Evidence for two-gap nodeless superconductivity in SmFeAsO1−xFx from point-contact Andreev-reflection spectroscopy

Point-contact Andreev-reflection spectroscopy measurements were performed in SmFeAsO_(1−x)F_x polycrystals with x=0.09 (Tc~42 K) and x=0.20 (Tc~52 K). In all cases the experimental conductance curves reproducibly exhibit low-energy peaks and higher-energy shoulders (at 4–6 and 16–20 meV, respectively, for x=0.20), which indicate the presence of two nodeless superconducting gaps. While the single-band Blonder-Tinkham-Klapwijk model can only reproduce a small central portion of a given conductance curve, the two-gap one accounts remarkably well for the shape of the whole experimental dI /dV vs V curve. The fit of the normalized curves gives Delta_1(0)=6.15+-0.45 meV and Delta _2(0)=18+-3 meV for x=0.20, while for x=0.09 the values Delta_1(0)=4.9+-0.5 meV and Delta_2(0)=15+-1 meV are obtained. In all cases, both gaps close at the same temperature and follow a BCS-like behavior.