Phonons in an entropic crystal

Hard spheres crystallize due to purely entropic forces. The underlying excluded-volume interaction is completely anharmonic and the nature of the phonon spectrum is therefore of interest. To measure the single-particle motion and the phonon spectrum by dynamic light scattering we have used a collection of novel techniques including multispeckle cross correlation on a CCD chip and the growth of large single crystals using a temperature gradient. The random hexagonal close packed crystal has a dispersion relation closer to hcp than to fcc.