Negative charge-transfer gap and even parity superconductivity in Sr2RuO4

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.