MICROWAVE CONDUCTIVITY OF FULLERENE INTERCALATION COMPOUNDS: MOTT TRANSITION DRIVEN BY THE ORTHORHOMBIC DISTORTION

摘要

By using a microwave cavity perturbation technique, we determined the microwave conductivity at 10.7 GHz for some ammoniated fullerides, including both superconductors (K_3C_(60), (NH_3)_xNaRb_2C_(60)) and antiferromagnets, (NH_3)K_(3-x)Rb_xC_(60). We confirmed that (NH_3)K_(3-x)Rb_xC_(60) remains an insulator at least up to 250 K. without a metal-insulator transition, and that the conductivity at 200 K is already 3-4 orders of magnitude smaller than that of K_3C_(60) and (NH_3)_xNaRb_2C_(60). The striking difference in the conductivity between these materials strongly suggests that the Mott-Hubbard transition in the ammoniated alkali fullerides is driven by the orthorhombic distortion, rather than by the magnetic ordering.