We present first principles calculations of the two-particle excitation spectrum of ${\mathrm{CrI}}_3$ using many-body perturbation theory including spin-orbit coupling. Specifically, we solve the Bethe-Salpeter equation, which is equivalent to summing up all ladder diagrams with static screening, and it is shown that excitons as well as magnons can be extracted seamlessly from the calculations. The resulting optical absorption spectrum as well as the magnon dispersion agree very well with recent measurements, and we extract the amplitude for optical excitation of magnons resulting from spin-orbit interactions. Importantly, the results do not rely on any assumptions of the microscopic magnetic interactions such as Dzyaloshinskii-Moriya (DM), Kitaev, or biquadratic interactions, and we obtain a model independent estimate of the gap between acoustic and optical magnons of 0.3 meV. In addition, we resolve the magnon wave function in terms of band transitions and show that the magnon carries a spin that is significantly smaller than $\hslash$. This highlights the importance of terms that do not commute with $S^z$ in any Heisenberg model description.

中文翻译：

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多体微扰理论对单层CrI3中磁子和激子的统一处理

我们提出了两粒子激发光谱的第一性原理计算 ${铬}_3$使用多体微扰理论，包括自旋轨道耦合。具体来说，我们求解了 Bethe-Salpeter 方程，这等效于将所有梯形图与静态筛选相加，并且表明可以从计算中无缝提取激子和磁振子。由此产生的光吸收光谱以及磁振子色散与最近的测量结果非常吻合，我们提取了自旋轨道相互作用产生的磁振子光激发的振幅。重要的是，结果不依赖于微观磁相互作用的任何假设，例如 Dzyaloshinskii-Moriya (DM)、Kitaev 或双二次相互作用，并且我们获得了 0.3 meV 声学和光学磁振子之间间隙的模型独立估计。此外，$\hslash$. 这突出了不通勤的术语的重要性${秒}^z$ 在任何海森堡模型描述中。