Effect of alloying in monolayer niobium dichalcogenide superconductors

When sulfur and silicon are incorporated in monolayer 2H-NbSe₂ the superconducting transition temperature, T_c, has been found to vary non-monotonically. This was assumed to be a manifestation of fractal superconductivity. Using first-principles calculations, we show that the nonmonotonic dependence of T_c is insufficient evidence for multifractality. A unifying aspect in our study are selenium vacancies in NbSe₂, which are magnetic pair-breaking defects that we propose can be present in considerable concentrations in as-grown NbSe₂. We show that sulfur and silicon can occupy the selenium sites and reduce the pair-breaking effect. Furthermore, when sulfur is incorporated in NbSe₂, the density of states at the Fermi level and the proximity to magnetism in the alloy are both reduced compared to the parent compound. Based on our results, we propose an alternative explanation of the non-monotonic change in T_c which does not require the conjecture of multifractality.