Valence-skipping and negative-U in the d-band from repulsive local Coulomb interaction

Weshow that repulsive local Coulomb interaction alone can drive valence-skipping charge disproportionation in the degenerate d-band, resulting in effective negative-U . This effect is shown to originate from anisotropic orbitalmultipole scattering, and it occurs only for d1, d4, d6, and d9 fillings (and their immediate surroundings). Explicit boundaries for valence-skipping are derived, and the paramagnetic phase diagram for d4 and d6 is calculated. We also establish that the valence-skipping metal is very different, in terms of its local valence distribution, compared to the atomiclike Hund’s metal. These findings explain why transition-metal compounds with the aforementioned d-band fillings are more prone to valence-skipping charge order and anomalous superconductivity.