Body
Phonon
physics
Written By:
* Sidney Perkowitz
<https://www.britannica.com/contributor/Sidney-Perkowitz/4558>
See Article History
<https://www.britannica.com/science/phonon#accordion-article-history>
*Phonon*, in condensed-matter physics
<https://www.britannica.com/science/condensed-matter-physics>, a unit of
vibrational energy that arises from oscillating atoms
<https://www.britannica.com/science/atom> within a crystal
<https://www.britannica.com/science/crystal>. Any solid crystal, such as
ordinary table salt <https://www.britannica.com/science/salt> (sodium
chloride), consists of atoms bound into a specific repeating
three-dimensional spatial pattern called a lattice. Because the atoms
behave as if they are connected by tiny springs, their own thermal
energy <https://www.britannica.com/science/thermal-energy> or outside
forces make the lattice vibrate. This generates mechanical waves that
carry heat <https://www.britannica.com/science/heat> and sound
<https://www.britannica.com/science/sound-physics> through the material.
A packet of these waves can travel throughout the crystal with a
definite energy <https://www.britannica.com/science/energy> and momentum
<https://www.britannica.com/science/momentum>, so in quantum mechanical
<https://www.britannica.com/science/quantum-mechanics-physics> terms the
waves can be treated as a particle, called a phonon. A phonon is a
definite discrete unit or quantum
<https://www.britannica.com/science/quantum> of vibrational mechanical
energy <https://www.britannica.com/science/mechanical-energy>, just as a
photon <https://www.britannica.com/science/photon> is a quantum
<https://www.merriam-webster.com/dictionary/quantum> of electromagnetic
<https://www.britannica.com/science/electromagnetism> or light energy.
Phonons and electrons <https://www.britannica.com/science/electron> are
the two main types of elementary particles or excitations in solids.
Whereas electrons are responsible for the electrical
<https://www.britannica.com/science/electricity> properties of
materials, phonons determine such things as the speed of sound
<https://www.britannica.com/science/speed-of-sound-physics> within a
material and how much heat it takes to change its temperature
<https://www.britannica.com/science/temperature>.
In addition to their importance in the thermal and acoustic
<https://www.britannica.com/science/acoustics> properties, phonons are
essential in the phenomenon of superconductivity
<https://www.britannica.com/science/superconductivity>—a process in
which certain metals such as lead
<https://www.britannica.com/science/lead-chemical-element> and aluminum
<https://www.britannica.com/science/aluminum> lose all their electrical
resistance
<https://www.britannica.com/technology/resistance-electronics> at
temperatures near absolute zero
<https://www.britannica.com/science/absolute-zero> (−273.15 °C; −459.67
°F). Ordinarily, electrons <https://www.britannica.com/science/electron>
collide with impurities as they move through a metal, which results in a
frictional <https://www.britannica.com/science/friction> loss of energy.
In superconducting metals at sufficiently low temperatures, however,
electrons—which ordinarily repel each other—slightly attract each other
through the intermediate effect of phonons. The result is that the
electrons move through the material as a coherent
<https://www.merriam-webster.com/dictionary/coherent> group and no
longer lose energy through individual collisions. Once this
superconducting state has been achieved, any initial flow of electrical
current will persist indefinitely.
In 1986 a new class of materials, called high-temperature
superconductors, was discovered; it is not known if the electron-phonon
interaction is the basis for the superconducting behaviour of these
materials. /See also/ low-temperature phenomena
<https://www.britannica.com/science/low-temperature-phenomenon>.