Theory of the metal-nonmagnetic Mott-Jahn-Teller insulator transition in A(4)C(60)

We study the unconventional insulating state in A(4)C(60) With a variety of approaches, including density-functional calculations and dynamical mean-field theory. While the former predicts a metallic state, in disagreement with experiment, the latter yields a (nonmagnetic) Mott-Jahn-Teller insulator. In that state, conduction between molecules is blocked by on-site Coulomb repulsion, magnetism is suppressed by the intramolecular Jahn-Teller effect, and important excitations (such as optical and spin gap) should be essentially intramolecular. Experimental gaps of 0.5 eV and 0.1 eV, respectively, compare well with molecular ion values, in agreement with this picture.