Phonon

phonon (fō`nŏn), quantum of vibrational energy. The atoms of any crystal are in a state of vibration, their average kinetic energy being measured by the absolute temperature of the crystal. In certain phenomena it becomes evident that this energy is divided into discrete bundles (see quantum theory); the energy bundles behave like particles in some respects and are termed phonons. These effects are most apparent at low temperatures where only a few phonons are present. For example, interactions between phonons and electrons are thought to be responsible for such phenomena as superconductivity.

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phonon

In solid-state physics, a quantum of lattice vibrational energy. In analogy to a photon (a quantum of light), a phonon is viewed as a wave packet with particlelike properties (see wave-particle duality). The way phonons behave determines or affects various properties of solids. Thermal conductivity, for instance, is explained by phonon interactions. Phonons also provide the basis for understanding superconductivity in certain metals.

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Phonon

A quantum of vibrational energy in a solid or other elastic medium. This vibrational energy can be transported by elastic waves. The energy content of each wave is quantized. For a wave of frequency f, the energy is (N + ½)hf, where N is an integer and h is Planck's constant. Apart from the zero-point energy, ½hf, there are N quanta of energy hf. In elastic or lattice waves, these quanta are called phonons. Quantization of energy is not related to the discreteness of the lattice, and also applies to waves in a continuum. See Quantum mechanics, Wave motion

The concept of phonons closely parallels that of photons, quanta of electromagnetic wave energy. The indirect consequences of quantization were established for phonons just as for photons in the early days of quantum mechanics—for example, the decrease of the specific heat of solids at low temperatures. Direct evidence that the energy of vibrational modes is changed one phonon at a time came much later than that for photons—for example, the photoelectric effect—because phonons exist only within a solid, are subject to strong attenuation and scattering, and have much lower quantum energy than optical or x-ray photons. See Photoemission

Like photons, phonons can be regarded as particles, each of energy hf and momentum proportional to the wave vector of the elastic or lattice wave. Such a particle can be said to transport energy, thus moving with a velocity equal to the group velocity of the underlying wave. See Lattice vibrations, Photon