Researchers have discovered that barium–iron–nickel–arsenic superconductors clearly deviate from the famous Ginzburg-Landau Theory developed in the 1960’s. According to this theory, superconductors should show a linear relationship between the magnetic field at which superconductivity is suppressed (known as the upper critical field) and the direction of the magnetic field. Using single crystals, researchers performed detailed experiments of the upper critical field as a function of temperature and the direction of the magnetic field. They were able to study the dependence on magnetic field direction to less than 0.1°. Plotting the inverse of the square of the upper critical field versus sin^{2}θ should have given a straight line according to the Ginzburg-Landau theory. It clearly did not. The non-linear dependence is speculated to reflect a variation in the superconducting gap along one axis of this superconductor. The upper critical fields of iron arsenic superconductors are very high leading to possible uses in high magnetic field applications. This work provides additional insight needed to find the best materials for such applications.

Angular-dependent Upper Critical Field of Overdoped Ba(Fe<sub>1-x</sub>Ni<sub>x</sub>)<sub>2</sub>As<sub>2</sub>