Theoretical Analysis On The Electromagnetic Properties Of Two-gap Superconductors NbSe₂ And TlBi₂Se₂

In 2011 Japanese scientists discovered that the binary compound MgB2 becomes superconducting at a high temperature about 39 K, then it was verified that MgB2 has two superconducting energy gaps by many experiments, such as specific heat measurement, electronic tunneling spectrum and angle-resolved photoemission spectrum. This discovery has aroused the huge interest of studying two-gap superconductors in this field all over the world, and various theoretical models of two-band superconductivity are proposed up to now. Based on two-band Ginzburg-Landau theory, we mainly study temperature dependences of upper critical field for simple layered compounds NbSe2 and TlNi2Se2, and it appears to involve heavy electrons in TlNi2Se2. We solve two-band Ginzburg-Landau equations by variational method, then perform the numerical calculations with Mathematica. The theoretical results of the upper critical field reproduce the experimental data well. It fully verifies the two-gap scenario for these materials.