— quantum field lattice vibrations. Ph. — quasiparticles with energy $\varepsilon=h \nu$ and momentum $\vec{p}=h \vec{k}$, where $\nu$ — the frequency, $\vec{k}$ — wave vector of the elastic waves propagating in the crystal, $h=\dfrac{h}{2 \pi}$, $h$ — Planck’s constant. Ph. the concept introduced by analogy with the quantum of the electromagnetic field — photons.