ANOMALOUS FEATURES OF THE CDW TRANSITION IN METALLIC 2H DICHALCOGENIDES

The metallic dichalcogenides of transition metals which crystallise in the trigonal prismatic 2H structure, are considered as a model family for the study of charge-density-wave transitions of the metal-metal t ype. We review different properties and their evolution in the series: transport, specific heat, magnetic susceptibility, soft phonon dynami cs. For temperatures higher than the transition temperature, T(d), man y characters underline the departure from the normal metal behaviour a s well as from the behaviour resulting from the presence of the simple soft mode predicted by weak-coupling theories. A phenomenological ana lysis rather suggests for this family a very strong coupling mechanism involving local vibronic pseudo spins, narrow-band character and prox imity to an order-disorder transition.