Massive spectral weight transfer and colossal magneto-optical effect in doped manganites

We calculate the value of the Frohlich electron-phonon interaction in manganites, cuprates, and some other charge-transfer insulators and show that this interaction (similar to 1 eV) is much stronger than any relevant magnetic interaction. This suggests that carriers in those systems are small (bi)polarons at all temperatures and doping levels, in agreement with the oxygen isotope effect and other data. 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 the exchange interaction with localized Mn spins during the transition. The corresponding current carrier density collapse in doped manganites leads to a colossal change of the optical conductivity in an external magnetic field at temperatures close to the ferromagnetic transition, in agreement with available experimental observations. (C) 2000 American Institute of Physics. [S0021-8979(00)53808-0].