Small polaron effect on carrier recombination in perovskite manganite thin films

Photoinduced ‘transient thermoelectric effect (TTE)’ in perovskite manganite La0.6Ca0.4MnO3 thin film has been measured under magnetic fields. The fast decay process of TTE signals is due to a recombination of photogenerated electron–hole pairs through Mn4+ ions as capture centers, whose evaluated cross section σ obeys the power law σ∝T−n (n=0.75) in the ferromagnetic phase far below Tc and in the paramagnetic phase. From the observed relaxation time τ1, we evaluated the parameter α characterizing a small polaron effect and the effective mass m∗; both are enhanced appreciably near Tc. Such anomaly is attributed to the change in the thermal velocity of diffusing holes with downspin due to a small polaron effect.