Two S-wave gap symmetry for single crystals of the superconductor BaFe1.8Co0.2As2

Abstract

To clarify the gap structure of the iron–pnictide superconductors, we synthesized optimally doped single crystals of BaFe1.8Co0.2As2, which had a critical temperature, Tc, of 23.6K. The initial M–H curve was used to find the lower critical field, Hc1. The full range of the temperature dependence of Hc1 was explained by using a two S-wave gap symmetry. We estimate the two gap as Δ1(0)=1.64±0.2meV for the small gap and Δ2(0)=6.20±0.2meV for the large gap.