On the Ginzburg-Landau Analysis of the Lower Critical Field Hc1 in MgB2

Temperature dependence of the lower critical field Hc1 (T) for the superconducting magnesium diboride, MgB2, is studied in the vicinity of Tc by using a two-band Ginzburg-Landau (G-L) theory. Theoretically calculated Hc1 (T) near Tc exhibits a negative curvature. The results are shown to be in a good agreement with the experimental data. In addition, two-band G-L theory calculations give a temperature dependence of the Ginzburg-Landau parameter, which varies little with temperature as in a similar manner of the microscopic single-band BSC theory.