umklappen

Umklappen is a German verb meaning to fold over, flip, or turn something so that a part lies on the other side. It is used for physical actions such as folding a lid, turning back a sleeve, or flipping a page. In everyday language the exact usage depends on context, and the verb can describe both literal folding and, more rarely, figurative reversals of a state or arrangement described by the surrounding sentence.

In physics, the Umklappenprozess, or Umklapp scattering, is a process in crystals in which the total crystal momentum of interacting quasiparticles is conserved only up to a reciprocal lattice vector. In practice, this means that momentum can be transferred to the lattice, reducing the net momentum of the carriers (electrons or phonons). This contrasts with Normal (N) processes, where momentum is conserved within the first Brillouin zone. Umklapp scattering becomes more significant at higher temperatures or in materials with strong lattice interactions and is a key factor limiting thermal conductivity and, in some cases, electrical resistivity.

The term is named after the German physicist Fritz Umklapp, who introduced the concept in the early development of the quantum theory of solids. Its study helps explain deviations from ideal heat conduction and is central to models of phonon transport and thermoelectric materials; in nanostructured materials, boundary scattering can suppress Umklapp contributions.