= Umklapp scattering
{c}
{title2=$\mathbf q_1+\mathbf q_2-\mathbf q_3=\mathbf G\neq0$}
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In a three-phonon <Umklapp scattering> process, the first-zone <wavevectors> obey the displayed equation for a nonzero <reciprocal lattice> vector $\mathbf G$, together with <conservation of energy>. The <crystal momentum> transferred to the lattice allows these processes to relax a phonon heat current. They are suppressed at sufficiently low <temperature> and often determine the approximate $1/T$ high-temperature <thermal conductivity> of a crystalline <electrical insulator>.
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