Generation and detection of coherent longitudinal acoustic phonons in the La_(0.67)Sr_(0.33)MnO_3 thin films by femtosecond light pulses

摘要

The authors report on an anomalous first-to-zero sound crossover in the colossal magnetoresistance compound La_(0. 67)Sr_(0 33)MnO_3 (LSMO) using ultrafast time-resolved optical techniques. Two-color pump-probe setup was employed to record the photoinduced reflectance oscillations due to excitations of longitudinal acoustic phonons. By changing the sample thickness and probe wavelength, they determine the longitudinal sound velocities of both the thin film and substrate. The values of the sound velocity in LSMO at gigahertz frequencies are more than 20% larger than those from ultrasound experiments in the megahertz regime near T_c. The results show the possible existence of dynamic polaron correlations in LSMO.