Kohn anomaly

A second set of features that should be mentioned in connection with the vibration spc ctra arc the Kohn anomalies (Kohn, 1959). We noted that i (q) had an infinite derivative with respect to q at q = 2k. . It follows immediately from Eqs. (17-6) and (17-22) that doy/dk must also be infinite when k = 2ki. In fact. 

Piscanec S, Lazzeri M, Mauri F, Ferrari AC, Robertson J (2004) Kohn anomalies and electron-phonon interactions in graphite. Phys Rev Lett 93 185503... 

Figure 19 Kohn anomaly observed by inelastic neutron scattering.

The Peierls transition and the giant Kohn anomaly are driven by the divergence of the polarization function x( ) for q = 2kp. Conditions for the divergence are understood intuitively in terms of the nesting of the Fermi surface Figure 20 shows the three dimensional representation of the Fermi surfaces of one- and two-dimensional electron systems. The divergence of xiq) at T = 0 is considered to be ascribed to the condition, - E = 0 stnd f E -f (E + q) 0. It is... 

The intense theoretical and experimental activity in the organic charge transfer crystals exemplified by TTF-TCNQ has led to several intriguing new results in the past year. In particular, the observation of the Kohn anomaly and Peierls instability in both x-ray and neutron... 

Some results of a X-ray diffuse scattering investigation on the charge transfer compounds TSeF-TCNQ and HMTSeF-TCNQ are presented. In the metallic state the results show the formation of a 1-D distortion or a Kohn anomaly in the phonon spectrum. For TSeF-TCNQ this is observaibelow T 238 K and corresponds to a periodicity of (3.15+ 0.05) Ibl along the chain axis. For HMTSeF-TCNQ the distortion was detected even at room temperature and has a period of (2-7+ 0,1) Icl In both materials the distortion has both transverse components. In the insulating state of TSeF-TCNQ below 29 K the 1-D distortion on each stack are correlated to font a 3-D super-lattice. 

Kohn anomaly at room temperature earlier reported by Hook and... 

The insert shows the transverse branches around 2kp at 84°K. Note that the Kohn anomaly occurs in the branch main polarized along c... 

Kinetic energy of electrons, 3 free-electron gas, 348f local approximation, 351, 377f, 541 Kleinman s internal displacement parameter, 198f tables, 196, 208, 220 Kohn anomalies, 395f Kohn-Sham exchange, 540 Koster-Slaler tables, 481 Kramers-Kronig relations, 99 Krypton, properties of. See Inert gas solids... 

Fig. 13. Phonon energy spectrum with soft mode (Kohn anomaly) at q — Inlia ikp n/ia).

Fig. 14. The Kohn anomaly in the phonon spectrum of K2Pt(CN)4Bro.3o(H20)3 as observed by neutron scattering (342).

In the Frohlich model (367), as in the Peierls transition and Kohn anomaly, the lattice vibration of wavevector Ikp produces a periodic variation in the lattice site positions. This new lattice periodicity results in an additional periodic... 

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