Research On The Upper Critical Field Of Two-band Superconductor NbSe₂

In2011, Japanese scientists discovered that MgB2crystal becomes superconducting at the temperature39K, which aroused a huge research interest in this field all over the world. The theoretical calculations indicate that MgB2is a superconductor with two superconducting gaps. Two-gap scenario of the superconducting MgB2has been testified in many experiments, such as specific heat measurement, nuclear magnetic resonance, electronic tunneling spectrum and angle-resolved photoemission spectrum. Thereafter, some other materials with multi-band superconductivity are discovered. Based on the two-band Ginzburg-Landau (GL) theory, we study the temperature dependence of the upper critical field for superconducting crystal NbSe2. We also calculate the temperature dependence of the anisotropy parameter for the upper critical field. We solve the two-band GL equations by variational method, and then perform the numerical calculations with Mathematica. Our calculations on the upper critical field and the temperature dependence of the anisotropy parameter for upper critical field fit the experimental data well in a broad temperature range. Our results thus support two-gap scenario of the superconducting NbSe2. The calculations also show that the effective mass anisotropy for the band with the larger gap is about10, the other band is almost1. Based on the experimental data from Fermi velocities and plasma frequencies, we expect that the smaller gap is most probably located on the bonding Nb Fermi sheets.