Evolution of superconducting gaps in Th-substituted Sm1-xThxOFeAs studied by multiple Andreev reflection spectroscopy
Using intrinsic multiple Andreev reflections effect spectroscopy, we studied SnS contacts in the layered oxypnictide superconductors Sm1-xThxOFeAs with various thorium doping and critical temperatures TC=21-54 K. We observe a scaling between both superconducting gaps and TC. The determined BCS ratio for the large gap 2ΔL/kBTC=5.0-5.7 and its eigen-BCS ratio (in a hypothetical case of zero interband coupling) 2ΔL/kBTCL=4.1-4.6 both exceeding the weak-coupling limit 3.52, and for the small gap 2ΔS/kBTC=1.2-1.6, remain nearly constant within all the TC range studied. The temperature dependences ΔL,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 Coulomb repulsion constants μ∗ are finite in Sm-based oxypnictides.