Sr2RuO4, like doped cuprates and barium bismuthate, is a negative charge-transfer gap even parity superconductor with 34-filled oxygen band
Abstract
A comprehensive theory of superconductivity in Sr2RuO4 must explain experiments that suggest even parity superconducting order and others that suggest broken time reversal symmetry. Completeness further requires that the theory applies to Ca2RuO4, a Mott-Hubbard semiconductor that exhibits an unprecedented insulator-to-metal transition driven by very small electric field, 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 [Phys. Rev. B 98, 205153] is 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 ionizaton energy in the low charge state. Metallic and superconducting ruthenates are 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 on the layer oxygen ions, which have an average charge -1.5. Spin singlet superconductivity evolves from the correlated lattice frustrated 3/4 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 tions uniquely at or very close to this filling [Phys. Rev. B 93, 165110 and 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.