\ensuremath{\delta}}$ realizes mixtures of such heavily hole-doped superconducting ${\mathrm{Ba}}₂\mathrm{Cu}{\mathrm{O}}₄$ and disordered ${\mathrm{Ba}}₂\mathrm{Cu}{\mathrm{O}}₃$ chains in a single-layer or predominately separated bilayer structure. Our findings suggest that unconventional cuprates with liberated orbitals as doped two-band Mott insulators can be a direction for realizing high-${T}_c$ superconductivity with enhanced transition temperature ${T}_c$.