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

Point-contact Andreev-reflection spectroscopy measurements were performed in SmFeAsO1-xFx polycrystals with x=0.09 (T-c similar or equal to 42 K) and x=0.20 (T-c similar or equal to 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.