Ionic-Bond-Driven Atom-Bridged Room-Temperature Cooper Pairing in Cuprates and Nickelates: a Theoretical Framework Supported by 32 Experimental Evidences

Unlike ordinary conductors and semiconductors, which conduct electricity through individual electrons, superconductors usually conduct electricity through electron pairs, known as Cooper pairs. Even after 4 decades of intense study, no one knows what holds electrons together in high-$T_c$ cuprates. Here, targeting the critical challenge of pairing mechanism behind high-$T_c$ superconductivity in oxides and considering the dominance of eV-scale ionic bonding, affinity of O$^-$ (1.46 eV) and O$^{2-}$ (-8.08 eV) and large two-electron ionization energy ($\sim$15-28 eV) of metal atoms, we propose an innovative idea of electron e$^-$ (hole h$^+$) pairing bridged by oxygen O (metal M) atoms, i.e., the ionic-bond-driven $\mathbf{e^--O-e^-}$ ($\mathbf{h^+-M-h^+}$) itinerant Cooper pairing formed at pseudogap temperature $T^*>T_c$, by following the principle of "tracing electron footprints to explore pairing mechanisms" and by standing on the solid foundation of chemical-bond$\rightarrow$structure$\rightarrow$property relationship. It is applicable to cuprates, nickelates, iron-based and other new ionic superconductors. Its correctness and universality are confirmed by 32 diverse experimental evidences, especially, the STM image in the CuO$_2$ plane combining with the small pair size. Any other sub-eV and covalent-binding pairing mechanisms would be doubtful. Our findings, which provide the missing link between ionic bonding and superconductivity, resolve a 40-year puzzle and validate the feasibility of room-temperature carrier-pairing in ionic superconductors. We further create a new theoretical framework rooted in our universal $\mathbf{e^--O-e^-}$ ($\mathbf{h^+-M-h^+}$) picture with the strongest pairing strength and Bose-Einstein condensation, which opens a new avenue for understanding high-$T_c$ mechanism and brings the dream of room-temperature superconductivity one step closer.