Measurements of the upper (HC2) and lower (HC1) critical fields in clean single crystals, and their anisotropies between the 2 principal crystallographic directions, were reviewed. Such crystals were far into the clean limit of type-II superconductivity, and for fields applied in the c-direction, the Ginzburg–Landau parameter κ was only about 3; just large enough for type-II behavior. Because μ0 HC2 was so low (about 3T for fields in the c-direction) MgB2 had to be modified for it to become useful for high-current applications. It was expected to be possible to increase HC2 by the introduction of strong electron scattering. However, because of the electronic structure and the double gap that resulted, the scatterers would have to be chosen carefully. In addition, pinning defects on the scale of a few nm would have to be engineered in order to enhance the critical current density at high fields.

Critical Fields and Critical Currents in MgB2. A.D.Caplin, Y.Bugoslavsky, L.F.Cohen, L.Cowey, J.Driscoll, J.Moore, G.K.Perkins: Superconductor Science and Technology, 2003, 16, 176-82