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

TL;DR

This study investigates how superconducting gaps in Th-substituted SmOFeAs vary with doping and temperature using multiple Andreev reflection spectroscopy, revealing strong intraband coupling and deviations from weak-coupling BCS theory.

Contribution

First detailed analysis of superconducting gap evolution in Th-doped SmOFeAs using IMARE spectroscopy, demonstrating strong intraband coupling and two-band BCS-like behavior.

Findings

Superconducting gaps scale with critical temperature $T_C$.

Large gap exceeds weak-coupling BCS ratio.

Temperature dependence fits two-band BCS model.

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 $2\Delta_L/k_BT_C = 5.0-5.7$ and its eigen BCS-ratio (in a hypothetical case of zero interband coupling) $2\Delta_L/k_BT_C^L = 4.1-4.6$ both exceeding the weak-coupling limit 3.52, and for the small gap $2\Delta_S/k_BT_C = 1.2-1.6$ 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.