The elementary CuO2 plane sustaining cuprate high temperature superconductivity occurs typically at the base of a periodic array of edge-sharing CuO5 pyramids. Virtual transitions of electrons between adjacent planar Cu and O atoms, occurring at a rate t/ℏ and across the charge-transfer energy gap ε, generate ‘superexchange’ spin-spin interactions of energy J≈4t4/ε3 in an antiferromagnetic correlated-insulator state. However, hole doping this CuO2 plane converts this into a very high temperature superconducting state whose electron-pairing is exceptional. A leading proposal for the mechanism of this intense electron-pairing is that, while hole doping destroys magnetic order it preserves pair-forming superexchange interactions governed by the charge-transfer energy scale ε. To explore this hypothesis directly at atomic-scale, we combine single-electron and electron-pair (Josephson) scanning tunneling microscopy to visualize both ε and the electron-pair density ns, in Bi2Sr2CaCu2O8+x. The responses of both ε and ns to alterations in the distance δ between planar Cu and apical O atoms are thus visualized. These data reveal the long-sought crux of microscopic theory for cuprate superconductivity, the response of condensate electron-pair density ns to varying the charge transfer energy ε. Concurrence of predictions from strong-coupling theory for hole-doped charge-transfer insulators with these observations, indicates that charge-transfer superexchange is the electron-pairing mechanism of superconductive Bi2Sr2CaCu2O8+x.
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