Anomalous High-Energy Electronic Interaction in Iron-Based   Superconductor
  Strong electron interactions in solids increase effective mass, and shrink the electronic bands [1]. One of the most unique and robust experimental facts about iron-based superconductors [2-4] is the renormalization of the conduction band by factor of 3 near the Fermi level [5-9]. Obviously related to superconductivity, this unusual behaviour remains unexplained. Here, by studying the momentum-resolved spectrum of the whole valence band in a representative material, we show that this phenomenon originates from electronic interaction on a much larger energy scale. We observe an abrupt depletion of the spectral weight in the middle of the Fe $3d$ band, which is accompanied by a drastic increase of the scattering rate. Remarkably, all spectral anomalies including the low-energy renormalization can be explained by coupling to excitations, strongly peaked at about 0.5 eV. Such high-energy interaction distinguishes all unconventional superconductors from common metals. 
D. V. Evtushinsky, A. N.Yaresko, V. B. Zabolotnyy, J. Maletz, T. K.   Kim, A. A. Kordyuk, M. S. Viazovska, M. Roslova, I. Mo...
cond-mat.supr-con