Introduced by Soviet physicist Igor Tamm in 1930 and named phonon by Yakov Frenkel from the Greek phonē ('sound'), each mode carries quantized energy E_n = (n + 1/2)ℏω_k.
A 3D crystal with N ≥ 2 atoms per primitive cell has 3 acoustic phonon branches (1 longitudinal LA, 2 transverse TA) and 3N − 3 optical branches (LO and TO).
Long-wavelength acoustic phonons propagate at the speed of sound, while out-of-phase optical phonons in ionic crystals couple directly to infrared light and mediate Cooper pairing in conventional superconductors.