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.