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- Kohn_anomaly abstract "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 is one explanation for charge density waves in solids. 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 ).In the phononic spectrum of a metal a Kohn anomaly is a discontinuity in the derivative of the dispersion relation that occurs at certain high symmetry points of the first Brillouin zone, produced by the abrupt change in the screening of lattice vibrations by conduction electrons. Kohn anomalies arise together with Friedel oscillations when one considers the Lindhard approximation instead of the Thomas-Fermi approximation in order to find an expression for the dielectric function of a homogeneous electron gas. The expression for the real part of the reciprocal space dielectric function obtained following the Lindhard model includes a logarithmic term that is singular at , where is the Fermi wavevector. Although this singularity is quite small in reciprocal space, if one takes the Fourier transform and passes into real space, the Gibbs phenomenon causes a strong oscillation of in the proximity of the singularity mentioned above. In the context of phonon dispersion relations, these oscillations appear as a vertical tangent in the plot of , the so-called Kohn anomalies.Many different systems exhibit Kohn anomalies, including graphene, bulk metals, and many low-dimensional systems (the reason involves the condition , which depends on the topology of the Fermi surface). However, it is important to emphasize that only materials showing metallic behaviour can exhibit a Kohn anomaly, as we are dealing with approximations that need a homogeneous electron gas.".
- Kohn_anomaly wikiPageExternalLink p1144_1.
- Kohn_anomaly wikiPageID "9585894".
- Kohn_anomaly wikiPageRevisionID "596283669".
- Kohn_anomaly context "October 2009".
- Kohn_anomaly hasPhotoCollection Kohn_anomaly.
- Kohn_anomaly orphan "February 2009".
- Kohn_anomaly refimprove "March 2007".
- Kohn_anomaly subject Category:Condensed_matter_physics.
- Kohn_anomaly comment "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.".
- Kohn_anomaly label "Kohn anomaly".
- Kohn_anomaly label "Kohn-Anomalie".
- Kohn_anomaly sameAs Kohn-Anomalie.
- Kohn_anomaly sameAs m.02pkyhg.
- Kohn_anomaly sameAs Q239719.
- Kohn_anomaly sameAs Q239719.
- Kohn_anomaly wasDerivedFrom Kohn_anomaly?oldid=596283669.
- Kohn_anomaly isPrimaryTopicOf Kohn_anomaly.