Lec 26: Vibrations in crystals with basis: optical modes

Continuing with study of vibrations of atoms in a crystal, the model developed in lecture 25 is extended to crystals with a basis. It is shown that a new mode of vibration - optical mode - arises in such cases in addition to the acoustic mode. Dispersion relation and displacement of atoms while oscillating in optical modes are discussed.
References: (i) Chapter 22: Ashcroft and Mermin
(ii) Chapters 4: Kittel