Temperature and angular dependences of upper critical fields for the layer structure superconductor 2H-NbSe ₂

The temperature and angular dependences of upper critical fields H _c2 have been measured for several 2H-NbSe ₂ single crystals by use of an electrical conduction method in magnetic fields up to 150 kOe. As the temperature approaches the transition temperature T _c , the value of H _{c2{norm of matrix}} (parallel to the layer planes) decreases with a positive curvature, while the value of H _c2⊥ (perpendicular to the layer planes) decreases almost linearly. The ratio of H _{c2{norm of matrix}} to H _c2⊥ increases monotonically from 2.4 near T _c with decreasing temperature and reaches the constant value of 3.2 at the lowest temperature. It becomes clear that the simple effective mass model based on the anisotropic Ginzburg-Landau theory does not explain our experimental results. The anisotropic behavior of H _c2 can be accounted for by the Takanaka theory, which includes anisotropies of both the Fermi velocity and the energy gap and the effect of nonlocality. Agreement between experimental results and the theoretical prediction is obtained by the use of values of 0.16≲ε ₁ ² ≲0.25 and -0.6≲ε ₂ ≲-0.3, where ε ₁ is the mass anisotropy parameter and ε ₂ the gap anisotropy parameter. The coupling strength between layers is too strong to be explained by the Josephson phase coupling model proposed for quasi-two-dimensional layer superconductors.