Bulk Fe-based superconductors exhibit high transition temperatures (Tc) and topological phases. They share a standard building block of a square lattice of Fe tetrahedrally coordinated to Pn or Ch ions, where the Tc is controlled by the local geometry of the FePn(Ch)₄ tetrahedron. By modulating carrier concentrations and applied pressure, the optimal Tc is achieved when the bond angle of Pn(Ch)–Fe–Pn(Ch) is tuned to that of a normal tetrahedron (109.5°) (for a review, see Hosono et al., Mater. Today 21, 278 (2018)).

Fe-chalcogenide films epitaxially grown on perovskite substrates offer a versatile platform for further tuning the Ch-Fe-Ch tetrahedron towards achieving higher Tc. In this talk, I will present our studies of epitaxial single-layer Fe-chalcogenide films on SrTiO₃ (STO) substrate, where we modify the Ch-Fe-Ch angle via UV light, chemical pressure, and substrate termination (SrO vs. TiO₂). From comprehensive studies using in-situ STM/ARPES, ex-situ TEM imaging and x-ray scattering, and DFT/DMFT calculations, we show a strong correlation between the Tc and the Ch-Fe-Ch angle in these single-layer films. I will also present our observation of Tomonaga-Luttinger liquid behavior in the topological edge channels in multilayer films and its tunability by chemical pressure. These findings indicate that epitaxial Fe-chalcogenide heterostructures are a highly tunable quantum system and shed light on the pairing symmetry and topological properties of Fe-chalcogenide superconductors.