Condensed Matter Physics- Lattice Vibrations : Phonons / Quantization of Lattice Vibrations
The energy of lattice vibrations is quantized. Drawing an analogy with the photon in context of electromagnetic waves, unit of quantum of energy a lattice vibration is called the phonon. The elastic waves in crystals are said to be made up of phonons. Thermal vibrations in crystals are caused by thermally excited phonons just analogous to the thermally excited photons of black body electromagnetic radiation. Similarly the quantized unit of a magnetic spin wave is called the magnon and that of the collective excitation of the electron gas is called the plasmon. These elementary excitations correspond to what are known as quasi particles. There is no direct experimental evidence of phonons. On the other hand,we do have direct experimental evidence of photons. The change in energy and momentum of X-rays and neutrons due to their scattering from a crystal prove the existence of phonons.