Ising superconductivity in NbSe₂ alloys

Monolayer NbSe₂ and NbS₂ are isostructural two-dimensional materials that exhibit contrasting superconducting properties. Monolayer NbSe₂ is an Ising superconductor,while there are no reports of superconductivity in monolayer NbS₂.Recent experiments on NbSe₂ alloyed with sulfur have shown T_c varies non-monotonically with sulfur content.This was interpreted as a manifestation of fractal superconductivity. In this talk we will show that the nonmonotonic dependence of T_c is insufficient evidence for fractal superconductivity. Using first-principles calculations we highlight how this non-monotonic change in T_c can arise due to magnetic point defects such as chalcogen vacancies, which we expect to be present in appreciable concentrations, along with changes in the density of states and proximity to magnetism as a function of alloying.b We use these insights to show how alloying on the Nb site with molybdenum may also lead to a non-monotonic change of T_c in NbSe₂.