title: Scanning tunneling spectroscopy and vortex imaging in the iron pnictide superconductor BaFe1.8Co0.2As2.
abstract: We present an atomic resolution scanning tunneling spectroscopy study of superconducting BaFe1.8Co0.2As2 single crystals in magnetic fields up to 9 T. At zero field, a single gap with coherence peaks at Delta=6.25 meV 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 xi=27.6+/-2.9 A, corresponding to an upper critical field Hc2=43 T.
authors.affiliation: Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.
authors.forename: Yi
authors.initials: Y
authors.lastname: Yin
authors.forename: M
authors.initials: M
authors.lastname: Zech
authors.forename: T L
authors.initials: TL
authors.lastname: Williams
authors.forename: X F
authors.initials: XF
authors.lastname: Wang
authors.forename: G
authors.initials: G
authors.lastname: Wu
authors.forename: X H
authors.initials: XH
authors.lastname: Chen
authors.forename: J E
authors.initials: JE
authors.lastname: Hoffman