Mechanism of Electron Pairing in Copper-Oxide High Temperature Superconductors

The CuO₂ plane supporting high temperature superconductivity in the cuprates typically occurs at the base of a periodic array of edge-sharing CuO₅ pyramids. In the undoped material, an antiferromagnetic insulator state is stabilized by hopping of electrons between neighboring Cu and O atoms at rate t/? and across the charge transfer energy gap E, generating ‘superexchange’ interactions of energy J ≈ (4t⁴)⁄E³. However, hole doping this CuO₂ plane produces a very high temperature superconducting state whose electron-pairing is exceptional. A proposed explanation for the electron-pairing is that hole doping destroys magnetic order, but preserves the pair-forming superexchange interactions which are determined by the charge transfer energy scale E. Combining single-electron and electron-pair (Josephson) scanning tunneling microscopy, we can explore this hypothesis directly by atomic-scale visualization of both n_P and E in Bi₂Sr₂CaCu₂O_8+x . Determining the responses of E and n_P to changes in the distance δ between the planar Cu and apical O reveals the response of the electron-pair condensate to controlled variations in the charge-transfer energy. Strong quantitative agreement between these observations and predictions from strong-correlation theory of hole-doped charge-transfer insulators indicates that charge-transfer superexchange is the electron-pairing mechanism of superconductive Bi₂Sr₂CaCu₂O_8+x.