Observation of Singular Behavior in the Focusing of Ballistic Phonons in Ge

Abstract
High-resolution heat-pulse experiments reveal that transverse, ballistic phonons in Ge are focused into sharply defined beams along the [001] and [110] axes. Angular distributions within these beams display fine-structure representing focusing singularities. It is shown that these features are dependent in a sensitive way on the topology of the phonon-frequency surface.