Formation of Hubbard-like bands as a fingerprint of strong electron-electron interactions in FeSe

We use angle-resolved photoemission spectroscopy (ARPES) to explore the electronic structure of single crystals of FeSe over a wide range of binding energies and study the effects of strong electron-electron correlations. We provide evidence for the existence of “Hubbard-like bands” at high binding energies consisting of incoherent many-body excitations originating from Fe $3d$ states in addition to the renormalized quasiparticle bands near the Fermi level. Many high-energy features of the observed ARPES data can be accounted for when incorporating the effects of strong local Coulomb interactions in calculations of the spectral function via dynamical mean-field theory, including the formation of a Hubbard-like band. This shows that over the energy scale of several eV, local correlations arising from the on-site Coulomb repulsion and Hund's coupling are essential for a proper understanding of the electronic structure of FeSe and other related iron-based superconductors.

Figure 1

(a)–(d) ARPES data in the $M-\mathrm{\Gamma }-M$ direction at 37 eV in linear vertical (LV) polarization at 10 K. In this geometry a holelike quasiparticle band with $d_{yz}$ orbital character dominates the photoemission spectrum. (b)–(d) Measurements in the same geometry at different incident photon energies. The data extend to high binding energies, where much broader features are found. (e) Schematic of the high-energy spectrum. (f) Integrated spectral weight from (d), showing features associated with the quasiparticle (QP), lower Hubbard-band (LHB) intensities, as well as a contribution from the Se $4p$ bands.

Figure 2

(a), (b), (d), (e) Comparison of ARPES spectra with DFT+DMFT calculations. The DFT+DMFT simulations are obtained by applying simple selection rules to the orbitally resolved spectral weight, to account for the experimental matrix element effects. Dashed lines are guides to the eye showing the location of experimental incoherent Fe $3d$ spectral weight. (c), (f) Schematic measurement geometries of the cuts shown in (a) and (b) (and also Fig. 1) and (d) and (e), respectively.

Figure 3

(a) Integrated spectral weights as calculated from DFT and DFT+DMFT. (b) Total intensity obtained by summation over the different cuts in Figs. 2, 2, and 2. (c) Orbital-resolved spectral weight. (d) Comparison of Fermi surfaces determined experimentally at 100 K in the tetragonal phase and (e) as calculated by DFT+DMFT projected in the $k_z=\pi$ plane.