  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
magnetoressitance (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.
