We derive the effective action for the collective spin modes in iron-based superconductors. We show that, due to the orbital-selective nature of spin fluctuations, the magnetic and nematic instabilities are controlled by the degrees of orbital nesting between electron and hole pockets. Within a prototypical three-pocket model the hole-electron orbital mismatch is found to boost spin-nematic order. This explains the enhancement of nematic order in FeSe as compared to 122 compounds, and its suppression under pressure, where the emergence of the second hole pocket compensates the orbital mismatch of the three-pocket configuration.

中文翻译：

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轨道失配加剧铁基超导体的向列不稳定性

我们得出铁基超导体中集体自旋模式的有效作用。我们表明，由于自旋涨落的轨道选择性质，磁性和向列不稳定性受电子和空穴腔之间的轨道嵌套程度控制。在典型的三腔模型中，发现空穴电子轨道失配会增强自旋向列顺序。这解释了与122种化合物相比，FeSe的向列顺序增强了，并在压力下得到抑制，其中第二个空穴的出现补偿了三口袋构型的轨道失配。