Dome – like variation of the superconducting gap anisotropy in Fe-based superconductors

Experiments performed on different iron – based superconductors suggest a variety of possible structures of the superconducting energy gap, both nodeless and nodal. To understand the pairing mechanisms, it is important to identify common features in the behavior of different materials. Measurements of the temperature – dependent London penetration depth provide important information on the structure of the superconducting gap. We show that despite significant differences between different iron – based superconductors, there is a universal trend: the gap is least anisotropic at the optimal doping and its anisotropy increases upon the departure towards underdoped and overdoped ends of the "superconducting dome". This trend is not related to the presence of the long – range magnetic order in the underdoped state.