Charge Ordering as Alternative to Jahn-Teller Distortion

We show that the Mott transition in orbitally degenerate systems can, and often does, proceed not in the standard “Mott insulator—weakly correlated metal” sequence, but via a novel intermediate phase with a charge (rather than orbital) ordering. Lifting an orbital degeneracy this way can be viewed as an alternative to a Jahn-Teller distortion. This may occur in a crossover between localized and itinerant regimes, if Hund’s rule coupling overcomes the on site Coulomb repulsion. We show both by calculations and by experiment that this scenario is realized in rare-earth nickelates, and argue that the same phenomenon takes place in many other systems.