INELASTIC NEUTRON-SCATTERING ON THE HIGH-FREQUENCY ULTRASOUND IN SINGLE-CRYSTALS

We have observed, for the first time, inelastic neutron scattering fro m the phonons externally excited in single crystals. We have used ultr asonic transducers (50-200 MHz) for the excitation and neutron spin-ec ho spectroscopy for observing acoustic phonons. This allowed us to tra ce evolution of the elastic Bragg intensity I(0) and of the phonon har monics up to the seventh order as a function of the excitation power. In contrast to the common belief, we have found that in perfect Si ult rasound does not seriously increase I(0), but an apparent increase in the Bragg intensity is entirely due to the onset of inelastic scatteri ng. At the highest power we observed a cross-over to a chaotic phonon behavior. In deformed Si I(0) strongly decreases at a low acoustic pow er. The same effect could be observed in KBr and graphite crystals whe re the ultrasonic field made the entire mosaic blocks vibrate. (C) 199 8 Elsevier Science B.V. All rights reserved.