Spin density wave instabilities in the NbS2 monolayer

TL;DR

This study uses first-principles calculations to reveal that monolayer NbS2 is close to a spin density wave instability, driven by Fermi surface nesting and strong Coulomb interactions, despite lacking observed magnetic order.

Contribution

It demonstrates that NbS2 monolayer exhibits potential for SDW order due to electronic instabilities, highlighting the role of nesting and Coulomb interactions in this 2D material.

Findings

Strong electronic instabilities at specific wavevectors.

Presence of large spin fluctuations near quantum criticality.

SDW tendencies enhanced by defects and nanoribbons.

Abstract

In the present work, we study the magnetic properties of the NbS2 monolayer by first-principles calculations. The transition metal dichalcogenides (TMDC) are a family of laminar materials presenting exciting properties such as charge density waves (CDW), superconductivity and metal-insulating transitions among others. 2H-NbS2 is a particular case within the family, because it is the only one that is superconductor without exhibiting a CDW order. Although no long range magnetic order was experimentally observed in the TMDC, 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 wave-like magnetic order is stabilized in the NbS2 monolayer in the presence of magnetic defects or within zig-zag nanoribbons, due to the presence of unpaired electrons. We calculate the real part of the bare electronic susceptibilty and the corresponding nesting function of the clean NbS2 monolayer, showing that there are strong electronic instabilities at the same wavevector asociated 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.