通过反转随机相位近似重正化q依赖自旋磁化率：对二维伊辛超导体NbSe $_{2}$中自旋涨落作用的定量评估的意义
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$_{2}$
摘要：
全动量自旋磁化率$χ(\mathbf{q}) $的精确测定对于磁性和超导性的描述是非常重要的。虽然在原则上的形式主义计算$χ(\mathbf{q})$的线性响应密度泛函理论（DFT）是建立良好的，几乎没有任何公开可用的代码包括这种能力。在这里，我们描述了一种替代方法来计算静态$χ(\mathbf{q})$，它可以应用于最常见的DFT代码，而无需额外的编程。该方法结合了标准的固定自旋力矩计算的$χ(\mathbf{0}) $与直接计算的自旋螺旋的能量稳定的人工哈伯德相互作用。从这些计算中，可以通过反转RPA公式来提取$χ_{DFT}(\mathbf{q} )$。我们将此配方应用于最近发现的NbSe $_2$单层中的伊辛超导性，这是近年来超导性中最令人兴奋的发现之一。有人提出，自旋涨落可能强烈影响的宇称的序参数。先前的估计表明接近铁磁性，$i.e.$，$χ(\mathbf{q})$在$\mathbf{q}=0$达到峰值。我们发现自旋涨落的结构更为复杂，涨落谱在$\mathbf{q}\approx (0.2,0)$处有一个尖锐的峰。这样的光谱会改变带间的配对相互作用，并对超导态产生很大的影响。
英文摘要：
Accurate determination of the full momentum-dependent spin susceptibility $χ(\mathbf{q}) $ is very important for the description of magnetism and superconductivity. While in principle the formalism for calculating $χ(\mathbf{q})$ 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 $χ(\mathbf{q})$, which can be applied to most common DFT codes without additional programming. The method combined standard fixed-spin-moment calculations of $χ(\mathbf{0}) $ with direct calculations of the energy of spin spirals stabilized by an artificial Hubbard interaction. From these calculations, $χ_{DFT}(\mathbf{q} )$ 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, $i.e.$, $χ(\mathbf{q})$ 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.
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