Observation of two-gap superconductivity in SrFe1.85Co0.15As2 single crystals by scanning tunneling microscopy and spectroscopy

Abstract

Superconducting properties of SrFe1.85Co0.15As2 single crystals and their parent material, SrFe2As2, were investigated by scanning tunneling microscopy and spectroscopy (STM/S). In the parent material, we modeled surface conditions on the in situ cleaved single crystals, based on the observation of 2x1 stripe patterns and p root 2 x root 2 square-lattice patterns in the atomic-resolution topography images and with the help of local density of states measurements. In the STM/S studies on SrFe1.85Co0.15As2, a robust superconducting gap (2 Delta(large) = 17.3 meV) was observed in the conductance spectra measured along a line on the SrFe1.85Co0.15As2 surface. Moreover, an additional small gap-like (2 Delta(small) = 2.9 meV) structure was simultaneously observed. Our observation corroborates the two-gap structures in iron-based superconductors.