重新研究XMg₂Bi₂（X = Ca、Sr、Ba、Yb和Eu）的拓扑性质
Revisiting the topological properties of XMg2Bi2 (X = Ca, Sr, Ba, Yb and Eu)
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中文总结 AI 辅助
本研究采用杂化泛函与拓扑不变量计算，发现XMg₂Bi₂类材料为拓扑平庸窄带隙半导体，实验观测的拓扑特征源于外禀效应，凸显了处理电子关联对评估拓扑性质的重要性。
中文摘要 AI 辅助
密度泛函理论（Density functional theory）被认为会低估半导体的带隙，且高估反转带隙，常夸大材料拓扑相图的预测尺寸。本研究采用杂化泛函（hybrid functionals）并计算拓扑不变量，重新研究结晶为CaAl₂Si₂型结构的化合物的拓扑性质，发现该晶体结构的化合物中反转带隙的高估尤为显著。其中，XMg₂Bi₂（X = Ca、Sr、Ba、Yb、Eu）类材料在所有考虑的阳离子下均为拓扑平庸态；计算显示这些材料为窄带隙半导体，直接带隙为0.24-0.34 eV，且随X元素原子量增加略有减小。本研究通过施加单轴应变和流体静压力验证了上述结果。研究强调，仅通过实验观测表面态不足以确立非平庸拓扑，因为拓扑平庸半导体也可能存在无表面狄拉克点的表面态；因此，由于这些材料本征为拓扑平庸态，实验观测到的任何拓扑特征都应归因于掺杂或表面重构等外禀效应。本研究结果凸显了在评估拓扑性质时准确处理电子关联的重要性，即使对于含铋等具有强自旋轨道耦合的重元素的材料亦是如此。
英文摘要
Density functional theory is known to underestimate band gaps in semiconductors and to over- estimate inverted band gaps, frequently exaggerating the predicted size of the topological phase diagram of materials. Employing hybrid functionals and calculating the topological invariants, we revisit the topological properties of compounds crystallizing in the CaAl2Si2-type and demonstrate that the overestimation of the inverted band gaps is particularly pronounced in compounds with this crystal structure. Among these, the class of XMg2Bi2 materials (X = Ca, Sr, Ba, Yb, and Eu) is topologically trivial for all considered cations. Our calculations show that these materi- als are narrow-gap semiconductors with direct band gaps of 0.24-0.34 eV, slightly decreasing with increasing the atomic weight of the element X. We confirm this by applying uniaxial strain and hydrostatic pressure, confirming these results. We emphasize that the experimental observation of surface states alone is insufficient to establish nontrivial topology, as trivial semiconductors may host surface states without a surface Dirac point. Consequently, since these materials are intrinsi- cally topologically trivial, any experimentally observed topological signatures should be attributed to extrinsic effects such as doping or surface reconstruction. Our results underscore the importance of accurately treating electronic correlations when assessing topological character, even in materials containing heavy elements with strong spin-orbit coupling, such as bismuth.
