J.C. Séamus Davis, On the Electron Pairing Mechanism of Copper-Oxide High Temperature Superconductivity

J.C. Séamus Davis. University of Oxford – University College Cork – Cornell University

  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/ℏand across the charge-transfer energy gap ε,  generate ‘superexchange’ spin-spin interactions of energy J≈4t^4/E^3 in an antiferromagnetic correlated-insulator state. However, hole doping this CuO₂ plane converts this into a 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 controlled by the charge-transfer energy scale ε. To explore this hypothesis directly at atomic-scale, we combine single-particle and electron-pair (Josephson) scanning tunneling microscopy to visualize both ε and the electron-pair density n_s, in Bi2Sr2CaCu2O8+x.  This technique determines the responses of both ε and n_s to alterations in the distance δ between planar Cu and apical O atoms, thereby revealing the impact of varying ε on n_s for a cuprate superconductor. Conformation of the 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 Bi2Sr2CaCu2O8+x.

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