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Dimer Impurity Scattering, "Reconstructed" Nesting and Density-Wave Diagnostics in Iron Pnictides

While the impurity-induced nanoscale electronic disorder has been extensively reported in the underdoped iron pnictides, its microscopic origins remain elusive. Recent scanning tunneling microscopy (STM) measurements reveal a dimer-type resonant structure induced by cobalt doping. These dimers are randomly distributed but uniformly aligned with the antiferromagnetic a axis. A theory of the impurity-induced quasiparticle interference patterns is presented that shows the local density of states developing an oscillatory pattern characterized by both geometry and orbital content of the {\em reconstructed} Fermi pockets, occasioned by the pocket density-wave (PoDW) order along the b axis. This pattern breaks the $C_4$ symmetry and its size and orientation compare well with the dimer resonances found in the STM experiments, hinting at the presence of a "hidden" PoDW order. More broadly, our theory spotlights such nanoscale structures as a useful diagnostic tool for various forms of order in iron pnictides.