Figure 1: Normal process (N-process) and Umklapp process (U-process). While during the normal process the phonon quasimomentum is preserved, during the transfer process, the phonon quasimomentum changes.
Figure 2 .: k- vectors outside the first Brillouin zone (red) can always be reduced to vectors in the first zone (black).
Transfer processes are collision processes between quasiparticles in crystals in which the law of conservation of momentum is satisfied up to the reciprocal lattice vector .
- {\ displaystyle \ hbar \ mathbf {K} _ {f} = \ hbar \ mathbf {K} _ {i} + \ hbar \ mathbf {g}} ,
,Where - Planck constant , Is the total initial wave vector of all quasiparticles, Is the total final wave vector of all quasiparticles, Is an arbitrary reciprocal lattice vector.
The transfer processes are important, for example, for such a phenomenon as the thermal conductivity of dielectrics . Thanks to these processes, quasiparticles can be scattered in large-angle collisions.