Kohn anomaly

The Kohn anomaly is an abnormality of the dispersion relation of phonons. It was discovered in 1959 by Walter Kohn. It is a logarithmic derivative of the divergence in the dispersion relation. This divergence coming from the interaction between electrons and phonons and always occur at the Fermi surface, so that it also gets information about the Fermi surface by the measurement of dispersion. The Kohn anomaly is also responsible for the Peierls transition in one -dimensional molecular chains such as Polyethin and the Jahn-Teller transition ( Jahn -Teller effect ) in three -dimensional crystals. It can cause a spontaneous breaking of lattice symmetry due to electronic energy minimization.