Ising superconductivity in monolayer niobium dichalcogenide alloys

Abstract

NbSe${}_2$ and NbS${}_2$ are isostructural two-dimensional materials that exhibit contrasting superconducting properties when reduced to the single monolayer limit. Monolayer NbSe${}_2$ is an Ising superconductor, while there have been no reports of superconductivity in monolayer NbS${}_2$. NbS${}_x$Se${}_{2-x}$ 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${}_2$ 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${}_x$Se${}_{2-x}$ 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${}_2$. Based on our results, we suggest an alternative explanation of the non-monotonic behavior the superconducting transition temperature in NbS${}_x$Se${}_{2-x}$ alloys, which does not require the conjecture of multifractality.

[v1] had minor typos in Figure 1 and the main text that are now fixed. Please use the corrected version, [v2]