LAYER COMPOUNDS. CHARGE DENSITY WAVES IN TRANSITIONS METAL COMPOUNDS.ELECTRONIC PROPERTIES OF TRANSITION METAL DICHALCOGENIDES : CONNECTION BETWEEN STRUCTURAL INSTABILITIES AND SUPERCONDUCTIVITY

Abstract

We discuss the electronic properties of the transition metal dichalcogenide layer compounds which have been investigated in detail because they undergo phase transitions accompanied by the occurence of charge density waves (CDW). The CDW state is favoured by the cylindrical nature of the Fermi surface and also by the possible existence of Van-Hove singularities in the band structure close to the Fermi level of the compounds TX2(T = Nb, Ta, X = S, Se). The anisotropy of the layer compounds is related to physical properties such as : compressibility, electrical conductivity and the superconducting properties. In the trigonal prismatic polytypes 2H-NbSe2, TaSe2, TaS2 the onset of a CDW is characterized by a weak anomaly in the resistivity ; whereas in the octahedral coordinated polytypes (1T-TaS2), the formation of a CDW induces a metal-insulator transition. A detailed study of the CDW state by NMR on 2H-NbSe2 will be presented. Below the onset temperature, a redistribution of the conduction electrons causes an inhomogeneous'broadening of the 93Nb quadrupolar satellites. From the interpretation of the line shape of the NMR lines we have deduced the existence of 3 incommensurate CDW in 2H-NbSe2 below 33 K. The amplitude of the CDW measured by NMR is proportional to the onset temperature as expected in a mean field theory. The pretransitional effects observed above the transition are related to static perturbations of the electron distribution induced by the charged impurities. A contribution of dynamical critical fluctuations above 33 K to the 93Nb spin-lattice relaxation rate has also been observed. The CDW states have been studied by resistivity measurements in other systems such as 2H-TaSe2 where a lock-in of the ICDW occurs through a 1st order phase transition between 87 and 93 K, or in the mixed polytype 4 HbTaS2 with a lock-in in 1T layers at 320 K and an onset in the 2H layers at 22 K. A pressure investigation shows a connection between the enhancement of Tc and the existence of a structural phase transition at T0 > Tc.The CDW phase transition will be discussed in terms of the phenomenological theory of McMillan. In this respect 4 HbTaS2 is of particular interest since the onset of CDW's occurs through a 1st order transition and therefore suggests that the coupling between the trigonal prismatic layers is relatively weaker in the mixed polytypes than in the pure 2H polytypes.