Metal-insulator transition in C₆₀ fullerides

Intercalated fullerides A₃C₆₀ (A = K and Rb) are well known BCS-type superconductors, but their superconductivity is easily destroyed by subtle chemical modification, resulting in the antiferromagnetically ordered states. This property is suggestive of the importance of electron correlation effect. Using microwave cavity perturbation technique, we found that the conductivity at 250 K of the antiferromagnet compounds is already 3-4 orders of magnitude smaller than those of superconducting compounds. These results strongly suggest that the Mott-Hubbard transition in the A₃C₆₀ systems is driven by reduction of lattice symmetry.