(001)*

Recent work shows evidence of topological superconductivity on the (001) surface of FeTeSe bulk crystals. In this work, we demonstrate both superconducting and topological phase transitions at higher temperature in FeTe_1-xSe_x thin films, by reducing its thickness down to one atomic layer. High quality single layer FeTe_1-xSe_x films are grown on SrTiO₃(001) substrate by molecular beam epitaxy and characterized by in situ scanning tunneling microscopy/spectroscopy and angle-resolved photoemission spectroscopy. We find the topological and superconducting properties to be strongly dependent on the Te concentration, which controls spin-orbit coupling. With increasing Te, the top of the valence band at the Γ point moves towards and then crosses the Fermi level, accompanied by a change of the band profile from parabolic to linear. Above a critical Te concentration of 80%, the linear valence band reverts back to parabolic and shifts back down below the Fermi level, characteristic of a topological phase transition. These findings and their implications for the emergence of topological phases in Fe-based superconductors at reduced dimensions will be presented at the meeting.