  The in-plane anisotropy of the electrical resistivity across the coupled
orthorhombic and magnetic transitions of the iron pnictides has been
extensively studied in the parent and electron-doped compounds. All these
studies universally show that the resistivity $\rho_{a}$ across the long
orthorhombic axis $a_{O}$ - along which the spins couple antiferromagnetically
below the magnetic transition temperature - is smaller than the resistivity
$\rho_{b}$ of the short orthorhombic axis $b_{O}$, i. e. $\rho_{a}<\rho_{b}$.
Here we report that in the hole-doped compounds
Ba$_{1-x}$K$_{x}$Fe$_{2}$As$_{2}$, as the doping level increases, the
resistivity anisotropy initially becomes vanishingly small, and eventually
changes sign for sufficiently large doping, i. e. $\rho_{b}<\rho_{a}$. This
observation is in agreement with a recent theoretical prediction that considers
the anisotropic scattering of electrons by spin-fluctuations in the
orthorhombic/nematic state.

Superconductivity