Ising superconductivity in monolayer niobium dichalcogenide alloys

Abstract: NbSe$<em>{2}$ and NbS$</em>{2}$ are isostructural two-dimensional materials that exhibit contrasting superconducting properties when reduced to the single monolayer limit. Monolayer NbSe$<em>{2}$ is an Ising superconductor, while there have been no reports of superconductivity in monolayer NbS$</em>{2}$. NbS$<em>{x}$Se$</em>{2-x}$ alloys exhibit an intriguing non-monotonic dependence of the superconducting transition temperature with sulfur content, which has been interpreted as a manifestation of fractal superconductivity. However, several key questions about this result are not known: (1) Does the electronic structure of the alloy differ from the parent compounds, (2) Are spin fluctuations which have been shown to be prominent in monolayer NbSe$<em>{2}$ also present in the alloys? Using first-principles calculations, we show that the density of states at the Fermi level and the proximity to magnetism in NbS$</em>{x}$Se$<em>{2-x}$ alloys are both reduced compared to the parent compound; the former would decrease the transition temperature while the latter would increase it. We also show that Se vacancies, which are likely magnetic pair-breaking defects, may form in large concentrations in NbSe$</em>{2}$. Based on our results, we suggest an alternative explanation of the non-monotonic behavior the superconducting transition temperature in NbS$<em>{x}$Se$</em>{2-x}$ alloys, which does not require the conjecture of multifractality.