SFB Colloquium

Scattering rates due to electron-electron interactions determine the transport and thermal properties of solids as well as the magnetic susceptibility and superconductivity. In this colloquium I will report on the determination of the energy and momentum dependence of scattering rates in various solids using angle-resolved photoemission spectroscopy. First, I will present results on the prototypical nearly-free electron metal Na. Then I will describe recent experiments on the ferromagnet Ni, from which we obtain evidence for spin dependent scattering rates due to a strong coupling to Stoner excitations. Finally, I will present studies of the momentum and energy dependent mass renormalizations and scattering rates of charge carriers in various iron-based superconductors. For the hole doped compounds, we obtain scattering rates well above the Planckian limit, indicating a highly incoherent normal state electronic structure. Furthermore, we receive information on the location of hot spots determining antiferromagnetism and superconductivity as well as on cold spots determining the normal state transport and optical properties. In this way we obtain a microscopic understanding of the electronic structure of these systems in the normal and in the superconducting state. The experimental results on ferropnictides are compared with DFT+DMFT calculations describing Hund’s metal behavior. There are remarkable differences between theory and experiment. The reason for this is possibly because DFT+DMFT uses only local correlation effects and does not include non-local spin fluctuation excitations.