Theoretical Study on the Upper Critical Field of a Layered Superconductor NbSe_2

Based on two-band Ginzburg-Landau theory, we study the temperature and angular dependences of the upper critical field for superconducting NbSe\(_2\) crystals. We introduce two variational parameters in our approach, and provide the explicit equations to determine the upper critical field in arbitrary direction. Our analyses strongly support previous scanning tunneling microscopy results and specific heat data, pointing to the existence of two energy gaps in this compound. Our results thus suggest that besides MgB\(_{2}\), NbSe\({_2}\) is another example of two-gap superconductors.