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Phonons:

Can phonons be thought of as a quantised wave of sound, in the same manner photons being a quantised light wave? Also, can they be taken as particles of sound in the same way that photons can be particles of light?

Crystal momentum:

My text states the following (assuming that this is a system with real-space lattice of ##a##):

"If 3 phonons each with crystal momentum ##\hbar (2/3) \pi /a## scatter off each other, they can produce 3 phonons each with a crystal momentum of ##\hbar (-2/3) \pi /a##"

Here, is crystal momentum conserved because the collision produces 3 extra phonons in addition to the original 3 and therefore gives a total final crystal momentum of ##0## (which is the same as having a crystal momentum of ##3 \hbar \frac{2\pi}{3 a}## in this system)? Or is crystal momentum because the 3 initial phonons "turn into" 3 phonons of crystal momentum ##3 \hbar \frac{-2\pi}{3 a}##, which is also the same as having the initial crystal momentum of ##3 \hbar \frac{2\pi}{3 a}##?