Ultrafast Conductivity Dynamics in Colossal Magnetoresistance Manganites

Abstract

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 ( $M\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}\mathrm{Ca}$, Sr) from 10 K to $\ensuremath{\sim}{0.9T}_{c}$ reveal a two-component relaxation. A fast, $\ensuremath{\sim}2\mathrm{ps}$, 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 $T\ensuremath{\ll}{T}_{c}$, $\ensuremath{\partial}\ensuremath{\sigma}/\ensuremath{\partial}T$ is primarily determined by thermally disordered phonons while spin fluctuations dominate near ${T}_{c}$.