Electronic correlations in iron-pnictide superconductors and beyond: lessons learned from optics

The Coulomb repulsion, impeding electrons' motion, has an important impact on the charge dynamics. It mainly causes a reduction in the effective metallic Drude weight (proportional to the so-called optical kinetic energy), encountered in the optical conductivity, with respect to the expectation within the nearly free electron limit (defining the so-called band kinetic energy), as evinced from band-structure calculations. In principle, the ratio between the optical and band kinetic energies allows one to define the degree of electronic correlations. Through spectral weight arguments on the excitation spectrum, we provide an experimental tool, free from any theoretical- or band-structure-based assumptions, to estimate the degree of electronic correlations in several systems. We first address the novel iron-pnictide superconductors, which serve to set the stage for our approach. We then revisit a large variety of materials, ranging from superconductors, to Kondo-like systems as well as materials close to the Mott-insulating state. For comparison, we also tackle materials where the electron-phonon coupling dominates. We establish a direct relationship between the strength of interaction and the resulting reduction in optical kinetic energy of the itinerant charge carriers.