Intermolecular polaron and bipolaron tunneling in C60.

Abstract

Intermolecular tunneling of polarons and bipolarons is studied in ${\mathrm{C}}_{60}$ in the limit of weak electron-electron interactions. It is shown that the effective mass of the polaron is renormalized only weakly by intramolecular electron-phonon interactions. However, using the adiabatic approximation, it is found that the Franck-Condon effect would essentially localize the bipolaron on a single molecule in the limit of narrow conduction bandwidth. Indeed, the effective mass of the bipolaron would be increased by more than a factor of ${10}^{3}$. Combined with the observed metallic behavior of doped ${\mathrm{C}}_{60}$, this suggests that either the electronic correlation effects are important or that the conduction band is not too narrow in order to suppress the artificially large mass renormalization or to inhibit the bipolaron formation altogether.