The effect of a charge-density wave transition on the transport properties of 2H-NbSe2

Single crystals of two-dimensional 2H-NbSe2, which undergoes a superconducting transition at T-c = 7.2 K and a chame-density wave (CDW) transition at T-CDW = 30 K. were synthesized. Measurements of the resistivity, Hall coefficient and magnetoresistance (MR) versus temperature were performed on NbSe2 crystals with different resistance residual ratios (RRR = R(300 K)/R(8 K)), chosen from different batches. The superconducting transition temperature hardly changes with the RRR. while the MR and Hall coefficient are strongly dependent on the RRR values of the samples. Moreover, the temperature and field dependence of the MR violate Kohler's rule, indicating that the scattering times of the charge carriers are no longer isotropic. A steep decrease of the Hall coefficient R-H below 50 K is only observed for the high quality sample: this can be interpreted in terms of a drastic increase of the mean free path for the electron-type charge carriers. The effect of the CDW transition on the Hall coefficient is discussed using a two-band model and a sharp change in the scattering rate on an electron-like orbit below T-CDW is suggested.