Ultrafast quasi-particle dynamics of charge/orbital ordered and ferromagnetic clusters in La0.7Ca0.3MnO3

We report on the quasi- particle dynamics of the charge/orbital ordered (COO) and ferromagnetic clusters in the optimally doped manganite, La0.7Ca0.3MnO3 (LCMO) single crystal by time-resolved two-color pump-probe spectroscopy. Pump photons with energies of similar to 1.55 and similar to 0.21 eV were employed in our transient optical spectroscopy to investigate the percolative phase separation including the COO and ferromagnetic clusters from 4 to 480 K. At 1.55 eV, the transient reflectivity change, Delta R/R, at Delta t = 0 shows a similar temperature dependence as that in resistivity and in neutron scattering intensity. We attribute the reflectivity signal to the characteristic optical response of the COO domains. We identify a new temperature scale T* similar to 400K for the clean limit to the formation of COO clusters in LCMO. In contrast, the temperature-dependent amplitude of the transient reflectivity change in the mid-infrared(IR) absorption band (similar to 5 mu m) scales with the volume fraction of the ferromagnetic metallic (FM) phases. Our results suggest ultrafast optical spectroscopy to be a powerful probe to reveal the correlated polarons and charge disorders in phase-separated manganites.