Ultrafast Conductivity Dynamics in Colossal Magnetoresistance Manganites

Ultrafast picosecond measurements of optically induced changes in the absolute conductivity (0.4–1.0 THz) of ${\mathrm{La}}_{0.7}{M}_{0.3}{\mathrm{MnO}}_3$ thin films ( $M=\mathrm{Ca}$, Sr) from 10 K to $\sim {0.9T}_c$ reveal a two-component relaxation. A fast, $\sim 2\mathrm{ps}$, conductivity decrease arises from an optically induced modification of the effective phonon temperature. The slower component, related to spin-lattice relaxation, has a lifetime that increases upon approaching $T_c$ from below in accordance with an increasing spin specific heat. We show that, for $T\ll T_c$, $\partial \sigma /\partial T$ is primarily determined by thermally disordered phonons while spin fluctuations dominate near $T_c$.