DFT-based analysis of substrate effects on the electronic structure of FeSe ultrathin films on CaF2

W e present a first-principles investigation of the electronic structure of FeSe thin films on a CaF 2 substrate, focusing on the effects of substrate interaction, magnetic ordering, and layer thick- ness. In the monolayer limit, the hole pocket at the Γ point, characteristic of free-standing FeSe, is absent due to a substrate-induced downward band shift, originating from surface reconstruction of the CaF2 slab. This behavior differs from FeSe/SrTiO3, where electron correlations are required to eliminate the Γ pocket. Incorporating antiferromagnetic (AFM) ordering further modifies the band structure, reinforcing the suppression of the hole pocket and introducing spin-related band splitting. As FeSe thickness increases, the valence band at Γ gradually approaches and eventually crosses the Fermi level, marking a crossover to bulk-like electronic behavior. These modifications to the Fermi surface topology may imply that superconductivity in ultrathin FeSe/CaF 2 may orig- inate from electron-pocket-only pairing scenarios, evolving toward a conventional multiband s ± mechanism in thicker films. These results highlight the tunability of superconducting properties of FeSe through substrate engineering, magnetic effects, and dimensional control.