Huan Yang, Zhenyu Wang, Delong Fang, Toshikaze Kariyado, Gengfu Chen, Masao Ogata, Tanmoy Das, A. V. Balatsky, Hai-Hu Wen
The pairing mechanism in the iron pnictides and chalcogenides remains as a hot and unresolved issue. The widely perceived picture is the S+- model. One of the puzzles concerning this model is why superconductivity survives up to a relatively high temperature in some doped samples, such as the Ba(Fe1-xTx)2As2 (T=Co, Ni, Ru, etc.), where the dopants are supposed to be the pair breakers. It has long been a desire to visualize microscopically how strong the scattering effect is in the iron pnictides. Here we show the spatially resolved scanning tunneling spectroscopy in Na(Fe0.95Co0.05)As. We successfully identified the Co-impurities and investigated the spatial distribution of LDOS at different energies. We see a continuum of the in-gap states which lift the zero-bias conductivity to a finite value, however the spectrum shows negligible variation when going through the Co impurity site. This dichotomy effect is explained with the possible S+- and S++ models.
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http://arxiv.org/abs/1203.3123
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