Question

1. What are phonons? What information can be obtained by studying the phonon spectrum of solids?

Answer 1

A phonon is a collective excitation in a periodic, elastic arrangement of atoms or molecules in condensed matter, like solids and some liquids. It is a quantized mode of vibration occurring in a rigid crystal lattice, such as the atomic lattice of a solid. Solids with one or more atoms exhibit two types of phonons. They are (1) acoustic phonons and (2) optical phonons. When a solid absorbs energy, the number of phonons changes. Macroscopic properties like thermal energy can be described by suitable statistical models. If a solid is viewed as a collection of distinguishable oscillating atoms, they would obey Maxwell-Boltzmann distribution. Since phonons are indistinguishable particles like photons, they follow Bose-Einstein distribution. There is no limit on the number of particles a boson state can occupy.

In solid-state physics, the study of phonons is important because phonons play an important role in many of the physical properties of solids, such as the specific heat, melting point, thermal conductivity, the electrical conductivity, and superconductivity.  The electrons are responsible for the majority of heat conduction in metal, while the phonons are the dominant carriers in the semiconductors and insulators, e.g. silicon, germanium and silica-based materials. In some cases, the phonon contribution to the specific heat or resistivity can be easily separated from other contributions and can provide important information about the lattice dynamical properties of a solid. There are studies made on the measurements of sound velocities using spectroscopic techniques which is quite useful since they provide the slopes of the acoustic phonon branches.