THEORY OF CHARGE DENSITY WAVES IN THE LAYERED COMPOUNDS

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 (<≈ 120 K) than in the 1T compounds. The present author and G. K. Scott [4] have proposed a model based on a two-dimensional band structure with saddle points at or near the Fermi level. The Fermi surface proposed by Wilson et al. [1] contained such points. It was shown that such a model band structure is unstable to CDW formation in a mariner similar to the more usual nesting model. However, in the saddle point model only a smail area of Fermi surface is truncated — that area near the saddle points. Indeed the saddle points are split and moved away from the Fermi level by the CDW. The saddle points are regions of low Fermi velocity and high density of states which can act as scattering sinks in the high temperature phase and their removal can enhance the conductivity. The saddle points also contribute strongly to the density of states at the Fermi level and can account for the temperature dependence of the magnetic susceptibility found in 2H-TaS2 and 2H-TaSe2. In summary, the saddle-point model appears to give a good qualitative description of the CDW transition in the 2H polytype.