Surface Structure and Superconductivity in Ba(Fe0.93Co0.07)2As2 Probed by Scanning Tunneling Microscopy/Spectroscopy

We present scanning tunneling microscopy/spectroscopy (STM/STS) on iron-based superconductor Ba(Fe-0.93-Co-0.07)(2)As-2 single crystals. The STM topography shows that the low-temperature cleaved surface consists of two types of ordered atomic structures: (1) the 2 x 1 surface, formed by half of the Ba atoms remaining on the As layer, and (2) the root 2 x root 2 p surface of the As layer from which Ba atoms are completely removed. The superconducting spectra with the energy gap (2 Delta similar or equal to 6.9k(B)T(c)) are observed only on the 2 x 1 surface. This value is approximately twice that for weak coupling s-wave BCS superconductors, indicating a strong pairing interaction similar to that observed in cuprate superconductors. We also find that vortices form a disordered structure and that the vortex core spectroscopy does not show a bound state, suggesting a strong disorder effect on the vortex pinning and quasiparticle scattering.