Fermi Surface Geometry of Heavily Hole Doped CaKFe₄As₄ Revealed by Angle-Resolved Photoemission Spectroscopy

We have investigated Fermi surface geometry and band structure of CaKFe₄As₄ in which the FeAs layer is sandwiched by Ca and K layers. Using angle-resolved photoemission spectroscopy, the three hole pockets are observed around the zone center as commonly seen in various Fe-based superconductors. The outer hole pocket is significantly expanded around the Γ point compared to that around the Z point. The area of the middle hole pocket is larger than that reported in the previous work [Mou et al., Phys. Rev. Lett. 117, 277001 (2016)] resulting in degraded Fermi surface nesting. The inner hole band is shifted downwards relative to the prediction of band structure calculations, and its top touches the Fermi level at the Γ point. The discrepancy can be assigned to orbital dependent band renormalization of the Fe 3d bands. Due to the order of K and Ca layers, the hole bands at the Z point of the 122 phase are folded to the Γ point in the 1144 phase increasing the interaction between the hole and electron bands irrespective of the poor Fermi surface nesting.