Origin of spatial variation of pairing gap in Bi-based high-Tc cuprates

TL;DR

This paper investigates how out-of-plane oxygens influence the spatial variation of the pairing gap in Bi-based high-Tc cuprates, combining theoretical mechanisms with experimental observations from scanning tunneling microscopy.

Contribution

It identifies two mechanisms—covalency and screening—that modulate the pairing interaction via apical oxygen position, explaining observed spatial gap variations.

Findings

Out-of-plane oxygens affect the pairing gap through covalency and screening.

The mechanisms depend strongly on apical oxygen position.

The combined effects explain the spatial variation observed in experiments.

Abstract

Recently, scanning tunneling microscopy on Bi-2212 cuprate superconductor has revealed a spatial variation of the energy gap that is directly correlated with a modulation of the apical oxygen position. We identify two mechanisms by which out-of-plane oxygens can modulate the pairing interaction within the CuO_2 layer: a covalency between the x^2-y^2 band and apical p-orbital, and a screening of correlation U by apical oxygen polarization. Both effects strongly depend on the apical oxygen position and their cooperative action explains the experiment.