Electrical Transport Properties in 2 H -NbS 2 , -NbSe 2 , -TaS 2 and -TaSe 2

Detailed measurements of the resistivity and the Hall coefficient have been made on high-quality single crystals of 2 H metallic transition-metal dichalcogenides between 4.2 K and 300 K, in order to investigate the carrier scattering mechanisms in a CDW system. The resistivity of 2 H -NbS 2 , which has no CDW transition, shows normal behavior. On the other hand, in 2 H -TaSe 2 which has the highest CDW transition temperature ( T o =122 K) of 2 H family, several anomalous features are observed. The temperature dependence of the resistivity in 2 H -TaSe 2 is ∼ T 5 ( T <15 K), ∼ T 2 (30 K< T <100 K) and A T + B ( T > T o ). The results were analyzed by including a “new” scattering mechanism due to phase fluctuations of the CDW (electron-phason scattering, phase disorder scattering). The analysis suggests that McMillan's phonon entropy model is more appropriate in 2 H -TaSe 2 than the conventional model.