ABSTRACT
This chapter discusses some of the implications for interaction with an electromagnetic field and introduces the mixed mode called a polariton and shows how its properties are manifested in the reflection of light from the bulk. It provides the ID monatomic crystal that in the long-wavelength limit the difference equations for the atomic displacements go over into the elastic wave equation. The chapter shows that inelastic light scattering is governed by the fluctuation spectrum, so that an evaluation of the relevant Green function is central to the theory of any given light-scattering experiment. It presents an introductory account of the theoretical models required to describe magnons in bulk ferromagnets and antiferromagnets. Ferromagnets are characterized by an interaction between neighbouring electronic spins that gives rise to a parallel alignment at low enough temperatures. The chapter also discusses the magnon spectra for three of the most important models, namely Heisenberg model, the dipole-exchange model and the Ising model in a transverse field.
