Kohn Anomaly

A Kohn anomaly is an anomaly in the dispersion relation of a phonon branch in a metal. For a specific wavevector, the frequency - and thus the energy - of the associated phonon is considerably lowered, and there is a discontinuity in its derivative. They have been first proposed by Walter Kohn in 1959. In extreme cases (that can happen in low-dimensional materials), the energy of this phonon is zero, meaning that a static distortion of the lattice appears. This one of the explanation for the apparition of charge density waves in solid. The wavevectors at which a Kohn anomaly is possible are the nesting vectors of the Fermi surface, that is vectors that connect a lot of points of the Fermi surface (for a one dimensional chain of atoms this vector would be ).