Lecture on Debye model for solids

I just finished preparing my undergraduate lecture on the Debye model for solids. Here are my slides. The course is a third-year course on statistical mechanics. In the previous lecture, they learned about blackbody radiation and derived the Planck spectrum.(Note. The derivations are not included, as I sketch them by hand on a document viewer. This slows down the pace of the lecture and engages the students as they detect mistakes I make. :) ) A few things I find interesting about the lecture content.The Debye theory is very similar to Planck's theory of blackbody radiation. The main difference is that for Debye there is a high-frequency cutoff for the phonon spectrum. This defines the Debye temperature.Given its simplicity, the success of Debye theory, qualitatively and quantitatively, is impressive. Arguably, this is because it is an effective theory that is constrained to be consistent with the long-wavelength physics (elasticity theory) and the classical limit (number of modes of…