Midinfrared Conductivity in Orientationally Disordered Doped Fullerides

TL;DR

This study investigates how orientational disorder in doped fullerides affects midinfrared conductivity by analyzing electron-phonon interactions and comparing theoretical models with experimental data.

Contribution

It introduces a detailed calculation of phonon self energies considering orientational disorder and links these to observable midinfrared conductivity spectra.

Findings

Disorder influences the symmetry and dispersion of phonon modes.

Renormalized phonon modes modulate the midinfrared conductivity.

Theoretical spectra align with experimental observations.

Abstract

The coupling between the intramolecular vibrational modes and the doped conduction electrons in $M_3C_{60}$ 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 $Fm \bar{3} m$ 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.