Dimer impurity scattering, reconstructed Fermi-surface 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 miscroscopy (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 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.