Electronic structure and transport properties of partially Ni-substituted superconducting iron chalcogenide FeSe_0.45Te_0.55

Iron chalcogenides bring together superconductivity, non-trivial electronic topology, and magnetism in a single material, making it a potential platform for hosting Majorana fermions which are the building blocks for topological quantum computing. We investigated the change of electronic structure and transport properties in a series of superconducting iron-nickel chalcogenides (FeNi)Se_0.45Te_0.55 by partial Ni substitution for Fe. Ni substitution increases the electron count. Ni 3d spin-orbit energy is ~ 50% higher than that of an Fe atom, leading to enhanced topological properties in this class of materials. From combined ARPES and transport measurements, we found that Ni substitution reduces superconducting transition temperature T_c and superconducting gap. Temperature dependent normal state resistivity changes from a weak metallic behavior to an insulating-like behavior with Ni-substitution level above 2% (nominal composition). A low level of Ni-substitution was found to enhance the critical current density in iron chalcogenide superconductors that peaks around 2% substitution level.