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

摘要

Neutron spectroscopy is used to probe transverse acoustic phonons near the (2, 2, 0) Bragg position in colossal magnetoresistive La0.7Ca0.3MnO3 and La0.7Sr0.3MnO3. Upon warming to temperatures near T-c = 257 K the phonon peaks in La0.7Ca0.3MnO3 soften and damp significantly with the phonon half width at half maximum approaching 2.5 meV for phonons at a reduced wave vector of (q) over right arrow = (0.2, 0.2, 0). Concurrently a quasielastic component develops that dominates the spectrum near the polaron position at high temperatures. This quasielastic scattering is approximate to 5 times more intense near T-c than in La0.7Sr0.3MnO3 despite comparable structural distortions in the two. The damping becomes more significant near the polaron position with a temperature dependence similar to that of polaron structural distortions. An applied magnetic field of 9.5 T only partially reverses the damping and quasielastic component, despite smaller fields being sufficient to drive the colossal magnetoresistive effect. The phonon energy, on the other hand, is unaffected by field. The damping in La0.7Sr0.3MnO3 near T-c at a reduced wave vector of (q) over right arrow = (0.25, 0.25, 0) is significantly smaller but displays a similar trend with an applied magnetic field.