Evolution of Superconducting Gaps in Th-Substituted Sm${}_{1-x}$Th${}_x$OFeAs Studied by Multiple Andreev Reflection Spectroscopy

Abstract

Using intrinsic multiple Andreev reflections effect (IMARE) spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm${}_{1-x}$Th${}_x$OFeAs with various thorium doping and critical temperatures $T_C=21-54$ K. We observe a scaling between both superconducting gaps and $T_C$. The determined BCS-ratio for the large gap $2{\Delta }_L/k_B{T}_C=5.0-5.7$ and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) $2{\Delta }_L/k_B{T}_C^L=4.1-4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2{\Delta }_S/k_B{T}_C=1.2-1.6$ remain nearly constant within all the $T_C$ range studied. The temperature dependences ${\Delta }_{L,S}(T)$ agree well with a two-band BCS-like Moskalenko and Suhl model. We prove intraband coupling to be stronger than interband coupling, whereas and Coulomb repulsion constants ${\mu }^{\ast }$ are finite in Sm-based oxypnictides.

arXiv comment12 pages, 14 figures

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