Introduction to Phonons

Phonons are quanta of vibrational energy in a lattice. This can be transverse, longitudinal, or a mixture.

There are two main types of phonon - optical and acoustic. In acoustic phonons displacement from lattice equilibrium for all particles is like a cosine function.

Optical phonons require at least two types of atom in the lattice. The two atoms behave slightly differently to the vibration, and the effect is like two overlapping cosine functions.

Schematic example: Optical and acoustic phonons

Frequency ~10¹²-10¹³ Hz (Infra red active)

In optical phonons, the two distinct particles move in opposite directions

In acoustic phonons, the wave is more simple.

Transverse

Frequency ~10³-10¹⁴ Hz (Not IR active)

Single phonon simulator animation: Click in the green box to give a 2d phonon wavevector, k

Red line follows dispacement of one ion

Optical

Transverse



Lattice spacing