Ising superconductivity in monolayer niobium dichalcogenide alloys

TL;DR

This study uses first-principles calculations to explore the electronic and magnetic properties of NbS$_{x}$Se$_{2-x}$ alloys, providing insights into their superconducting behavior and challenging previous multifractality interpretations.

Contribution

The paper offers a detailed theoretical analysis of alloy electronic structures and proposes an alternative explanation for the non-monotonic superconducting transition temperature.

Findings

Density of states at the Fermi level is reduced in alloys.

Se vacancies may act as magnetic pair-breaking defects.

The non-monotonic T_c behavior can be explained without multifractality.

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.