The degenerate 3-band Hubbard model with "anti-Hund's rule" interactions; a model for AxC60

TL;DR

This paper analyzes a 3-band Hubbard model with unique interactions favoring low spin and orbital angular momentum, explaining key features of Mott insulators and metallic fullerides like AxC60, including gaps and doping effects.

Contribution

It introduces a model capturing the essential physics of fullerides with anti-Hund's rule interactions, explaining their insulating and metallic behaviors.

Findings

Mott insulator with non-degenerate ground state at even filling

Single particle spectrum rescaled at low doping

Doping suppresses density of states and superconductivity

Abstract

We consider the orbitally degenerate 3-band Hubbard model with on-site interactions which favor low spin and low orbital angular momentum using standard second order perturbation theory in the large Hubbard-U limit. At even integer filling this model is a Mott insulator with a non-degenerate ground state that allows for a simple description of particle-hole excitations as well as gapped spin and orbital modes. We find that the Mott gap is generally indirect and that the single particle spectrum at low doping reappears close to even filling but rescaled by a factor 2/3 or 1/3. The model captures the basic phenomenology of the Mott insulating and metallic fullerides AxC60. This includes the existence of a smaller spin gap and larger charge gap at even integer filling, the fact that odd integer stoichiometries are generally metallic while even are insulating, as well as the rapid suppression of the density of states and superconducting transition temperatures with doping away from x=3.