Localization of Large Polarons in the Disordered Holstein Model

We solve the disordered Holstein model via the DMRG method to investigate the combined roles of electron-phonon coupling and disorder on the localization of a single charge or exciton. The parameter regimes chosen, namely the adiabatic regime, $\hbar\omega/4t_0 = \omega' < 1$, and the `large' polaron regime, $\lambda < 1$, are applicable to most conjugated polymers. We show that as a consequence of the polaron effective mass diverging in the adiabatic limit (defined as $\omega' \to 0$ subject to fixed $\lambda$) self-localized, symmetry breaking solutions are predicted by the quantum Holstein model for infinitesimal disorder -- in complete agreement with the predictions of the Born-Oppenheimer Holstein model. For other parts of the ($\omega'$, $\lambda$) parameter space, however, self-localized Born-Oppenheimer solutions are not expected. If $\omega'$ is not small enough and $\lambda$ is not large enough, then the polaron is predominately localized by Anderson disorder, albeit more than for a free particle, because of the enhanced effective mass. Alternatively, for very small electron-nuclear coupling ($\lambda \ll 1$) the disorder-induced localization length is always smaller than the classical polaron size, $2/\lambda$, so that disorder always dominates. We comment on the implication of our results on the electronic properties of conjugated polymers.