Low-energy modes and Debye behavior in a colloidal crystal

► The vibrational spectrum of a two-dimensional section of a three-dimensional colloidal crystal is studied by confocal microscopy. ► The scaling of the density of states differs from the standard Debye form in two dimensions or three dimensions. ► The results are confirmed by numerical simulations. ► The observation is explained by an effective theory for the projections of modes onto two dimensions.