On the Electron Pairing Mechanism of Copper-Oxide High Temperature Superconductivity.

Hole doping the CuO₂ plane generates the transition from a charge-transfer superexchange mediated antiferromagnetic insulator, to a high temperature superconducting state whose electron-pairing is exceptional. A leading proposal for the mechanism of this intense electron-pairing is that hole doping destroys magnetic order, but preserves superexchange interactions with a charge-transfer energy scale ε. In Bi₂Sr₂CaCu₂O_8+x, we use both single-particle and electron-pair (Josephson) STM to visualize ε and the electron-pair density n_p. We determine their responses to the modulations in the distance δ between planar Cu and apical O atoms. The responses we determine of ε and n_p to δ, and crucially of n_p to ε conform closely to strong-coupling theories in which charge-transfer superexchange is the electron-pairing mechanism (arXiv:2108.03655).