Surface Structure and Superconductivity in Ba(Fe0.93Co0.07)2As2Probed 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 2×1 surface, formed by half of the Ba atoms remaining on the As layer, and (2) the \(\sqrt{2}\times\sqrt{2}\) surface of the As layer from which Ba atoms are completely removed. The superconducting spectra with the energy gap (2Δ≃6.9 k B T c ) are observed only on the 2×1 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.