Theoretical Analysis On The Upper Critical Field Of Several Iron-based Superconductors

In2001, Japanese scientists discovered that superconductivity appeared at39K in binary compound MgB2, which aroused the huge interest of study in this field all over the world. Then it was verified that MgB2has two superconducting energy gaps by many experiments, such as specific heat measurement, electronic tunneling spectrum and angle-resolved photoemission spectrum. In2006, the group of professor Hideo Hosono at Tokyo Institute of Technology found the superconducting compound LaFeOP, of which the iron plays a major role in superconductivity. The discovery broke the notion that the Fe is not beneficial to induce the superconductivity. Since then, multigap superconductivity was also discovered in Fe-based materials, and KFe2As2and Ba(Fe1-xNix)2As2are two of them. Based on two-band Ginzburg-Landau theory, we principally study the temperature dependences of upper critical fields for two types of layered iron-based superconducting crystals, KFe2As2and Ba(Fe1-xNix)2As2(x=0.054and x=0.072). The results reproduce the experimental data in a broad temperature range and directly underlie the multigap superconductivity in these crystals. We solve Ginzburg-Landau equations by variational method, then perform the numerical calculations with Mathematica and finally plot the theoretical results on graphics with Origin. For KFe2As2, our calculations also indicate that the specific-heat jump at the superconducting critical temperature is about0.77, in accordance with the experimental data. We also study the angular dependences of upper critical fields at different temperatures for Ba(Fe1-xNix)2As2. The specific-heat jumps of these two materials are0.75(x=0.054) and0.79(x=0.072) respectively. All our theoretical analyses strongly point to the existence of two superconducting gaps in these systems.