Microwave conductivity of superconducting and insulating fullerides

Superconductivity of A₃ C₆₀ (A = K and Rb) is destroyed by the intercalation of neutral ammonia molecules, and is replaced by the antiferromagnetic ground state. These properties suggest the importance of the electronic correlation in fullerides, which have been believed to be simple BCS superconductors. By using a microwave cavity perturbation technique, we confirmed that the conductivity of the antiferromagnet (NH₃) K_3-x Rb_x C₆₀ at 200K is already 3-4 orders of magnitude smaller than those of superconductors, K₃ C₆₀ and (NH₃)_x NaRb₂ C₆₀, and that the antiferromagnetic compounds are insulators without metal-insulator transition at the Néel temperature. These results indicate that the Mott-Hubbard transition in the A₃ C₆₀ systems is driven by a reduction of lattice symmetry from face-centered-cubic (fcc) to face-centered-orthorhombic (fco), rather than by the magnetic ordering.