Highly Isotropic In-plane Upper Critical Field in the Anisotropic s -Wave Superconductor 2 H -NbSe 2

Resistivity measurements for a layered transition metal dichalcogenide 2 H -NbSe 2 were performed to investigate the in-plane anisotropy of the upper critical field ( H c2 ) for magnetic fields up to 14.5 T at temperatures down to 2.0 K. For fields rotated within the basal plane, the in-plane anisotropy of a characteristic field H ∗ (defined as the field strength at which the resistivity reaches zero) showed twofold symmetry at low temperatures. On the basis of this result, the in-plane anisotropy of H c2 and the superconducting gap structure in 2 H -NbSe 2 are discussed.