Ising and XY paramagnons in two-dimensional 2H-NbSe$_2$

TL;DR

This paper investigates the spin anisotropy of paramagnons in monolayer 2H-NbSe$_2$, revealing a strong off-plane magnetic anisotropy and proximity to an in-plane spin density wave instability, with implications for superconductivity.

Contribution

It provides the first detailed analysis of spin anisotropy of paramagnons in 2H-NbSe$_2$ monolayers using a DFT-derived tight-binding model, highlighting their anisotropic behavior and proximity to magnetic instabilities.

Findings

Paramagnons have energies around 1 meV with strong off-plane uniaxial anisotropy.

XY fluctuations dominate at finite wave vectors in the Brillouin zone.

2H-NbSe$_2$ is close to an in-plane spin density wave instability.

Abstract

Paramagnons are the collective modes that govern the spin response of nearly magnetic conductors. In some cases they mediate electron pairing leading to superconductivity. This scenario may occur in 2H-NbSe$_2$ monolayers, that feature spin-valley coupling on account of spin-orbit interactions and their lack of inversion symmetry. Here we explore spin anisotropy of paramagnons both for non-centrosymmetric Kane-Mele-Hubbard models for 2H-NbSe$_2$ monolayers described with a DFT-derived tight-binding model. In the infinite wavelength limit we find spatially uniform paramagnons with energies around $1$~meV that feature a colossal off-plane uniaxial magnetic anisotropy, with quenched transversal spin response. At finite wave vectors, longitudinal and transverse channels reverse roles: XY fluctuations dominate within a significant portion of the Brillouin zone. Our results show that 2H-NbSe$_2$ is close to a Coulomb-driven in-plane (XY) spin density wave instability.