Phonon

Atoms in a solid are not motionless: they vibrate around equilibrium positions. In a crystal, where atoms occupy a repeating lattice, their motions are coupled, so a disturbance can propagate as a collective vibrational wave. Quantum mechanics assigns these normal modes discrete energies. A phonon is one quantum of a lattice-vibration mode, with energy proportional to its frequency (E = ħω). It is a quasiparticle: not a separate atom or tiny object moving through empty space, but a compact way to describe the quantized collective motion of many atoms. The analogy with a photon is useful because each is a quantum of a wave-like mode, although photons describe electromagnetic radiation while phonons describe mechanical vibrations in matter.

A solid supports many vibrational modes, differing in frequency, wavelength and the relative motion of atoms. At long wavelengths, neighboring parts of a lattice can move in phase in a way that resembles ordinary sound; these are acoustic modes. Crystals whose unit cells contain multiple atoms can also support optical modes, in which atoms within a cell move relative to one another. The name reflects how some such modes can interact with infrared light, not that they are visible-light waves. A real material at nonzero temperature contains a distribution of phonons rather than one vibration alone. Phonons can scatter from other phonons, defects, boundaries and electrons, changing how energy and vibrational disturbances propagate.

Phonons help account for heat transport in crystalline solids. In many insulators and semiconductors, lattice vibrations carry a substantial share of heat; electrons can also carry heat in electrically conducting materials. The distance phonons travel before scattering, as well as their frequencies and speeds, helps determine thermal conductivity. This is why nanoscale structure, defects and interfaces can alter how efficiently heat crosses a material. Phonons are also a microscopic description of sound in a solid: ordinary acoustic waves correspond to collective vibrational modes, and their quantum treatment is useful when energy exchange must be described at small scales. Phonons do not replace the macroscopic concepts of temperature or heat flow; they offer a microscopic framework for understanding them.

In conventional superconductors, lattice vibrations can mediate an effective attraction between electrons. Under suitable conditions, this interaction contributes to the formation of paired electronic states and superconductivity. This mechanism is important in many conventional superconductors, but it should not be assumed to explain every superconducting material: Britannica notes that the role of electron–phonon interaction in high-temperature superconductors was not established in its account. Phonons are therefore one important part of condensed-matter physics, not a universal explanation for all material behavior.