Electronic structure and two-band superconductivity in unconventional high-$T_c$ cuprates Ba${}_2$CuO${}_{3+\delta }$

Abstract

The recently discovered cuprate superconductor Ba${}_2$CuO${}_{3+\delta }$ exhibits a high $T_c\simeq 73$K at $\delta \simeq 0.2$. The polycrystal grown under high pressure has a structure similar to La${}_2$CuO${}_4$, but with dramatically different lattice parameters due to the CuO${}_6$ octahedron compression. The crystal field in the compressed Ba${}_2$CuO${}_4$ leads to an inverted Cu $3d$ $e_g$ complex with the $d_{x^2-y^2}$ orbital sitting below the $d_{3z^2-r^2}$ 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 $x=2\delta$ 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 $d_{3z^2-r^2}$ 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 Ba${}_2$CuO${}_{3+\delta }$ realizes mixtures of such heavily hole-doped superconducting Ba${}_2$CuO${}_4$ and disordered Ba${}_2$CuO${}_3$ 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 $T_c$.

arXiv comment12 pages, 8 figures

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• Superconducting dome with $extended$ $s$-$wave$ pairing symmetry in the heavily hole-overdoped copper-oxide planes
• Lattice Quantum Geometry Controlling 118 K Multigap Superconductivity in Heavily Overdoped CuBa2Ca3Cu4O10+d
• Band crossover and magnetic phase diagram of high-Tc superconducting compound Ba2CuO4-δ
• Nodeless $s$-Wave Superconducting Phases in Cuprates with Ba${}_2$CuO${}_3$-Type Structure
• Correlated electronic structure of high-temperature superconductor Ba${}_2$CuO${}_{3+\delta }$