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Neutron spectroscopy is used to probe transverse acoustic phonons near the(2, 2, 0) Bragg position in colossal magnetoresistive La0.7Ca0.3MnO3 andLa0.7Sr0.3MnO3. Upon warming to temperatures near Tc = 257 K the phonon peaksin La0.7Ca0.3MnO3 soften and damp significantly with the phonon half width athalf maximum approaching 2.5 meV for phonons at a reduced wave vector of q =(0.2, 0.2, 0). Concurrently a quasielastic component develops that dominatesthe spectrum near the polaron position at high temperatures. This quasielasticscattering is ~5 times more intense near Tc than in La0.7Sr0.3MnO3 despitecomparable structural distortions in the two. The damping becomes moresignificant near the polaron position with a temperature dependence similar tothat of polaron structural distortions. An applied magnetic field of 9.5 T onlypartially reverses the damping and quasielastic component, despite smallerfields being sufficient to drive the colossal magnetoresistive effect. Thephonon energy, on the other hand, is unaffected by field. The damping inLa0.7Sr0.3MnO3 near Tc at a reduced wave vector of q = (0.25, 0.25, 0) issignificantly smaller but displays a similar trend with an applied magneticfield.

Damping and softening of transverse acoustic phonons in colossal magnetoresistive La$_{0.7}$Ca$_{0.3}$MnO$_3$ and La$_{0.7}$Sr$_{0.3}$MnO$_3$