  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 $t/{\hbar}$ and across the
charge-transfer energy gap E, generate 'superexchange' spin-spin interactions
of energy $J\approx4t^4/E^3$ in an antiferromagnetic correlated-insulator
state. However, Hole doping the 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 E. To
explore this hypothesis directly at atomic-scale, we combine single-electron
and electron-pair (Josephson) scanning tunneling microscopy to visualize the
interplay of E and the electron-pair density nP in ${Bi_2Sr_2CaCu_2O_{8+x}}$.
The responses of both E and nP to alterations in the distance {\delta} between
planar Cu and apical O atoms are then determined. These data reveal the
empirical crux of strongly correlated superconductivity in CuO2, the response
of the electron-pair condensate to varying the charge transfer energy.
Concurrence of predictions from strong-correlation theory for hole-doped
charge-transfer insulators with these observations, indicates that
charge-transfer superexchange is the electron-pairing mechanism of
superconductive ${Bi_2Sr_2CaCu_2O_{8+x}}$.
