Assume the low frequency modes of a lattice are simple plane waves so that                                     Is a good approximation, and determine the low tempera-ture behavior of a harmonic solid....

Assume the low frequency modes of a lattice are simple plane waves so that

Is a good approximation, and determine the low tempera-ture behavior of a harmonic solid. (This picture of a lattice is called the Debye model, and the cut-off frequency, ω₀, is called the Debye frequency.)