Resistivity Anisotropy and Novel Impurity-Induced States in Fe-based Superconductors

We investigate emergent impurity-induced states arising from point-like scatterers in the spin-density wave (SDW) phase of iron-based superconductors within a microscopic five-band model [1]. Independent of the details of the band-structure and disorder potential, it is shown how stable magnetic (pi,pi) unidirectional nematogens are formed locally by the impurities. Interestingly, these nematogens exhibit a dimer structure in the electronic density, are directed along the antiferromagnetic a-axis, and have typical lengths of 10 lattice constants in excellent agreement with recent scanning tunnelling experiments [2]. These electronic dimers provide a natural explanation of the dopant-induced transport anisotropy found e.g. in the 122 iron pnictides [3]. We also study the extension of the (pi,0) SDW state above the putative Neel transition temperature T$_{\mathrm{N}}$ by addition of magnetic impurities. This study is relevant for recent neutron scattering studies showing induced magnetic high-temperature phases for sufficient amounts of Mn substitution in 122 materials [4]. Below T$_{\mathrm{N}}$ neutron studies have found enhanced (pi,pi) scattering which also can be reproduced within our scenario [5]. \\[4pt] [1] M. N. Gastiasoro \textit{et al.}, arXiv:1307.4913 (2013).\\[0pt] [2] M. P. Allan \textit{et al.,} Nat. Phys. \textbf{9}, 220 (2013).\\[0pt] [3] S. Ishida \textit{et al.,} Phys. Rev. Lett. \textbf{101}, 207001 (2013).\\[0pt] [4] D. S. Inosov \textit{et al., }Phys. Rev. B \textbf{87}, 224425 (2013).\\[0pt] [5] G. S. Tucker \textit{et al., }Phys. Rev. B \textbf{86}, 020503 (2012).