Ya. G. Ponomarev, S. A. Kuzmichev, T. E. Kuzmicheva, M. G. Mikheev, M. V. Sudakova, S. N. Tchesnokov, O. S. Volkova, A. N. Vasiliev, Yu. F. Eltsev, V. M. Pudalov, A. V. Sadakov, A. S. Usol'tsev, Th. Wolf, E. P. Khlybov
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 2\Delta_L/kTc = 4.2 - 5.9 and 2\Delta_S/kTc << 3.52, respectively. Temperature dependences \Delta_{L,S}(T) in FeSe indicate a k-space proximity effect between two superconducting condensates. A linear relation between the gap \Delta_L and magnetic resonance energy E_{res} ~ 2\Delta_L is found to be valid for iron-based superconductors.
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http://arxiv.org/abs/1302.5320
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