Ultrafast conductivity dynamics in colossal magnetoresistance manganites.

Ultrafast picosecond measurements of optically induced changes in the absolute conductivity (0.4-1.0 THz) of La0.7M0.3MnO3 thin films ( M = Ca, Sr) from 10 K to approximately 0.9 Tc reveal a two-component relaxation. A fast, approximately 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 Tc from below in accordance with an increasing spin specific heat. We show that, for T<<Tc, partial differential sigma/ partial differential T is primarily determined by thermally disordered phonons while spin fluctuations dominate near Tc.