Negative charge-transfer gap and even parity superconductivity in Sr2RuO4.

A comprehensive theory of superconductivity (SC) in Sr2RuO4 must simultaneously explain experiments
that suggest even parity superconducting order and others that claim broken
time reversal symmetry. Completeness requires that the theory is applicable to isoelectronic Ca2RuO4.
The valence transition model, previously proposed for cuprate SC [1]
is extended to insulating Ca2RuO4 and superconducting Sr2RuO4 [2]. Within the model
Ru(4+)-to-Ru(3+) valence transition accompanies insulator-metal transition, the driving
force behind which is the unusually large ionization energy of the half-filled high-spin Ru(3+)
ion. Superconducting ruthenates are 2-component systems
in which high-spin Ru(3+) ions determine the magnetic behavior but not transport,
while the charge carriers are entirely on the layer O-ions, which have average charge
-1.5. Spin-singlet SC in Sr2RuO4 occurs in the correlated lattice-frustrated
3/4-filled band of layer O-ions, in agreement with many-body calculations that
find enhancement by the Hubbard U of superconducting pair correlations uniquely at this filling [3]. Several model-specific experimental predictions are made [2].
[1] S. Mazumdar, Phys. Rev B 98, 205153.
[2] S. Mazumdar Phys Rev Res 2, 023382.
[3. N. Gomes et al. 93, 165110.