Recent developments in higherorder topological phases have revealed the relationship between nontrivial multipole moments in the bulk and the emergence of fractionally quantized charges at the boundary. Here, we put forth a proposal of the three-dimensional octupole insulator with fractionally quantized corner charges $\pm e/8$: sodium chloride, commonly known as table salt. The fractional quantization of the corner charge is unaffected by the quantum fluctuation of the electric charge per ion, as we demonstrate explicitly with ab initio calculations. We further show that the electrostatic signature of the fractional charge is well preserved even when the ideal crystal is sprinkled with defects, and provide a systematic analysis on how the corner charge contribution could be isolated from the electric-field corrections originating from realistic surface relaxation. Our results suggest that the observation of corner charges from quantized multipole moments in solids might be imminent.

中文翻译：

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氯化钠的角电荷分数

高阶拓扑相的最新发展揭示了体积中的非平凡多极矩与边界处部分量子化电荷的出现之间的关系。在这里，我们提出了具有分数量化角电荷的三维八极绝缘体的建议$\pm \mathrm{电子}/8$：氯化钠，俗称食盐。正如我们用ab initio计算明确证明的那样，角电荷的分数量子化不受每个离子电荷的量子波动的影响。我们进一步表明，即使在理想的晶体布满缺陷时，分数电荷的静电特征也能很好地保留下来，并提供了系统分析如何将角电荷贡献与源自真实表面弛豫的电场校正隔离开来。我们的结果表明，从固体中的量化多极矩观察角电荷可能迫在眉睫。