Quantitative assessment of the role of spin fluctuations in 2D Ising superconductor NbSe2

Accurate determination of the full momentum-dependent spin susceptibility $\chi(\mathbf{q}) $ is very important for description magnetism and superconductivity. While in principle formalism calculating $\chi(\mathbf{q})$ linear response density functional theory (DFT) well established, hardly any publicly available code includes this capability. Here, we describe an alternative way to calculate static $\chi(\mathbf{q})$, which can be applied most common DFT codes without additional programming. The method combined standard fixed-spin-moment calculations $\chi(\mathbf{0}) with direct energy spirals stabilized by artificial Hubbard interaction. From these calculations, $\chi_{DFT}(\mathbf{q} )$ extracted inverting RPA formula. We apply recipe recently discovered Ising superconductivity NbSe$_2$ monolayer, one exciting findings recent years. It was proposed that fluctuations may strongly affect parity order parameter. Previous estimates suggested proximity ferromagnetism, $i.e.$, peaked at $\mathbf{q}=0$. find structure more complicated, fluctuation spectrum sharply $\mathbf{q}\approx (0.2,0)$. Such a would change interband pairing interaction considerably superconducting state.