Possible mechanism of enhanced pairing correlation near dopant oxygen in cuprate

Recent experiments on Bi-based cuprate superconductors have revealed an unexpected enhancement of the pairing correlations near the interstitial dopant oxygens. We propose a mechanism by which the dopant oxygens strongly enhance the interaction $J$ locally [1]. We notice that there is a strong covalency between the dopant oxygen and closely located apical oxygens, forming a molecular orbital complex. By considering virtual $p$-$d$ and $d$-$d$ charge transitions within the Cu-O-Cu bond that lead to the spin exchange $J$, we will show that the corresponding excitation energies are screened by the polarization of molecular orbitals hence enhancing $J$. The effect is greatly amplified due to cooperative response of the spatially extended oxygens complex. We will also show, by an exact diagonalization of the $t$-$J$ model, that local enhancement of $J$ leads to the spatial variations in density of electronic states observed in STM experiments. Our findings suggest an interesting possibility of quantum-chemistry control of the key interaction $J$ in cuprates. \\[4pt] [1] G. Khaliullin, M. Mori, T. Tohyama, and S. Maekawa, arXiv:1008.0435