PHONON

in solid-state physics, quantum of lattice vibrational energy. In analogy to a photon (a quantum of light), the phonon can be viewed as a wave packet with particle-like properties. Its behaviour characteristics determine or affect various properties of solids. The concept of the phonon is, for example, particularly useful in the theory of the thermal conductivity of insulators. It has been found that ionic crystals are better thermal insulators than metals. In such a crystal, so-called anharmonic forces exist that cause different phonons to interact with one another. This interaction in turn enables the phonons to transmit heat energy to the crystal as a whole, resulting in the establishment of a smooth temperature variation throughout the solid. Phonons also provide a basis for understanding the property of superconductivity in certain metals. In most metallic solids phonons, along with impurities, scatter individual electrons. Under certain conditions, however, the interaction between phonons and electrons has quite a different effect and leads to an interaction between the electrons themselves. This action couples together electrons with energies near the Fermi level. At temperatures near absolute zero (-273.15 C), its effects are sufficient to cause these electrons to move as a coherent group through the medium. Thus, after an electric current has been set up, phonons must produce a change in the motion of the entire group of coupled electrons rather than simply scatter individual electrons. Because the relatively weak, thermally excited phonons are not able to do so, they move through the lattice without scattering. As a consequence, a current set up in the material persists indefinitely, and the metal becomes a superconductor of zero resistance.