Strong coupling to magnetic fluctuations in the charge dynamics of iron-based superconductors

We present a comprehensive comparison of the infrared charge response of two systems, characteristic of classes of the 122 pnictide (SrFe2As2) and 11 chalcogenide (Fe1.087Te) Fe compounds with magnetically ordered ground states. In the 122 system, the magnetic phase shows a decreased plasma frequency and scattering, and associated appearance of strong midinfrared features. The 11 system, with a different magnetic-ordering pattern, also shows decreased scattering, but an increase in the plasma frequency while no clear midinfrared features appear below the ordering temperature. We suggest how this marked contrast can be understood in terms of the diverse magnetic-ordering patterns of the ground state and conclude that while the high-temperature phases of these systems are similar, magnetic order strongly affects the charge dynamical response. In addition, we propose an optical-absorption mechanism which appears to be consistent with information gained from several different experiments.