Temperature dependence of the lower critical field Hc1 in SmFeASO0.9F0.1 and Ba0.6K0.4Fe2As2 iron-arsenide superconductors

We report detailed measurements of the temperature dependence of the lower critical field H_c1 of the FeAs-based superconductor SmFeAsO_0.9F_0.1 (Sm-1111) and Ba_0.6K_{0. 4}Fe₂As₂ (BaK-122) by global and local magnetization measurements. It is found that the obtained H_c1 for both kinds of samples show a weak temperature dependence in low temperature region. We argue that this weak T-dependence of H_c1 of Sm-1111 does not indicate a conventional s-wave state, instead it satisfies a T² dependence up to a temperature of (0.5–0.6) T_c. In contrast, the data of H_c1(T) of BaK-122 superconductor clearly show a multigap behavior. Excellent fitting to the data can be reached with two s-wave superconducting gaps. Comparison of the absolute values of H_c1(0) between Sm-1111 and BaK-122 shows a relatively large superfluid density for the latter. This may indicate the distinction between the electron doped and hole-doped FeAs-based superconductors.