  We present the synthesis and the tunneling spectroscopy study of
superconducting FeSe0.5Te0.5 (Tc = 14 K), SmFeAsO0.85 (Tc = 45 K) and
SmFeAsO0.9F0.1 (Tc = 52 K). The samples were characterized by Rietveld
refinement of X-ray diffraction patterns and transport measurements. Tunneling
experiments on FeSe0.5Te0.5 revealed a single superconducting gap of ~1 meV in
BCS-like tunnelling conductance spectra. In SmFeAsO0.85 and SmFeAsO0.9F0.1
however, more complex spectra were observed characterized by two gap-like
structures at ~4 meV and ~10 meV. These spectra are qualitatively understood
assuming a two-band superconductor with a 's\pm' order parameter. We show that
depending on the sign relation between the pairing amplitudes in the two bands,
the interband quasiparticle scattering has a crucial effect on the shape of the
tunneling spectra. Detailed analysis of the tunneling spectroscopy data
supports the 's\pm' model, but single-gap spectra found in FeSe0.5Te0.5 are
more compatible with a disorder-induced 's'-wave gap, due to the Se-Te
substitution.
