Renormalized q-dependent Spin Susceptibility by inverting the Random Phase Approximation: Implications for quantitative assessment of the role of spin fluctuations in 2D Ising superconductor NbSe$_{2}$

TL;DR

This paper introduces a practical method to compute the full momentum-dependent spin susceptibility in DFT, applied to NbSe$_{2}$, revealing complex spin fluctuation structures that influence its superconducting properties.

Contribution

The authors develop an accessible approach to invert the RPA formula for calculating $oldsymbol{ ext{q}}$-dependent spin susceptibility using standard DFT calculations, applied to NbSe$_{2}$.

Findings

Spin fluctuations in NbSe$_{2}$ are peaked at $oldsymbol{ ext{q}} eq 0$

The fluctuation spectrum significantly impacts the superconducting pairing

Method enables broader application without additional coding

Abstract

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