Studies of the gap structure of iron-based superconductors using magnetic penetration depth

This article reviews experiments to determine the structure of the superconducting gap in the iron-based superconductors. It focuses on insights provided by measurements of the temperature dependence of magnetic penetration depth but also discusses the information provided by other measurements especially thermal conductivity. The article includes background theory on the types of gap structure suggested for the iron-based superconductors as well as the general theory of penetration depth in conventional and unconventional superconductors. The effect of impurity scattering on each candidate gap structure is important for the interpretation of data and so will also be discussed. Penetration depth data for single crystals of the following compounds is reviewed: SmFeAsO0.8F0.2, FeTe0.5Se0.5, LaFePO, KFe2As2 and BaFe2(As0.67P0.33)2 along with discussion on related materials such as K- and Co-doped BaFe2As2.