Multiple Andreev Reflections Spectroscopy of Two-Gap 1111-and 11 Fe-Based Superconductors

Using the break-junction technique, we prepared and studied superconductor-constriction-superconductor (ScS) nanocontacts in polycrystalline samples of F superconductors (CeOF0.12FeAs)-F-0.88 (Ce-1111; T-C(bulk) = 41 +/- 1 K), LaO0.9F0.1FeAs (La-1111; T-C(bulk) = 28 +/- 1 K), and FeSe (T-C(bulk) = 12 +/- 1 K). We detected two subharmonic gap structures related with multiple Andreev reflections, indicating the presence of two superconducting gaps with the BCS-ratios 2 Delta(L)/k(B)T(C) = 4.2 divided by 5.9 and 2 Delta(S)/k(B)T(C) similar to 1 << 3.52, respectively. Temperature dependences of the two gaps Delta(L, S)(T) in FeSe indicate a k-space proximity effect between two superconducting condensates. For the studied iron-based superconductors, we found a linear relation between the gap Delta(L) and magnetic resonance energy, E-res approximate to 2 Delta(L).