Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxides

Abstract

We present a valence transition model for electron-and hole-doped cuprates, within which there occurs a discrete jump in ionicity Cu 2+ → Cu 1+ in both families upon doping, at or near optimal doping in the conventionally prepared electron-doped compounds and at the pseudogap phase transition in the hole-doped materials. In thin films of the T compounds, the valence transition has occurred already in the undoped state. The phenomenology of the valence transition is closely related to that of the neutral-to-ionic transition in mixed-stack organic charge-transfer solids. Doped cuprates have negative charge-transfer gaps, just as rare earth nickelates and BaBiO3. The unusually high ionization energy of the closed shell Cu 1+ ion, taken together with the doping-driven reduction in three-dimensional Madelung energy and gain in two-dimensional delocalization energy in the negative charge transfer gap state drives the transition in the cuprates. The combined effects of strong correlations and small d − p electron hoppings ensure that the systems behave as effective 1 2 -filled Cu-band with the closed shell electronically inactive O 2− ions in the undoped state, and as correlated two-dimensional geometrically frustrated 1 4 -filled oxygen hole-band, now with electronically inactive closed-shell Cu 1+ ions, in the doped state. The model thus gives microscopic justification for the two-fluid models suggested by many authors. The theory gives the simplest yet most comprehensive understanding of experiments in the normal states. The robust commensurate antiferromagnetism in the conventional T crystals, the strong role of oxygen deficiency in driving superconductivity and charge carrier sign corresponding to holes at optimal doping are all manifestations of the same quantum state. In the hole-doped pseudogapped state, there occurs a biaxial commensurate period 4 charge density wave state consisting of O 1− -Cu 1+ -O 1− spin-singlets, that coexists with broken rotational C4 symmetry due to intraunit cell oxygen inequivalence. Finite domains of this broken symmetry state will exhibit two-dimensional chirality and the polar Kerr effect. Superconductivity within the model results from a destabilization of the 1 4 -filled band paired Wigner crystal [Phys. Rev. B 93, 165110 and 93, 205111]. We posit that a similar valence transition, Ir 4+ → Ir 3+ , occurs upon electron doping SrIr2O4. We make testable experimental predictions on cuprates including superoxygenated La2CuO 4+δ and iridates. Finally, as indirect evidence for the valence bond theory of superconductivity proposed here, we note that there exist an unusually large number of unconventional superconductors that exhibit superconductivity proximate to exotic charge ordered states, whose bandfillings are universally 1 4 or 3 4 , exactly where the paired Wigner crystal is most stable.

High-T$$_c$$ Cuprates: a Story of Two Electronic Subsystems

“…The local structure around one hole need not be compatible with the one around another, creating a large amount of small patches, or short-range HTT/LTT/LTO disorder, which can rearrange themselves with external fields and temperature changes. It has also been proposed that they order as a Wigner crystal, which contains pre-formed zero-coupled pairs [91]. Others have identified pre-formed pairs with the corresponding bipolarons [16].…”

Superconductivity due to cooperation of electron-electron and electron-phonon interactions at quarter filling

“…The PEC is often adjacent to SC in the organic charge-transfer solids (CTS), which are all ρ = 0.5 materials [18]. Besides the CTS many other ρ = 0.5 superconductors are known [18], and this mechanism of SC has been further generalized to the cuprates [23]. E-p interactions are required to realize the PEC because of the bond distortion that simultaneously occurs with n.n.…”

High-Temperature Superconductors as Ionic Metals

“…The exceptionally large effort involved, both experimentally and theoretically, without a resolution over thirty years, naturally raises questions about the effort itself. Several recent synthetic works [3,4,5] indicate that mainstream theoretical approaches may have adopted a reductionist attitude too soon. In particular, the much-emphasized strong electronic correlations in the cuprates were dominantly studied in the minimal two-dimensional, one-band model of intraelectronic copper-site repulsion [6].…”

“…The exceptionally large effort involved, both experimentally and theoretically, without a resolution over thirty years, naturally raises questions about the effort itself. Several recent synthetic works [3,4,5] indicate that mainstream theoretical approaches may have adopted a reductionist attitude too soon.…”