Superconductivity and topology of monolayer Fe(Se,Te)*

The origin of enhanced superconductivity over 50 K in the recently discovered FeSe monolayer films grown on SrTiO₃ (STO), as compared to 8 K in bulk FeSe, is intensely debated. Many mechanisms, like lattice strain, interfacial coupling, electron doping, have been discussed as possible reasons which can change the superconductivity. Using molecular beam epitaxy (MBE), we are able to manipulate several parameters in monolayer Fe(Se, Te)/STO. In-situ high-resolution angle-resolved photoemission spectroscopy (ARPES) measurement displays how the lattice strain, electron, and chemical doping affect physical properties of monolayer FeSe. We observed enhanced superconductivity accompanying Lifshitz transition in both FeSe and Fe(Se, Te). Moreover, our results provide a clear experimental indication that the Fe(Se, Te) materials are high-temperature topological superconductors in which band topology and superconductivity are integrated intrinsically.