Scanning Tunneling Spectroscopy and Vortex Imaging in the Iron Pnictide Superconductor BaFe1.8Co0.2As2

We present an atomic resolution scanning tunneling spectroscopy study of superconducting BaFe_1.8Co_0.2As₂ single crystals in magnetic fields up to 9 T. At zero field, a single gap with coherence peaks at Δ¯=6.25meV is observed in the density of states. At 9 and 6 T, we image a disordered vortex lattice, consistent with isotropic, single flux quantum vortices. Vortex locations are uncorrelated with strong-scattering surface impurities, demonstrating bulk pinning. The vortex-induced subgap density of states fits an exponential decay from the vortex center, from which we extract a coherence length ξ=27.6±2.9Å, corresponding to an upper critical field H_c2=43T.