Electrodynamics of Solids: Low-Energy Spectroscopy of Correlated Electrons

Electronic correlations in solids, though often ne-glected for simple materials, can become decisive effects leading to novel states of matter. Spectroscopic investigations have to adjust to the small energy scales of relevance here, μmeV or meV. Hence, the crucial optical experiments are conducted in the far-infrared range, at THz and microwave frequencies or even below. In recent years, significant advances in methods, materials and understanding allowed us to shed new light on the electrodynamic prop-erties of correlated electron systems, answer pertinent questions and reveal unexpected properties.