Type: Article
Publication Date: 2002-02-12
Citations: 34
DOI: https://doi.org/10.1103/physrevlett.88.096401
Microwave study using the cavity perturbation technique revealed that the conductivity of the antiferromagnet $({\mathrm{NH}}_{3}){\mathrm{K}}_{3\ensuremath{-}x}{\mathrm{Rb}}_{x}{\mathrm{C}}_{60}$ at 200 K is already 3--4 orders of magnitude smaller than those of superconductors, ${\mathrm{K}}_{3}{\mathrm{C}}_{60}$ and $({\mathrm{NH}}_{3}{)}_{x}{\mathrm{NaRb}}_{2}{\mathrm{C}}_{60}$, and that the antiferromagnetic compounds are insulators below 250 K without metal-insulator transitions. The striking difference in the magnitude of the conductivity between these materials strongly suggests that the Mott-Hubbard transition in the ammoniated alkali fullerides is driven by a reduction of lattice symmetry from face-centered-cubic to face-centered-orthorhombic, rather than by the magnetic ordering.