The Helfand and Werthamer theory developed in 1960s predicts the magnetic field at which a superconductor turns into a normal metal if certain details of the electronic structure are known. When new superconductors are discovered, their upper critical field is usually analyzed using this theory, even though it has a well-known shortcoming — it assumes that the electronic properties of the superconductor are the same in all directions and lead to an isotropic upper critical field. A new generalization and expansion of the Helfand and Werthamer theory includes anisotropy and allows for more than one electron energy band. Researchers used the new theory to look at recently discovered anisotropic superconductors, including magnesium diboride and iron–based compounds. Previously it was thought that the cause of the temperature dependence found for the anisotropy of the upper critical field was multiple bands.However, the new scheme rules out this commonly held belief. This improved analysis method greatly advances our understanding of new superconductors.
Orbital Upper Critical Field and its Anisotropy of Clean One- and Two-band Superconductors