This curve shows the phonon density of states for the fcc lattice, assuming nearest neighbor spring forces between atoms. The model used is the same as in Qu. 2 and Qu. 3 homework 2. The frequency is in units of the maximum phonon frequency given by 2 sqrt(2K/M). Also shown is the density of states in the Debye theory, where the Debye frequency has been adjusted to give the correct density of states at low frequency. The cut-off in the Debye theory is then determined by the requirement that the area under the curve is equal to 3.
