Multiple Andreev reflections spectroscopy of two-gap 1111- and 11 Fe-based superconductors

The "break-junction" technique was used to prepare superconductor - constriction - superconductor (ScS) nanocontacts in polycrystalline samples of Fe-based superconductors CeO$_0.88$F$_0.12$FeAs (Ce-1111; Tc^bulk = 40 - 42 K), LaO$_0.9$F$_0.1$FeAs (La-1111; Tc^bulk = 27 - 29 K), and FeSe (Tc^bulk = 10 - 12 K). Two sets of subharmonic gap structures were detected due to multiple Andreev reflections effect, indicating the presence of two nodeless superconducting gaps with the BCS-ratios 2Δ_L/kTc = 4.2 - 5.9 and 2Δ_S/kTc << 3.52, respectively. Temperature dependences Δ_L,S(T) in FeSe indicate a k-space proximity effect between two superconducting condensates. A linear relation between the gap Δ_L and magnetic resonance energy E_res ~ 2Δ_L is found to be valid for iron-based superconductors.