Spin density wave instabilities in the NbS2 monolayer

Abstract

In the present work, we study the magnetic properties of the NbS2 monolayer by first-principles calculations. The transition metal dichalcogenides (TMDCs) are a family of laminar materials presenting exciting properties such as charge density waves (CDWs), superconductivity, and metal-insulating transitions. 2H-NbS2 is a particular case within the family, because it is the only one that is a superconductor without exhibiting a CDW order. Although no long-range magnetic order was experimentally observed in the TMDCs, we show here that the single monolayer of NbS2 is on the verge of a spin density wave (SDW) phase. Our calculations indicate that a wavelike magnetic order is stabilized in the NbS2 monolayer in the presence of magnetic defects or within zigzag nanoribbons, due to the presence of unpaired electrons. We calculate the real part of the bare electronic susceptibility and the corresponding nesting function of the clean NbS2 monolayer, showing that there are strong electronic instabilities at the same wave vector associated with the calculated SDWs, also corresponding with one of the main nesting vectors of the Fermi surface. We conclude that the physical mechanism behind the spin-wave instabilities are the nesting properties, accentuated by the quasi-2D character of this system, and the rather strong Coulomb interactions of the 4d band of the Nb atom. We also estimate the amplitude of the spin fluctuations and find that they are rather large, as expected for a system on the verge of a quantum critical transition.