Abstract
Physics In crystalline materials, atoms are arranged in an ordered lattice but can still wiggle around their equilibrium positions in concert with one another. These collective oscillations—phonons—are easy to describe in perfect crystals. In the real world, however, materials have nanoscale defects that can influence how phonons move through the crystal. To see these effects directly, Cremons et al. optically excited phonons in samples of WSe2 and Ge and watched them propagate. The phonons caused minute rearrangements of the lattice, which were captured by an ultrafast electron microscope. The resulting movies indicate that the phonons were generated at the step edges of the samples and that their motion was influenced by the local morphology. Nat. Commun. 7 , 11230 (2016).
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