Evidence for the immobile bipolaron formation in the paramagnetic state ofthe magnetoresistive manganites

Recent research suggests that the charge carriers in the paramagnetic state of the magnetoresistive manganites are small polarons. Here we report stud ies of the oxygen-isotope effects on the intrinsic resistivity and thermoel ectric power in several ferromagnetic manganites. The precise measurements of these isotope effects allow us to make a quantitative data analysis. Our results do not support a simple small-polaron model, but rather provide co mpelling evidence for the presence of small immobile bipolarons, i.e., pair s of small polarons.