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Hoffman |
Lab |
Yi Yin,1 M. Zech,1 T. L. Williams,1 K. F.
Wang,2 G. Wu,2 X. H. Chen,2 and J. E. Hoffman1
1Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA
2Hefei National Laboratory for Physical Science at Microscale and Department of Physics, University of Science and Technology of China, Hefei, Anhui 230026, People's Republic of China
Phys. Rev. Lett. Volume 102, 097002 (3 March 2009).
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 of 27.6±2.9 Å, corresponding to an upper critical field Hc2=43 T.
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