The essential interactions in oxides and spectral weight transfer in dopedmanganites

We calculate the value of the Frohlich electron-phonon interaction in manga nites, cuprates, and some other charge-transfer insulators and show that th is interaction is much stronger than any relevant magnetic interaction. A p olaron shift due to the Frohlich interaction, which is about 1 eV, 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 da ta. An opposite conclusion, recently suggested in the literature, is shown to be incorrect. The frequency and temperature dependence of the optical co nductivity of ferromagnetic manganites is explained within the framework of the bipolaron theory.