Anomalous features of the CDW transition in metallic 2H dichalcogenides

Abstract
The metallic dichalcogenides of transition metals which cystallise in the trigonal prismatic 2H structure, are considered as a model family for the study of charge-density-wave transitions of the metal-metal type. We review different properfies and their evolution in the series : transport, specific heat, rnagnetic susceptibility, soft phonon dynamics. For temperatures higher than the transition temperature, T_d many characters underline the departure from the normal metal behaviour as well as from the behaviour resulting from the presence of the simple soft mode predicted by weak-coupling theories. A phenomenological analysis rather suggests for this family a very strong coupling mechanism involving local vibronic pseudo spins, narrow-band character and proximity to an order-disorder transition.