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Kohn anomaly (Read 2099 times)
Gerrit-Jan Linker
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Kohn anomaly
08.03.13 at 08:08:16
 
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 2kF).
 
Source:
http://en.wikipedia.org/wiki/Kohn_anomaly
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Gerrit-Jan Linker
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