Recent research suggests that the charge carriers in the paramagnetic state of the magnetoresistive manganites are small polarons. Here we report studies of the oxygen-isotope effects on the intrinsic resistivity and thermoelectric 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 compelling evidence for the presence of small immobile bipolarons, i.e., pairs of small polarons. Since the bipolarons in the manganites are immobile, the present result alone appears not to give a positive support to the bipolaronic superconductivity theory for the copper-based perovskites.

The modern multipolaron theory predicts a polaronic Fermi-liquid state, but such a state has not been unambigously confirmed by experiment so far. Here we report theoretical and experimental studies of the isotope effects on the low temperature kinetic properties of the doped ferromagnetic manganites. Our results provide clear evidence for a polaronic Fermi-liquid state in doped ferromagnetic manganites.

Micro-Raman spectra were recorded for two samples of polycrystalline La 0.67 Ca 0.33 MnO 3 , which were synthesized from oxides with oxygen isotopes 16 O and 18 O. Isotope Raman shifts were measured and compared with the theoretical prediction. In this way the oxygen-related Raman modes were clearly identified. Temperature-related blue Raman shifts for 16 O ( 18 O) vibration modes at 230 cm −1 (216 cm −1 ) were unexpectedly observed. These blue Raman shifts were compared with the blue shift due to lattice parameter variation as the temperature is lowered. The large blue Raman shift that occurs with decrease in temperature demonstrates that the Jahn-Teller distortion may play an important role. The appearance of new peaks at 440-480 and 410-450 cm −1 for 16 O and 18 O samples, respectively, as the temperature decreases to 140-100 K indicates a possible charge-orbit ordering transition. A continuous low-wavenumber signal was observed which is superimposed on the broad phonon Raman lines. It was found that the scattering intensity of the low-wavenumber signal is dramatically reduced as the temperature is lowered. The nature of this excitation is not clear.