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

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We present scanning tunneling microscopy/spectroscopy (STM/STS) on iron-based superconductor Ba(Fe0.93 Co0.07)2As2 single crystals. The STM topography shows that the low-temperature cleaved surface consists of two types of ordered atomic structures: (1) the 2\times 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 (2\Delta\simeq 6.9 k_{\text{B}}T_{\text{c}}) are observed only on the 2\times 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.
2011-01-15