摘要
本文首先介绍了密度泛函理论和基于2n+1定理的密度泛函微扰理论,陈述了研究材料电光张量的意义及其在密度泛函微扰理论中的分解表达式(根据玻恩—奥本海默近似,电光张量由电子和离子两部分的贡献构成)。为了分析电光材料巨电光系数的来源,我们选择了结构相同且同为III-V族化合物但有着迥然不同电光特性的砷化镓(GaAs)和立方氮化硼(c-BN)进行对比分析。我们使用基于第一原理密度泛函微扰理论的方法对GaAs和c-BN的电光张量进行了分析计算,并分别得到了电子和离子对其电光系数的贡献。通过比较可以看出,作为电光材料的GaAs有比c-BN高得多的电光张量绝对值。通过对电光张量中电子、离子分解的分析可以看出这种效应主要来自于电子的贡献,而且计算的结果与材料结构点群的电光系数具有的对称性一致。在电光张量的计算过程中我们得到了物质的平衡特性、压电系数、格点动力学特性—声子色散曲线和Born有效电荷等结果,并进行了相应的分析。
In this paper, we introduce the density-functional theory (DFT) and the density-functional perturbation theory (DFPT) based on the 2n+1 theorem. The significance of the research on the electro-optic tensors of materials is proposed, and the decomposed expressions based on the DFPT of the electro-optic tensors are presented (according to the Born-oppenheimer approximation, the electro-optic tensor can be decomposed by the contribution of the ion and electron, respectively). To analyze the large electro-optic coefficients of the electro-optic materials, we select GaAs and c-BN that come from the III-V group. They have the same structure, but completely different electro-optic coefficients. The electro-optic tensors of GaAs and c-BN are calculated through the first-principles DFT. The absolute values of the electro-optic coefficients of GaAs are much higher than that of c-BN. By analyzing the composition of the electro-optic tensor, we find out that the contribution from electronics is the dominant factor. The results also indicate that the electro-optic tensor is according with the symmetry of the dot group. In addition, other relative features, such as the ground-state properties, piezoelectricity, lattice dynamics properties—phonon dispersion relation and Born effective charge are also obtained and analyzed.
引文
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