London penetration depth in iron-based superconductors

Measurements of London penetration depth, a sensitive tool to study multiband superconductivity, have provided several important insights into the behavior of Fe-based superconductors. We first briefly review the 'experimentalist-friendly' self-consistent Eilenberger two-band model that relates the measurable superfluid density and temperature dependences of the superconducting gaps. Then we focus on BaFe2As2-derived materials, for which the results are consistent with (1) two distinct superconducting gaps; (2) development of strong in-plane gap anisotropy with departure from optimal doping; (3) development of gap nodes along the c direction in a highly overdoped regime; (4) significant pair-breaking, presumably due to charge doping; (5) fully gapped intrinsic behavior (exponential at low temperatures) at optimal doping if the scattering is removed (probed in the 'self-doped' stoichiometric LiFeAs); (6) competition between the magnetically ordered state and superconductivity, which do coexist in underdoped compounds. Overall, it appears that while there are common trends in the behavior of Fe-based superconductors, the gap structure is non-universal and is quite sensitive to the doping level. It is plausible that the rich variety of possible gap structures within the general s(+/-) framework is responsible for the observed behavior.