Negative charge-transfer gap and even parity superconductivity in Sr2RuO4

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2 Scopus citations


A comprehensive theory of superconductivity in Sr2RuO4 must simultaneously explain experiments that suggest even-parity superconducting order and yet others that have suggested broken time-reversal symmetry. Completeness further requires that the theory is applicable to isoelectronic Ca2RuO4, a Mott-Hubbard semiconductor that exhibits an unprecedented insulator-to-metal transition which can be driven by very small electric field or current, and also by doping with very small concentration of electrons, leading to a metallic state proximate to ferromagnetism. A valence transition model, previously proposed for superconducting cuprates [Mazumdar, Phys. Rev. B 98, 205153 (2018)2469-995010.1103/PhysRevB.98.205153], is here extended to Sr2RuO4 and Ca2RuO4. The insulator-to-metal transition is distinct from that expected from the simple melting of the Mott-Hubbard semiconductor. Rather, the Ru ions occur as low spin Ru4+ in the semiconductor, and as high spin Ru3+ in the metal, the driving force behind the valence transition being the strong spin-charge coupling and consequent large ionization energy in the low charge state. Metallic and superconducting ruthenates are thus two-component systems in which the half-filled high spin Ru3+ ions determine the magnetic behavior but not transport, while the charge carriers are entirely on the layer oxygen ions, which have an average charge of -1.5. Spin-singlet superconductivity in Sr2RuO4 evolves from the correlated lattice-frustrated 34-filled band of layer oxygen ions alone, in agreement with quantum many-body calculations that have demonstrated enhancement by electron-electron interactions of superconducting pair-pair correlations uniquely at or very close to this filling [Gomes, Wasanthi De Silva, Dutta, Clay, and Mazumdar, Phys. Rev. B 93, 165110 (2016)2469-995010.1103/PhysRevB.93.165110; Wasanthi De Silva, Gomes, Mazumdar, and Clay, Phys. Rev. B 93, 205111 (2016)2469-995010.1103/PhysRevB.93.205111]. Several model-specific experimental predictions are made, including that spin susceptibility due to Ru ions will remain unchanged as Sr2RuO4 is taken through superconducting Tc.

Original languageEnglish (US)
Article number023382
JournalPhysical Review Research
Issue number2
StatePublished - Jun 2020

ASJC Scopus subject areas

  • Physics and Astronomy(all)


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