Te-vacancy enhanced superconductivity in hybrid interface FeTe_1-xSe_x/Bi₂Te₃ grown by molecular beam epitaxy

Hybrid interfaces of topological insulators and s-wave superconductors are great candidates for realizing Majorana bound states (MBSs) which have been projected to have paradigm-changing possibilities in quantum computing. The epitaxial FeTe_1-xSe_x/Bi₂Te₃ platform possess the necessary parameters for topological states, and provides wide range of tunability. Recently, monolayer of superconducting FeTe_1-xSe_x (x=0.25) grown on the Bi₂Te₃ was reported to exhibit emergent topological interfacial Dirac states at the Fermi energy. Pushing to lower Se levels reduces disorder which is critical for interrogating Majorana bound states, yet pure FeTe is not superconducting. Here we systematically investigate how the molecular beam epitaxy growth of Bi₂Te₃ and FeTe_1-xSe_x enable tailoring both superconductivity and topological properties at low Se doping levels. Low temperature transport measurement, angle resolved photoemission spectroscopy, Rutherford backscattering spectrometry and X-ray diffraction are combined to unravel the roles of band structure, crystallinity, composition and superconductivity which can be tailored as a function of growth conditions. This study will reveal the complex relation of strain and charge at FeTe_1-xSe_x/Bi₂Te₃ interface which will hopefully create a robust platform for realizing MBSs and advancing quantum devices.