Unconventional superconductivity usually originates from several strongly coupled degrees of freedom, such as magnetic, charge and elastic. A highly anisotropic electronic phase, not driven by lattice degrees of freedom, has been proposed in some of these superconductors, from cuprates to iron-based compounds. In the iron pnictide BaFe 2 As 2, this nematic phase arises in the paramagnetic phase and is present for wide doping and temperature ranges. Here we probe the in-plane electronic anisotropy of electron- and hole-doped BaFe 2 As 2 compounds. Unlike other materials, the resistivity anisotropy behaves very differently for electron- and hole-type dopants and even changes sign on the hole-doped side. This behaviour is explained by Fermi surface reconstruction in the magnetic phase and spin-fluctuation scattering in the paramagnetic phase. This unique transport anisotropy unveils the primary role played by magnetic scattering, demonstrating the close connection between magnetism, nematicity and unconventional superconductivity.
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We thank A.V. Chubukov for useful comments, suggestions and critical reading of the manuscript. R.M.F. and J.S. acknowledge useful discussions with E. Abrahams. Work at The Ames Laboratory was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under contract no. DE-AC02-07CH11358. The work in China was supported by the NSF of China, the Ministry of Science and Technology of China (973 projects: 2011CBA00102, 2012CB821403). R.M.F. acknowledges the support of the NSF Partnerships for International Research and Education (PIRE) programme OISE-0968226.