Phonons and Magnons

Abstract

We now discuss some additional examples of Bose systems of interest in condensed matter physics. Phonons are quasi-particles produced by the quantization of the elastic vibrations of a crystalline lattice. Magnons are quasi-particles associated with the elementary magnetic excitations above a ferromagnetic ground state (although there are magnons associated with antiferromagnetic and even more complex magnetic orderings). Models of noninteracting quasi-particles are useful to explain transport and thermodynamic properties at low temperatures (as the behavior of the specific heat of crystalline solids). Noninteracting phonons are related to harmonic elastic potentials. Free magnons correspond to an approximate treatment, presumably correct at low temperatures, which takes into account only the lowest excited energy states above a magnetically ordered ground state. In the last section of this chapter, we examine the role of interactions in a Bose system. Just as a curiosity, we sketch a theory to account for superfluidity in liquid helium.