Ising paramagnons

Paramagnons are the collective modes that govern the spin response of nearly ferromagnetic conductors. Their interactions with quasiparticles can induce spin-triplet superconductivity, a scenario that may occur in spin-valley coupled two dimensional transition metal dichalcogenides such as 2H-NbSe$_2$. This motivates this work exploring paramagnons in systems with spin split bands due to spin orbit coupling leading to spin-valley coupling. We use both a Kane-Mele-Hubbard model and a tight-binding model derived from DFT calculations for a monolayer of 2H-NbSe$_2$. We find paramagnons with energies around 1 meV that feature a colossal magnetic anisotropy. In the longitudinal (spin preserving) channel, we obtain the conventional paramagnon enhancement of the spin response, whereas spin response is quenched in the transverse channel. We discuss a possible connection of these Ising paramagnon with Ising superconductivity observed in these materials.