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

TL;DR

This study uses Andreev reflection spectroscopy to identify two superconducting gaps in Fe-based superconductors, revealing their temperature dependence and a linear relation between the larger gap and magnetic resonance energy, advancing understanding of their pairing mechanisms.

Contribution

It provides direct spectroscopic evidence of two-gap superconductivity in Fe-based superconductors and explores their temperature behavior and relation to magnetic resonance.

Findings

Detection of two superconducting gaps via multiple Andreev reflections.

Observation of a linear relation between the larger gap and magnetic resonance energy.

Temperature dependence of gaps indicating a k-space proximity effect.

Abstract

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; $T_C^{\rm bulk} = 28 \pm 1 K$), and FeSe ($T_C^{\rm bulk} = 12 \pm 1 K$). 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 $\Delta_{L,S}(T)$ 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, $E_{\rm res} \approx 2\Delta_L$.