The Valence Transition Model of Pseudogap, Charge-Order and Superconductivity in Electron- and Hole-Doped Copper Oxides.

I propose an integrated theoretical approach to spatial broken symmetries and
superconductivity in electron- and hole-doped cuprates, as well as many other strongly correlated systems [1]. I propose that that there occurs a discrete jump in ionicity Cu(2+)-to-Cu(1+) at optimal doping in the conventionally prepared electron-doped compounds and at the pseudogap phase transition in the hole-doped materials; the unusually high ionization energy of the closed shell Cu(1+)-ion, taken together with the doping-driven reduction in the 3D Madelung energy drives the transition. The undoped states behave as effective 1/2-filled Cu-band with the closed shell electronically inactive O(2-) ions; the doped states behave as correlated two-dimensional geometrically frustrated 1/4-filled oxygen hole-bands, now with electronically inactive closed-shell Cu(1+) ions. The charge-ordered state in the frustrated O-band is a period 4 paired Wigner crystal of spin singlets with broken C4 symmetry. Correlated-electron superconductivity results from the destabilization of the paired Wigner crystal. The theory gives the simplest yet most comprehensive understanding of experiments in the normal states, and is easily extended to many other unconventional superconductors.

[1] S. Mazumdar, arXiv:1807.08772.