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Ultrafast picosecond measurements of optically induced changes in the absolute conductivity (0.4--1.0 THz) of La0.7M0.3MnO₃ thin films ( M0ex0ex=0ex0exCa, Sr) from 10 K to ~0.9Tc reveal a two-component relaxation. A fast, ~2ps, 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 TTc, ∂σ/∂T is primarily determined by thermally disordered phonons while spin fluctuations dominate near Tc.
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Averitt et al. (2001) studied this question.
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