Explanation

The change of phonon state caused by phonon bouncing. The phonon is not a real particle. It is called a quasiparticle. It reflects the excitation unit of the collective motion state of lattice atoms. It represents the normal vibration of crystal lattice. Under the simple harmonic approximation, there is no interaction between phonon, that is, between normal modes. The harmonic oscillators of lattice atomic vibration are linearly independent and cannot exchange energy. Therefore, there is no phonon masking. The phonon state is a stationary state, and phonon masking will occur only under the anharmonic interaction. Since the interaction force between atoms is not strictly proportional to the displacement of atoms, when the cubic and higher terms need to be considered in the potential energy expansion of crystal, the vibration of lattice atoms cannot be described by strictly linear independent harmonic oscillators. Generally, the cubic and higher terms are called anharmonic terms, and the perturbation of anharmonic terms can lead to transitions between phonon states, This transition between phonon states is called phonon phonon interaction, or collision or scattering between phonon. The interaction between phonon follows the conservation of quasi momentum and energy. Only when the anharmonic interaction exists, the phonon system can reach thermal equilibrium through collision and internal exchange of energy, which satisfies Einstein Bose statistical law.

Classification