Electronic structure and two-band superconductivity in unconventional high-<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi>T</mml:mi><mml:mi>c</mml:mi></mml:msub></mml:math> cuprates <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msub><mml:mi mathvariant="normal">Ba</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mo> </mml:mo><mml:msub><mml:mi mathvariant="normal">CuO</mml:mi><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo><mml:mi>δ</mml:mi></mml:mrow></mml:msub></mml:math>
Kun Jiang, Congcong Le, Yinxiang Li, Shengshan Qin, Ziqiang Wang, Fu‐Chun Zhang, Jiangping Hu
Abstract
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 ${T}_{c}$.