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What is a phonon?
A phonon is a collective vibrational mode in an ideal crystalline solid. A single phonon is pure vibrational mode. It is direct analogy to a pure vibrational mode in a music…al instrument such as a string of a guitar. It can be said that it is a sound wave, but the vibrations allowed that are pure single mode vibrations are more extensive than simple sound waves. Phonons, in their original and normal sense, occur in perfect crystalline structures where each atom has a specific equilibrium location that is repeated regularly in space. In a pure single phonon mode there is a periodic vibration (i.e. displacement) of each atom that is described as a sinusoidal variation in space and time. One specifies a particular phonon with a wavelength, direction and frequency, just as with a sound wave. In real solids, there are imperfections, but they are near enough to idea that the idealized concept of a phonon is usually completely adequate. Finally, phonons can be described as classical vibrations as is natural in classical mechanics and they can be described as quantum vibrations using quantum mechanics. The quantum description is fundamentally correct, but the classical description is very useful and convenient in many cases. Some people would say that you should not call the collective vibrations of a solid phonons unless you are describing them as a quantum phenomena but other people would say that is too picky. Usually, however, the term phonon implies that quantum nature of the vibration of a crystal. ( Full Answer )