Phonons in an aperiodic alkane/urea composite crystal studied by inelastic x-ray scattering

Aperiodic composite crystals belong to a new state of matter that possesses long-range order without translational symmetry. We report measurements of low-lying phonon branches in an $n$-alkane/urea compound that provide insight into the urea sublattice and the commensurate basal plane dynamics. Focusing on the thoroughly studied $n$-nonadecane/urea compound makes a comparison with other inelastic scattering results possible, allowing the observation of two distinct aperiodic elastic signatures. Using three techniques, a slight but reproducible difference in sound velocities is observed between the transverse mode propagating along the aperiodic direction and polarized in the commensurate basal plane, and the one polarized along the aperiodic direction propagating in the basal plane.