2 and 3 Phonon Interactions, whats the difference?

[JimJim]

I am doing a basic introduction to solid state physics and I have come across a question which asks What the difference between two phonon interactions and three phonon intereactions.

I know that Normal colission and Umklapp colissions are part of the three phono interactions since it tells me this in Kittel's Introduction to solid state.

K1 + K2 = K3 (normal)

K1 + K2 = K3 + G (Umklapp)

So what would a 2 phonon interaction be?

At first I thought the two phonon interaction would be the intereaction between a phonon and its surroundings resulting in a new phonon of different momentum but the more reading I did left me rather confused.

Thanks :)

 

[Valerie Park]

A two-phonon interaction is a process involving the simultaneous exchange of two phonons. It can be seen as an interaction between two particles, one of which is a phonon and the other is a quasiparticle formed by the combination of two phonons. This process can lead to scattering of phonons, and can be used to model the thermal conductivity of solids.