Abstract – Séamus Davis: ‘On the Electron Pairing Mechanism of Copper-Oxide High Temperature Superconductivity’
Abstract: The mechanism of high temperature superconductivity has been a fundamental mystery of physics for almost 40 years. The 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. Since 1987, the leading proposal for the mechanism of this intense electron-pairing was 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 nP in Bi2Sr2CaCu2O8+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 nP to alterations in δ that occur naturally in Bi2Sr2CaCu2O8+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 cuprate superconductivity indicate that charge-transfer superexchange is the electron-pairing mechanism (S. O’Mahony et al PNAS 119, 2207449119 (2022)).