Doping influence on Sm$_{1-x}$Th$_x$OFeAs superconducting properties: Observation of the effect of intrinsic multiple Andreev reflections and determination of the superconducting parameters

We studied SNS and S-N-S-N-$\dots$-S contacts (where S is a superconductor and N is a normal metal) formed by “break-junction” technique in polycrystalline Sm$_{1-x}$Th$_x$OFeAs superconductor samples with critical temperatures $T_c=(34{-}45)\,$K. In such contacts (intrinsic) multiple Andreev reflections effects were observed. Using spectroscopies based on these effects, we detected two independent bulk order parameters and determined their magnitudes. Theoretical analysis of the large and the small gap temperature dependences revealed superconducting properties of Sm$_{1-x}$Th$_x$OFeAs to be driven by intraband coupling, and $\sqrt{V_{11}V_{22}}/V_{12}\approx14$ (where $V_{ij}$ are the electron-boson interaction matrix elements), whereas the ratio between density of states for the bands with the small and the large gap, $N_2/N_1$, correspondingly, was roughly of an order. We estimated “solo” BCS-ratio values in a hypothetic case of zero interband coupling ($V_{i\neq j}=0$) for each condensate as $2\Delta_{L,S}/k_{\text{B}}T_c^{\text{L,S}}\leq4.5$. The values are constant within the range of critical temperatures studied, and correspond to a case of strong intraband electron-phonon coupling.