Abstract
A comprehensive theory of superconductivity in Sr$_2$RuO$_4$ 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 Ca$_2$RuO$_4$, 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 {\bf 98}, 205153] is extended to Sr$_2$RuO$_4$ and Ca$_2$RuO$_4$. 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 Ru$^{4+}$ in the semiconductor, and as high spin Ru$^{3+}$ 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 Ru$^{3+}$ 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 {\bf 93}, 165110 and {\bf 93}, 205111]. Several model specific experimental predictions are made, including that spin susceptibility due to Ru ions will remain unchanged as Sr$_2$RuO$_4$ is taken through superconducting Tc.
Highlights
Sr2RuO4 has long been thought of as a chiral spin-triplet superconductor, with orbital parity px ± ipy [1]
In the present theoretical paper, which is an extension of a valence transition model [10] recently postulated for superconducting cuprates and doped barium bismuthate, (Ba, K)BiO3, I posit that the peculiarities observed in Sr2RuO4 and the isoelectronic Ca2RuO4 should not be considered in isolation, but that the unconventional behaviors of all these superconducting perovskite oxides, along with the pseudogaplike features [11,12,13] observed in electron-doped Sr2IrO4, can be understood within a common theoretical model
II that are difficult to understand with low spin Ru4+ are obtained within the negative charge-transfer gap model in which Ru ions occur as high spin Ru3+ which contribute to magnetism but not transport
Summary
Sr2RuO4 has long been thought of as a chiral spin-triplet superconductor, with orbital parity px ± ipy [1]. The approach here is to treat both systems on an equal footing III the theory of what I term as the type-I negative charge-transfer gap, as observed in doped cuprates, BaBiO3 and (Ba, K)BiO3, and doped Sr2IrO4 in an octahedral environment Much of this has already been presented in the earlier work [10], it is necessary to repeat this briefly here to point out the unique common feature shared by Cu1+, Bi3+, Ir3+ in the octahedral environment, and Ru3+.
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