ARPES view on electronic correlations in the iron-based superconductor RbFe₂As₂ – kinks, Hubbard bands, and electron localization

The electronic structure of iron-based superconductors is characterized by an interplay of Coulomb interaction and Hund’s rule coupling acting on a multiband system. It is currently debated which role electronic correlations play in the complex phase diagrams that contain superconducting, nematic, and magnetic phases as well as strange metal behavior. Correlation effects are particularly pronounced in the hole-doped AFe₂As₂ (A=K,Rb,Cs), which show large effective masses. We present a comprehensive ARPES study on RbFe₂As₂ and compare the results to DMFT. Our data reveal the existence of Hubbard bands. Their intensity increases with increasing temperature at the expense of quasiparticle spectral weight. The quasiparticle peak of the Fe 3d_xy orbital is absent above 90K. We demonstrate that the electrons localize into the Hubbard bands across this coherent-incoherent transition. In addition, we identify two kinks in the dispersion of the d_xz/yz orbital. The low energy kink strongly renormalizes the quasiparticle dispersion well beyond a weak-coupling description. We will discuss possible origins. The high-energy kink initiates a waterfall structure that separates the quasiparticle peak from the Hubbard bands. The kink is induced by electronic interactions. Our study reveals key signatures of electronic correlations in the spectral function of RbFe₂As₂ and provides a link between iron-based superconductors and other correlated systems such as cuprates, nickelates, and heavy fermions.