Phonons in an entropic crystal

Hard spheres crystallize due to purely entropic forces. The underlying excl uded-volume interaction is completely anharmonic and the nature of the phon on spectrum is therefore of interest. To measure the single-particle motion and the phonon spectrum by dynamic light scattering we have used a collect ion of novel techniques including multispeckle cross correlation on a CCD c hip and the growth of large single crystals using a temperature gradient. T he random hexagonal close packed crystal has a dispersion relation closer t o hcp than to fee.