Midinfrared conductivity in orientationally disordered doped fullerides

The coupling between the intramolecular vibrational modes and the doped conduction electrons in ${\mathit{M}}_3$ ${\mathrm{C}}_{60}$ is studied by a calculation of the electronic contributions to the phonon self-energies. The calculations are carried out for an orientationally ordered reference solid with symmetry Fm3¯m and for a model with quenched orientational disorder on the fullerene sites. In both cases, the dispersion and symmetry of the renormalized modes is governed by the electronic contributions. The current-current correlation functions and frequency-dependent conductivity through the midinfrared are calculated for both models. In the disordered structures, the renormalized modes derived from even-parity intramolecular phonons are resonant with the dipole excited single-particle spectrum, and modulate the predicted midinfrared conductivity. The spectra for this coupled system are calculated for several recently proposed microscopic models for the electron-phonon coupling, and a comparison is made with recent experimental data which demonstrate this effect.