LOW-ENERGY ELECTRONIC EXCITATIONS AND FANO RESONANCE IN K-DOPED C-60 FROM RAMAN-SCATTERING EXCITED AT 1.16EV

We present a Raman scattering study of pristine and K doped C60 at various doping levels by exciting in the near-IR at 1.16 eV. The normal metallic state of K3C60 is characterized by a broad scattering background and by the resonance of low energy phonons in the range of 250-500 cm-1. We assign the broad background to an electronic Raman scattering due to low energy electronic excitations. This spectral feature is indicative of an anomalous normal state behaviour and is similar to the case of high temperature ceramic superconductors. In the overdoped K6C60 the squashing mode at 278 cm-1 shows a Fano resonance with the electronic scattering associated with localized electronic excitations which are characteristic of isolated regions of K3C60 into the matrix of K6C60 as a result of inhomogeneous doping. The Fano resonance indicates a specific electron-phonon coupling of this Jahn-Teller mode with low energy excitations and suggests that the symmetry of this electronic excitation is hg (i.e. the same of the coupled phonon mode). We discuss the nature of the anomalous electronic Raman scattering in terms of scattering from low energy excitations involving a low lying singlet band resulting from electron correlation and/or dynamical J-T distortion caused by the squashing mode.