MIDINFRARED CONDUCTIVITY IN ORIENTATIONALLY DISORDERED DOPED FULLERIDES

The coupling between the intramolecular vibrational modes and the dope d conduction electrons in M3C60 is studied by a calculation of the ele ctronic contributions to the phonon self-energies. The calculations ar e carried out for an orientationally ordered reference solid with symm etry Fm3BARm and for a model with quenched orientational disorder on t he fullerene sites. In both cases, the dispersion and symmetry of the renormalized modes is governed by the electronic contributions. The cu rrent-current correlation functions and frequency-dependent conductivi ty through the midinfrared are calculated for both models. In the diso rdered structures, the renormalized modes derived from even-parity int ramolecular phonons are resonant with the dipole excited single-partic le spectrum, and modulate the predicted midinfrared conductivity. The spectra for this coupled system are calculated for several recently pr oposed microscopic models for the electron-phonon coupling, and a comp arison is made with recent experimental data which demonstrate this ef fect.