Doping Influence on Sm$_{1-x}$Th$_x$OFeAs Superconducting Properties: Observation of Intrinsic Multiple Andreev Reflections Effect and Superconducting Parameters Determination
We studied SNS- and S-N-S-N-...-S contacts (where S - superconductor, N - normal metal) formed by "break-junction" technique in polycrystalline Sm$_{1-x}$Th$_x$OFeAs superconductor samples with critical temperatures $T_C = (34 \div 45)$ 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 $\sqrt{V_{11}V_{22}}/V_{12} \approx 14$ (where $V_{ij}$ - 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 ($V_{i \ne j} = 0$) for each condensate as $2\Delta_{L,S}/k_BT_C^{L,S} \le 4.5$. The values are constant within the range of critical temperatures studied, and correspond to a case of strong intraband electron-phonon coupling.