Thermal Conductivity of Manganite Perovskites: Colossal Magnetoresistance as a Lattice-Dynamics Transition

We present the thermal conductivity (κ) of several manganite perovskite compounds at temperatures 10--350 K and fields up to 6 T. In the metallic phase, κ behaves as expected for a crystalline solid. In the high-temperature insulating phase, however, dκ/dT>0, the behavior of an amorphous solid. Unlike amorphous solids, dκ/dT>0 here is related not to quenched structural disorder but rather to unusually large dynamic lattice distortions accompanying charge transport. Thus, ``colossal magnetoresistance'' in the manganite perovskites is characterized as a transition between regimes of different lattice dynamics.