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

Abstract

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.

Figures (5)

• Figure 1: Formation process of ionic oxides with low-valent ions: carrier migration and gathering. (a) We take high-$T_c$ superconductor YBa2Cu3O7 (YBCO) as an example to describe the crystal formation process in order to explore the pairing mechanism of electrons by tracing their footprints. (b) Energy levels of neutral metal atoms Y, Ba, Cu and their respective cations Y^n+, Ba^n+, Cu^n+ (n=1,2), and the oxygen atom O, its anions (O^-, O^2-) and cation (O+) Atkins2010Kittel2005. Here, the levels of Y, Ba, Cu and O$^-$ are set to zero, respectively. We find that the strength of ionic bonds is in the magnitude of eV, about 2 orders of magnitude larger than antiferromagnetic coupling in YBCO ($J_c$=12 meV, $J_{ab}$=120 meV) Mourachkine2002 and electron-phonon interaction in cuprates ($\sim$40-80 meV) Yan2023Lanzara2001. The large two-electron ionization energy ($\sim$15-28 eV) of metal atoms and the electron affinity (-8.08 eV) of O^2- ensure that ions can only exist in the low-valent states, which is a key for us to solve the high-$T_c$ mechanism in cuprates and nickelates.
• Figure 2: From the electron (hole) clouds to the atom-bridged Cooper pairing picture e--O-e- (h+-Cu-h+). As the most basic constituent ions in ionic oxides, the O$^{x-}$ anion surrounded by the electron cloud and metal (M) cation together with its inner-shell electrons surrounded by the hole cloud due to the strong ionic bonds are shown in (a) and (c), respectively. (b) Due to the low-valent ions, the conduction electrons, i.e., the $d$-electrons (the origin and solid evidence of $d$-wave superconductivity) donated by metal atoms, which are not captured by oxygen atoms, are forced to gather towards oxygen atoms by the eV-scale ionic bonding and the strong attraction of oxygen nucleus to them, forming itinerant superconducting $d$-wave electron pairs above $T_c$Mourachkine2002Xiang2022Gomes2007Kondo2011. It is this O-bridged electron pair e^--O-e^-, featured with large pseudogap and small pair size comparable to the lattice constant, that dominates the high-$T_c$ superconductivity of oxides. The two pairs of electrons are bridged by two respective oxygen anions here. (d) Same as in (b), but for the Cu-bridged $d$-wave hole pairs h+-Cu-h+. The key data of the affinity of O$^-$ and O$^{2-}$ and the large two-electron ionization energy of metal atoms are taken from textbooks Atkins2010Kittel2005, respectively. The spatial symmetry of wavefunctions necessitates that two electrons (holes) approach each other to form Cooper pairs with opposite spins Sakurai1994.
• Figure 3: Energy scales in high-$T_c$ cuprates from two repulsive holes to the hole-pair formation and condensation into the superconducting state. Here, we set the interaction energy between two free holes to zero. Considering the short coherence length of $\xi_c$=1-2.5 $\textrm{\AA}$ of the hole Cooper pairs Mourachkine2002, as a lower limit, we estimate the corresponding Coulomb repulsion energy about 6.08-0.67 eV, confirmed by Refs. Anderson1959Derriche2025Alexandrov2013Ohta1991, between the two holes forming a Cooper pair under the strongest screening with the maximum screening electron concentration of $1 \times 10^{21}$ cm$^{-3}$ at the same level as the hole concentration, in which the Thomas-Fermi screening is included with the screening length of 1.16 $\textrm{\AA}$ as the major screening mechanism and the dielectric screening is minor due to the smaller coherence length than the lattice constant ($\sim$4-5 $\textrm{\AA}$). It is evident that only the eV-scale ionic bonds can overcome the strong Coulomb repulsion between two holes, and confine hole pairs within the CuO2 plane (Evidence 2 Supplemental), which can be regarded as an ironclad proof of h+-Cu-h+ picture (Fig. \ref{['fig:2']}(d)). According to experiments of high-$T_c$ cuprates Hüfner2008, we adopt a pseudogap of approximately 0.08 eV and a superconducting gap of about 0.04 eV, smaller than the pseudogap, for the optimally doped Bi2Sr2CaCu2O_8+δ (Bi2212) as references for the pseudogap and superconducting gap, corresponding to the formation and condensation of Cooper pairs (see Evidences: 7-12 Supplemental), respectively‌. For other cuprates with different doping concentrations, their pseudogap and superconducting gap exhibit certain fluctuations around these two reference values‌.
• Figure 4: Cu-bridged hole and O-bridged electron pairing h^+-Cu-h^+ and e^--O-e^-, and their charge-stripe phases. (a) The STM constant-current topographic image of Na-CCOC Kohsaka2007, an irrefutable evidence of our h^+-Cu-h^+ picture. It indicates that holes highly gather around Cu ($+$) cations and the 4$a$-period hole-stripe phase is formed Vojta2008, as Cooper-pair density wave and hole localization states. (b) The 4$a$-period O-bridged e^--O-e^- stripe phase in the CuO2 plane. (c) Probability density distribution of the center-of-mass of the hole and electron pairs along the -Cu-O-Cu- chain. The subscripts $h$ and $e$ in $\Theta_0 (\bm{R})$ represent holes and electrons, respectively.
• Figure 5: Energy levels of neutral oxygen atom O and its anions O$^-$ and O$^{2-}$Atkins2010.