Sign-reversal of the in-plane resistivity anisotropy in iron pnictides

The concept of an electronically-driven breaking of the rotational symmetry of a crystal, without involving magnetic order, has found experimental support in several systems, from semiconductor heterostructures and ruthenates, to cuprate and iron-pnictide superconductors. In the pnictide BaFe$_{2}$As$_{2}$, such an "electronic nematic state" appears above the magnetic transition dome, over a temperature range that can be controlled by external strain. Here, by measuring the in-plane resistivity anisotropy, we probe the electronic anisotropy of this material over the entire nematic/magnetic dome, whose end points coincide with the optimal superconducting transition temperatures. Counter-intuitively, we find that, unlike other materials, the resistivity anisotropy in BaFe$_{2}$As$_{2}$ changes sign across the doping phase diagram, even though the signs of the magnetic, nematic, and orthorhombic order parameters are kept fixed. This behavior is explained by the Fermi surface reconstruction in the magnetic phase and spin-fluctuation scattering in the nematic phase. The unique behavior of the transport anisotropy unveils that the primary role is played by magnetic scattering in the normal state transport properties of the iron pnictides, suggesting a close connection between magnetism, nematicity, and unconventional superconductivity.