No abstract

We have studied the sub-picosecond quasiparticle dynamics in the perovskite manganite La0.7Ca0.3MnO3 and the layered manganite La1.4Sr1.6Mn2O7 using ultrafast optical spectroscopy. We found that for T ≥ TC, initial relaxation proceeds on the time scale of several hundred femtoseconds and corresponds to the redressing of a photoexcited electron to its polaronic ground state. The temperature and dimensionality dependence of this polaron redressing time provides insight into the relationship between polaronic motion and spin dynamics on a sub-picosecond time scale. We also observe a crossover to a more conventional electron-phonon relaxation in the ferromagnetic metallic phase below T c .Although interest in the unusual electronic and magnetic properties of "Colossal" negative magnetoresistance (CMR) materials has led to a tremendous research effort both theoretically [1] and experimentally [2], there are still many unresolved issues related to the behavior of CMR materials. Basic mechanisms of the ferromagnetic metallic state emergence in manganites have been understood within the framework of double exchange theory [3] as shown in Fig. 1(a). In addition to double exchange, mechanisms such as the Jahn-Teller (JT) distortion with polaron

No abstract