✍ Scribed by M. CriŞan; Z. Gulacsi
No coin nor oath required. For personal study only.
Electron-phonon coupling is considered in the strong coupling approximation in order to explain the formation of the charge-density-wave state. The theory gives the expression for the critical temperature as a function of the effective electron-phonon coupling, and the isotopic effect in the critical temperature can be evaluated.
The appearance of condensed states in two-band systems (electronhole systems) remains one of the greatest challenges in the physics of phase transitions. Overhauser 1 pointed out the possibility of the appearance of charge-density waves (CDW) in the electron gas.
Fedders and Martin 2 considered a two-band model and the spin states of the electron-hole pairs, known as spin-density waves (SDW), to explain the itinerant-electron antiferromagnetism.
The existence of CDW states has been discovered in the transitionmetal dichalcogenides and intensively studied by Wilson et al. 3 On the other hand, it is well known that structural phase transitions are connected with the electron-phonon interaction. In a special Fermi-surface geometry, this interaction gives the Kohn anomaly 4 in the phonon spectrum. Roth et al. s pointed out that the Kohn anomaly gives rise to the CDW state in the electron system. The Green's function treatment of this problem uses the Gor'kov method from the theory of superconductivity and was given by Keldys and Kopaev, 6 Jerome et al., 7 Zittartz, 8 and Maki and Sakurai. 9 The electron-phonon interaction was considered by Volkov et al. 1~ and Entel 1~ in the mean field approximation.
Recently, Fenton and Leavens ~3 reconsidered the problem of electronhole pairing and calculated the effects of phonon pair-breaking on the SDW 399
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