TY - JOUR
T1 - Valence transition model of the pseudogap, charge order, and superconductivity in electron-doped and hole-doped copper oxides
AU - Mazumdar, Sumit
N1 - Funding Information:
The author acknowledges partial support from NSF (US) CHE-1764152 and is grateful to R. Torsten Clay (Mississippi State University) and Charles Stafford (University of Arizona) for their careful reading of the manuscript and valuable suggestions. The author also acknowledges close interactions and collaborations through the years with David Campbell (Boston University) and R. Torsten Clay (Mississippi State University). Much of the early work on the spatial broken symmetries in the 1/4-filled band was done in collaboration with D. Campbell. Establishing the concept of the paired-electron crystal in the frustrated 2D 1/4-filled band, the numerical demonstrations of the enhancement of superconducting pair-pair correlations in such lattices, and the application of these concepts to organic charge-transfer solids would not have been possible without the continued collaboration with R. T. Clay. The author is grateful to T. Saha-Dasgupta (Indian Association for the Cultivation of Science, Kolkata) for drawing his attention to the literature on negative charge-transfer gaps in nickelates and bismuthates.
Publisher Copyright:
© 2018 American Physical Society.
PY - 2018/11/30
Y1 - 2018/11/30
N2 - We present a valence transition model for electron- and hole-doped cuprates, within which there occurs a discrete jump in ionicity Cu2+→Cu1+ 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 Cu1+ 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 O2- ions in the undoped state, and as correlated two-dimensional geometrically frustrated 1/4-filled oxygen hole band, now with electronically inactive closed-shell Cu1+ 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 O1 - Cu1+-O1- 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 (2016)2469-995010.1103/PhysRevB.93.165110 and Phys. Rev. B 93, 205111 (2016)2469-995010.1103/PhysRevB.93.205111]. We posit that a similar valence transition, Ir4+→Ir3+, occurs upon electron doping Sr2IrO4. We make testable experimental predictions in cuprates including superoxygenated La2CuO4+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 band fillings are universally 1/4 or 3/4, exactly where the paired Wigner crystal is most stable.
AB - We present a valence transition model for electron- and hole-doped cuprates, within which there occurs a discrete jump in ionicity Cu2+→Cu1+ 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 Cu1+ 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 O2- ions in the undoped state, and as correlated two-dimensional geometrically frustrated 1/4-filled oxygen hole band, now with electronically inactive closed-shell Cu1+ 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 O1 - Cu1+-O1- 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 (2016)2469-995010.1103/PhysRevB.93.165110 and Phys. Rev. B 93, 205111 (2016)2469-995010.1103/PhysRevB.93.205111]. We posit that a similar valence transition, Ir4+→Ir3+, occurs upon electron doping Sr2IrO4. We make testable experimental predictions in cuprates including superoxygenated La2CuO4+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 band fillings are universally 1/4 or 3/4, exactly where the paired Wigner crystal is most stable.
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U2 - 10.1103/PhysRevB.98.205153
DO - 10.1103/PhysRevB.98.205153
M3 - Article
AN - SCOPUS:85057738347
VL - 98
JO - Physical Review B-Condensed Matter
JF - Physical Review B-Condensed Matter
SN - 0163-1829
IS - 20
M1 - 205153
ER -