Direct observation of two-gap superconductivity in SrFe$_{1.85}$Co$_{0.15}$As$_{2}$ single crystals by scanning tunneling microscopy and spectroscopy

Scanning tunneling microscopy and spectroscopy (STM/S) studies on the recently discovered superconductor, SrFe$_{1.85}$Co$_{0.15}$As$_{2 }$single crystal, and its parent material SrFe$_{2}$As$_{2}$ reveal a 0.8 nm stripe pattern in the atomic resolution topographic images of both samples. In the conductance spectra taken along a line on the SrFe$_{1.85}$Co$_{0.15}$As$_{2}$\underline { }surface robust superconducting gaps (2$\Delta _{large}$ = 17.3 meV) as well as additional small gap (2$\Delta _{small}$ = 2.9 meV) structures were simultaneously observed. To the best of our knowledge, these are the first observations of such two-gap structures in pnictides in STS/M studies.