A phonon is the vibration quantum
of a crystalline lattice
. Even at absolute zero
(0K), the basis atoms of a crystal
vibrate about their average equilibrium
positions. The concept of a crystal
having a precisely-known, static structure is thus only an approximation. Solid-state
physicists find it very useful to make this approximation, known as the Born-Oppenheimer Approximation
approximation, when calculating bandstructure
--the electron eigenstate
s and eigenenergies
in a solid. The treatment of crystalline vibrations as quantized packets (phonons) allowed physicists to explain why the specific heat
of solids is proportional to T3
at very low temperatures.
Phonons are important for less academic reasons. The vibrations of a crystal act to scatter electrons (and holes). In a silicon MOSFET (a type of transistor), phonons are the fundamental barrier to current flow. The speed of a digital integrated circuit such as a microprocessor is dependent on how fast MOSFETs can charge and discharge capacitors. Since phonons limit current, they greatly degrade circuit speed. At lower temperatures, crystals vibrate less. For this reason, microprocessors could run many times faster if cooled (by liquid nitrogen for instance).