The frequency and temperature dependence of the optical conductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory. As these materials are cooled below the Curie temperature, the colossal magnetoresistance (CMR) is accompanied by a massive transfer of the spectral weight of the optical conductivity to lower frequencies. As with the CMR itself, this change in the optical conductivity is explained by the dissociation of bipolarons into small polarons by exchange interaction with the localized Mn spins during the transition to the low temperature ferromagnetic phase.
This is a pre-print. It is also available at: http://arxiv.org/abs/cond-mat/9901340.