Phonon
A quantum of lattice vibration — the particle-like packet of sound and heat that carries thermal energy through a solid.
Definition
A phonon is a quantum of collective vibrational energy in a crystal lattice, the vibrational analogue of the photon. Just as light comes in quanta of energy ħω, the vibrations of a solid's atoms come in discrete packets ħω of definite frequency and wavevector. Treating lattice vibrations as a gas of phonons turns the messy many-atom problem of a vibrating solid into the tractable statistics of weakly interacting quasiparticles.
Phonons carry both sound and heat. Their number is not conserved — heating a solid simply creates more of them — and they obey Bose–Einstein statistics. The temperature dependence of how many phonon modes are excited explains why the heat capacity of a solid falls below the classical Dulong–Petit value beneath the Debye temperature, vanishing as T³ near absolute zero.
Phonons are central to condensed-matter physics well beyond heat capacity: they scatter electrons to produce electrical resistance, mediate the attractive interaction behind conventional superconductivity, and limit thermal conductivity. The full theory belongs to the condensed-matter branch; here the phonon is the quantum that rescues the heat capacity of solids.