Iron-based high-temperature superconductivity develops when the ‘parent’ antiferromagnetic/orthorhombic phase is suppressed, typically by introduction of dopant atoms1. But their impact on atomic-scale electronic structure, although in theory rather complex2,3,4,5,6,7,8,9,10,11,12,13, is unknown experimentally. What is known is that a strong transport anisotropy14,15,16,17,18,19,20,21,22,23,24,25 with its resistivity maximum along the crystal b axis14,15,16,17,18,19,20,21,22,23,24,25, develops with increasing concentration of dopant atoms14,20,21,22,23,24,25; this ‘nematicity’vanishes when the parent phase disappears near the maximum superconducting Tc. The interplay between the electronic structure surrounding each dopant atom, quasiparticle scattering therefrom and the transport nematicity has therefore become a pivotal focus7,8,12,22,23 of research into these materials. Here, by directly visualizing the atomic-scale electronic structure, we show that substituting Co for Fe atoms in underdoped Ca(Fe1−xCox)2As2 generates a dense population of identical anisotropic impurity states. Each is ∼ 8 Fe–Fe unit cells in length, and all are distributed randomly but aligned with the antiferromagnetic a axis. By imaging their surrounding interference patterns, we further demonstrate that these impurity states scatter quasiparticles in a highly anisotropic manner, with the maximum scattering rate concentrated along the b axis. These data provide direct support for the recent proposals7,8,12,22,23 that it is primarily anisotropic scattering by dopant-induced impurity states that generates the transport nematicity; they also yield simple explanations for the enhancement of the nematicity proportional to the dopant density14,20,21,22,23,24,25 and for the occurrence of the highest resistivity along the b axis14,15,16,17,18,19,20,21,22,23,24,25.
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We acknowledge and thank F. Baumberger, P. Dai, P. J. Hirschfeld, J. E. Hoffman, A. Kaminski, E-A. Kim, D-H. Lee, J. Orenstein, A. Pasupathy, G. Sawatzky, D. J. Scalapino, J. Schmalian and S. Uchida for helpful discussions and communications. We are grateful to N. D. Loh for insightful suggestions and to A. W. Rost for proposing the analysis and presentation in Fig. 4b. The reported studies are supported by the Center for Emergent Superconductivity, a DOE Energy Frontier Research Center headquartered at Brookhaven National Laboratory. Work at the Ames National Laboratory was supported by the DOE, Basic Energy Sciences under Contract no. DE-AC02- 07CH11358; the ARPES experiments were supported by the DOE DE-FG02-03ER46066 through the University of Colorado, and were performed at the Advanced Light Source supported by the DOE Office of Science. Further support came from NSF/DMR-0654118 through the National High Magnetic Field (T-M.C.), the Cornell Center for Materials Research under NSF/DMR-0520404 (Y.X.); the UK Engineering and Physical Sciences Research Council and the Scottish Funding Council under the PhD plus program (M.P.A.); and the Foundation for Fundamental Research on Matter (FOM) of the Netherlands Organization for Scientific Research (F.M. and M.S.G.).
The authors wish to dedicate this study to the memory of Prof. Zlatko Tešanović, some of whose recent research was focused on issues addressed herein.