We study the linear and nonlinear [second-harmonic generation (SHG)] spectra of metal monoxide $\mathrm{MO}$ ($M$=Mg,Ca,Cd) monolayers by using a first-principles approach based on many-body theory. Our results indicate that the strongly bound excitons in $\mathrm{MO}$ with planar structure characterize the optical properties, including both linear and SHG spectra. The optical bandgap $E_{\text{opt}}$ is 4.63, 3.71, and 1.27 eV, while the exciton binding energy is as large as 2.49, 2.37, and 1.13 eV, for MgO, CaO, and CdO, respectively. Moreover, the largest SHG coefficient of 586 pm/V is found in CdO monolayer with the smallest bandgap among the three $\mathrm{MO}$ materials. Most importantly, we find a linear relationship between the SHG coefficient and $\mathrm{Qd}/E_{\text{opt}}$ ($Q$ is the transferred Bader charge and $d$ is the bond length) which can be used as a descriptor to search for new two-dimensional nonlinear materials.

中文翻译：

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来自第一性原理计算的金属一氧化层MO(M=Mg,Ca,Cd)中激子驱动的线性和非线性光学响应

我们研究金属一氧化物的线性和非线性 [二次谐波产生 (SHG)] 光谱 $摩$ ($米$=Mg,Ca,Cd) 单层，使用基于多体理论的第一性原理方法。我们的结果表明，强束缚激子$摩$平面结构表征光学特性，包括线性和 SHG 光谱。光学带隙${乙}_{\text{选择}}$MgO、CaO 和 CdO 的激子结合能分别为 2.49、2.37 和 1.13 eV，分别为 4.63、3.71 和 1.27 eV。此外，在 CdO 单层中发现了最大的 SHG 系数 586 pm/V，带隙最小的三个$摩$材料。最重要的是，我们发现 SHG 系数和$\mathrm{量子点}/{乙}_{\text{选择}}$ ($问$ 是转移的贝德电荷和 $d$ 是键长），可用作描述子来搜索新的二维非线性材料。