Enhanced Pairing Correlations near Oxygen Dopants in Cuprate Superconductors

Recent experiments on Bi-based cuprate superconductors have revealed an unexpected enhancement of the pairing correlations near the interstitial oxygen dopant ions. Here we propose a possible mechanism—based on local screening effects—by which the oxygen dopants do modify the electronic parameters within the ${\mathrm{CuO}}_2$ planes and strongly increase the superexchange coupling $J$. This enhances the spin pairing effects locally and may explain the observed spatial variations of the density of states and the pairing gap.

Figure 1

(a) The ${\mathrm{O}}_a–{\mathrm{O}}_d–{\mathrm{O}}_a$ molecular complex formed by a dopant ${\mathrm{O}}_d$ and its two apical ${\mathrm{O}}_a$ oxygen neighbors. $2p$ and $3s$ orbitals involved in the polarization process are depicted. (b) The level structure and hopping pathways of these orbitals. (c) Charge disproportionation due to the $p\mathrm{\text{−}}d$ (left) and $d\mathrm{\text{−}}d$ (right) electron transfer within the ${\mathrm{CuO}}_2$ plane. The corresponding excitation energies ${\Delta }_{pd}$ and $U$ are screened by the high-energy transitions between ${\mathrm{O}}_a–{\mathrm{O}}_d–{\mathrm{O}}_a$ molecular orbitals.

Figure 2

(a) Reduction of the ${\Delta }_{pd}$ and $U$ excitation energies due to a polarization of the ${\mathrm{O}}_a–{\mathrm{O}}_d–{\mathrm{O}}_a$ molecular complex, as a function of hopping $T_{pp}$. (b) Enhancement of a virtual hopping $t=t_{pd}^2/{\Delta }_{pd}$ and the superexchange coupling $J$ on the Cu-Cu bond below a dopant oxygen.

Figure 3

(a) Reduction of the ${\Delta }_{pd}$ and $U$ excitation energies as a function of $3s-2p$ energy level separation $E_s-E_p$. (b) Enhancement of $t$ and $J$ values on the Cu-Cu bond below a dopant ion.

Figure 4

Exact diagonalization results for the local DOS (per spin direction) on different sites $i$ ($=0,1,\dots$) of the $t-t^{\prime }-J$ model cluster shown in the inset. The periodic boundary conditions are applied. The next-nearest-neighbor hopping $t^{\prime }=-0.4t$. One particular exchange bond indicated by an arrow has an enhanced $J=1.7J_0$ (compared to $J_0=0.4t$ in the ”bulk”). On the corresponding sites $i=0$ or $i=15$, the spectral weight is suppressed at the Fermi level and transferred to the higher binding energies. The result for $J=J_0$ case is represented by a dotted line.