Ultrafast conductivity dynamics in colossal magnetoresistance manganites

Ultrafast picosecond measurements of optically induced changes in the absolute conductivity (0.4-1.0 THz) of La_0.7M_0.3MnO₃ thin films (M = Ca, Sr) from 10 K to ∼0.9T_c reveal a two-component relaxation. A fast, ∼2 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≪T_c, ∂σ/∂T is primarily determined by thermally disordered phonons while spin fluctuations dominate near T_c.