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Abstract : The series of compounds formed by a group V transition element and the chalcogens S or Se are of particular interest because of the recent discovery of charge density wave CDW phase transitions in these materials 1, 2. Although CDW transitions occur in both the octahedral or 1T polytype, and the 2H or trigonal prismatic polytype, the effect of the CDW transitions is strikingly different 1. In the 1T polytypes of TaS2 and TaSe2, the resistivity rises dramatically and the magnetic susceptibility drops abruptly. At low temperatures 1T-TaS2 is a diamagnet with resistivity ≈ 10-1 Ω cm. Wilson and co-workers l have proposed that the Fermi surface which is composed of three ellipsoids is unstable against a nesting CDW instability across each of the ellipsoids. Such a model accounts for the almost complete truncation of the Fermi surface in the presence of the CDW. It is important to point out, however, that the very large energy gaps observed optically in these compounds 3 and the high transition temperatures ~ 500 K imply that a weak coupling model is inadequate. A strong coupling theory in which the change in entropy of all phonon modes is included is required but remains yet to be developed. By contrast, the CDW transitions in the 2H polytypes of TaSe2, TaS2 and NbSe2 are essentially metal to metal transitions. The resistivity of TaSe2 for example, drops after the formation of the CDW. The paramagnetic susceptibility drops a little at the onset of the CDW but remains high. Further, these materials become superconductors at low temperatures. The CDW transition temperatures also are much lower



Autor: T. Rice

Fuente: https://hal.archives-ouvertes.fr/



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