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 nanocontacts in polycrystalline samples of Fe-based superconductors CeO${}_{0.88}$F${}_{0.12}$FeAs (Ce-1111; $T_C^{\rm bulk} = 41 \pm 1 K$),LaO${0.9}$F${0.1}$FeAs(La-1111;$T_C^{\rm bulk} = 28 \pm 1 K$),andFeSe($T_C^{\rm bulk} = 12 \pm 1 K$). We detected two subharmonic gap structures related with multiple Andreev reflections, indicating the presence of two superconducting gaps with the BCS-ratios $2\Delta_L/k_BT_C = 4.2 \div 5.9$and$2\Delta_S/k_BT_C\sim 1 \ll 3.52$, respectively. Temperature dependences of the two gaps ${\mathrm{\Delta }}_{L,S}(T)$ 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 ${\mathrm{\Delta }}_L$ and magnetic resonance energy, $E_{\mathrm{r}\mathrm{e}\mathrm{s}}\approx 2{\mathrm{\Delta }}_L$.