Scanning Tunneling Microscopy and Spectroscopy of Surface Impurities in the Unconventional Superconductor FeSe_1-xTe_x

Bulk FeSe_1-xTe_x hosts unconventional superconductivity and nematicity, and may have non-trivial topological surface states. Resonant states from impurities provide a powerful local probe of electronic structure, and can be used to search for possible Majorana zero modes (MZMs) and give important insight into electronic order. Here we study the impurity effect in FeSe_1-xTe_x with low-temperature scanning tunneling microscopy and spectroscopy. We image Fe-site and Se/Te-site surface impurities, all of which create low-energy resonant states inside the superconducting gap. A detailed comparison of these in-gap impurity states can provide insight into the superconducting order parameter, and is necessary to further clarify whether these resonances are MZMs. We also track nematicity in this material using quasiparticle interference (QPI) obtained at energies larger than the superconducting gap.