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

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, T-c, of 23.6 K. The initial M-H curve was used to find the lower critical field, H-c1. The full range of the temperature dependence of Fir, was explained by using a two S-wave gap symmetry. We estimate the two gap as Delta(1)(0) = 1.64 +/- 0.2 meV for the small gap and Delta(2)(0) = 6.20 +/- 0.2 meV for the large gap. (C) 2009 Elsevier B.V. All rights reserved.