Optical spectroscopy on alkali-metal doped fullerene compounds

Abstract

We report our optical investigations on the alkali-metal-doped C₆₀ over a broad energy spectral range, extending from the far-infrared up to the ultraviolet. The occurrence of superconductivity in A₃C₆₀ compounds has raised quite a bit of controversy with respect to arguments, favouring pairing mechanism based on electron-phonon interactions with low frequency intermolecular vibrations or with high frequency intramolecular modes. We discuss our experimental results within the standard Eliashberg electron-phonon theory of superconductivity, in order to single out the relevant phonon excitation for the pairing mechanism. The Eliashberg calculation strongly supports a pairing mechanism mediated by high-frequency intramolecular phonon modes, in accord with the implications of the weak coupling BCS limit. Moreover, we present the electrodynamic response of the quasi-one-dimensional A₁C₆₀ compounds. Our experimental findings demonstrate that Rb₁C₆₀ and Cs₁C₆₀ undergo a metal-insulator phase transition below about 50 K, while K₁C₆₀ remains, however, metallic at all temperatures. The possibility of a broken symmetry ground state due to the formation of a spin-density-wave (SDW) condensate for the Rb and Cs compound, as suggested by the magnetic properties, is discussed.