Metallic conductivity and metal-insulator transition in (AC60)n(A=K, Rb, and Cs) linear polymer fullerides

Magnetic spin susceptibility and optical conductivity from 1 to 100 meV and at 9 GHz are presented for the recently discovered alkali-metal fulleride linear polymers, (A${\mathrm{C}}_{60}$ ${)}_{\mathit{n}}$ (A=K, Rb, and Cs). We find that (${\mathrm{KC}}_{60}$ ${)}_{\mathit{n}}$ is a metal with an enhanced Pauli susceptibility and a low-frequency conductivity decreasing with increasing temperature. (${\mathrm{RbC}}_{60}$ ${)}_{\mathit{n}}$ and (${\mathrm{CsC}}_{60}$ ${)}_{\mathit{n}}$ are also conducting at high temperatures but have a metal-insulator transition at 50 and 40 K, respectively. The data are suggestive of a quasi-one-dimensional instability towards a condensed spin-density-wave state.