Charge density wave and superconductivity in 2H-and 4H-NbSe2: A revisit

Good-quality hexagonal NbSe2 single crystals were prepared. In 2H-NbSe2, superconducting and charge density wave (CDW) transitions were found at T-s = 7.4 K and T-c, = 35 K respectively as reported previously. We have noticed that these two transitions are changed to T-c, = 42 K and T-s = 6.5 K, in 4H-NbSe2. Thermopower has shown clear anomaly at CDW transitions. The anisotropic upper critical field was calculated as similar to 3 and 6.3 for 2H- and 4H-single crystals around t = 0.81, where t = T/T-s, from resistivity and explained in terms of coherence length. From the relation, H-c2(T) = H-c2(0)[1- t(2)], H-c2(l)(0) was calculated as similar to 8.15 T and 16.98 T at t = 0.84 in 2H-NbSe2 and 4H-NbSe2 respectively. However, H-c2(t)(0) = 2.68 T for both single crystals.