  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.
