单层Fe$_3$GeTe$_2$中的少数自旋磁振子与模式分支
Minority magnons and mode branching in monolayer Fe$_3$GeTe$_2$
AI总结：
本文通过第一性原理和含时密度泛函理论预测单层Fe₃GeTe₂中存在使磁矩增大的新型少数自旋磁振子，并通过模式分析区分相干集体激发与混合Stoner激发，揭示其丰富的磁激发谱。
摘要：
我们利用第一性原理计算预测了单层Fe$_3$GeTe$_2$中存在少数自旋磁振子。少数自旋磁振子构成一种新型集体磁激发，它会使磁矩增大——与使其降低的普通（多数自旋）磁振子相反——从而在动态磁化率$χ^{-+}(ω)$中产生自旋升高的极点。Fe$_3$GeTe$_2$非平庸的铁磁能带结构使这类准粒子的存在成为可能。我们使用含时密度泛函理论计算磁化率，并进行详细的模式分析，从而能够识别和研究单个磁振子模式以及构成多体谱的Stoner激发。分析揭示了大量多数自旋和少数自旋激发，由于体系的电子巡游性，除主要磁振子分支外，还包括卫星磁振子、谷磁振子和自旋反转磁振子。关键的是，该分析使我们能够将具有相干集体性质的峰与具有混合磁振子/Stoner性质的峰区分开来。为此，我们预测单层Fe$_3$GeTe$_2$的最低能量少数自旋磁振子模式在长波极限下确实构成一个相干的集体准粒子，并针对观测到的少数自旋磁振子增强引入了一个简化高斯模型。其背后的物理绝不限于Fe$_3$GeTe$_2$，因此少数自旋磁振子有望在其他复杂铁磁材料中同样被观测到。
英文摘要：
We predict the presence of minority magnons in monolayer Fe$_3$GeTe$_2$ using first principles calculations. Minority magnons constitute a new type of collective magnetic excitation which increase the magnetic moment -- contrary to ordinary (majority) magnons which lower it -- giving rise to spin-raising poles in the dynamic susceptibility $χ^{-+}(ω)$. The presence of such quasi-particles is made possible by the nontrivial ferromagnetic band structure of Fe$_3$GeTe$_2$. We calculate the susceptibility using time-dependent density functional theory and perform a detailed mode analysis, which allows us to identify and investigate individual magnon modes as well as the Stoner excitations that constitute the many-body spectrum. The analysis reveals a plethora of both majority and minority excitations, which in addition to the main magnon branches include both satellite, valley and spin-inversion magnons thanks to the electron itinerancy of the system. Crucially, the analysis allows us to separate peaks of a coherent collective nature from those of mixed magnon/Stoner nature. To this end, we predict that the lowest energy minority magnon mode of monolayer Fe$_3$GeTe$_2$ indeed constitutes a coherent collective quasi-particle at long wavelengths and introduce a simplistic gaussian model for the observed minority magnon enhancement. The underlying physics is in no way restricted to Fe$_3$GeTe$_2$, and minority magnons are thus expected to be observable in other complex ferromagnetic materials as well.
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