Interplay of strong correlation and Jahn-Teller effect in orbitally degenerate systems

We study the unconventional insulating state in A(4)C(60) by means of the dynamical mean-held theory. The interplay between the electron-electron correlation and the Jahn-Teller interaction determines the properties of these compounds. The system is a Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by intra-molecular Jahn-Teller effect, and important excitations (such as optical and spin gap) are found to be essentially intra-molecular. Experimental values of the optical and spin gaps are recovered by our calculations.