Phonon Mechanism of the Ferromagnetic Transition in La1-xSrxMnO3

Temperature dependence of longitudinal optical phonons with oxygen character was measured in La1-xSrxMnO3 (x=0.2, 0.3) by inelastic neutron scattering in the (1 0 0) cubic direction. The zone center mode intensity is consistent with the Debye-Waller factor. However the intensity of the bond-stretching phonons half way to the zone boundary and near the zone boundary decreases dramatically as the temperature increases through the ferromagnetic (FM) transition. We found evidence that the lost phonon spectral weight might shift into polaron scattering at the same wavevectors. The temperature evolution starts well below the onset of the FM transition suggesting that the transition is driven by phonon renormalization rather than by magnetic fluctuations.