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

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 E, generate ‘superexchange’ spin-spin interactions of energy J≈4t⁴/E³ 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 E.

To explore this hypothesis directly at atomic-scale, we developed high-voltage single-electron and electron-pair (Josephson) scanning tunneling microscopy, to visualize the interplay of E and the electron-pair density n_P in Bi₂Sr₂CaCu₂O_8+x. Changing the distance δ between each pyramid’s apical O atom and the CuO₂ plane below, should alter the energy levels of the planar Cu and O orbitals and thus vary E.  Hence, the responses of both E and n_P to alterations in δ that occur naturally in Bi₂Sr₂CaCu₂O_8+x were visualized. These data revealed, directly at atomic scale, the crux of strongly correlated superconductivity in CuO₂: the response of the electron-pair condensate to varying the charge transfer energy. Strong concurrence between these observations and recent three-band Hubbard model DMFT predictions for superconductivity in hole-doped Bi₂Sr₂CaCu₂O_8+x (PNAS 118, e2106476118 (2021)) indicate that charge-transfer superexchange is the electron-pairing mechanism (PNAS 119, 2207449119 (2022)).

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