What is a phonon?

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We have studied elastic waves in solids, standing waves in solid, heat capacity due to oscillation modes of individual atoms in a solid etc etc. And all these are put in the chapter called phonons in my book. So what are phonons basically? Are they just any kind of wave oscillatory motion that can occur within a solid? Because surely any wave in a solid need to be standing etc etc.
 

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vanhees71
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Loosely speaking photons are "quantized sound waves", i.e., the quantum description of collective lattice vibrations of a solid. Wikipedia gives a pretty nice starting point:

http://en.wikipedia.org/wiki/Phonons
 
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They are quanta of sound waves just as photons are quanta of electromagnetic waves
 
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Phonons are bundles [quanta] of energy associated with lattice vibrations...that is, oscillations associated with the degrees of freedom in the structure of a material....

From Zapper:
https://www.physicsforums.com/showthread.php?p=899393#post899393 [Broken]

.....When atoms and molecules form a solid, they start to lose most of their individual identity and form a "collective behavior" with other atoms. It is as the result of this collective behavior that one obtains a metal, insulator, semiconductor, etc. Almost all of the properties of solids that we are familiar with are the results of the collective properties of the solid as a whole, not the properties of the individual atoms. The same applies to how a photon moves through a solid.

A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is.
 
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