Fermi surfaces and band structures of the 2H metallic transition-metal dichalcogenides

Non-relativistic calculations on the band structures of the 2H polytype of NbSe2, NbS2, TaSe2 and TaS2 are reported. The slight qualitative differences which occur seem to be determined by the chalcogen rather than the metal. The Fermi surfaces are presented, together with a simple and accurate Fourier fit involving three parameters per sheet. The hole orbits are infinitely extended in the direction normal to the layers. A possible connection of these results to the observed charge-density waves is discussed. Some comments concerning the technique (layer method) are included. Special attention is devoted to the convergence of the method and the use of group theory.