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中国物理学会期刊

反铁磁轴子绝缘体候选材料EuIn2As2的表面原子排布和电子结构

CSTR: 32037.14.aps.70.20210783

Surface and electronic structure of antiferromagnetic axion insulator candidate EuIn2As2

CSTR: 32037.14.aps.70.20210783
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  • 非平庸的能带拓扑性与磁性结合可以产生丰富的量子现象, 包括量子反常霍尔效应、轴子绝缘体态等. 不同于磁性掺杂和异质结方案, 内禀磁性拓扑绝缘体避免了掺杂带来的无序, 且制备工艺通常比异质结更加简单, 因此对研究和利用磁性拓扑绝缘体都有重要的意义. 最近, EuIn2As2被认为是内禀反铁磁轴子绝缘体, 本文使用低温扫描隧道显微镜研究了它的解理表面的原子排布和电子结构. 结合原子分辨形貌图、晶格对称性分析以及局域态密度等信息, 认为观测到的表面条纹结构来源于Eu截止面50%覆盖度的 1\times 2 表面重构. 通过条纹面的局域态密度测量, 发现4 K时费米能附近态密度存在非对称的谷-峰特征, 该特征随温度升高逐渐变弱, 在反铁磁相变温度以上完全消失, 表明其与反铁磁序密切相关. 此外, 在某些台阶附近, 伴随有少量迷宫状的结构, 进一步分析认为可能是Eu原子形成的翘曲结构导致的. 这些结果为理解EuIn2As2的表面能带结构和拓扑性质提供了重要信息.

     

    The interplay between non-trivial band topology and magnetic order can induce exotic quantum phenomena, such as the quantum anomalous Hall effect and axion insulator state. A prevalent approach to realizing such topological states is either by magnetic doping or through heterostructure engineering, while the former will bring in inhomogeneity and the latter requires complex procedures. Intrinsic magnetic topological insulators are expected to avoid the aforementioned disadvantages, which is of great significance in both studying and practically using these exotic quantum phenomena. Recently, a Zintl compound EuIn2As2 is predicted to be an intrinsic antiferromagnetic axion insulator. The bulk magnetic order of EuIn2As2 has been reported in a lot of experiments, while the topological nature has not yet been confirmed. The surface properties of intrinsic magnetic topological insulators play an important role in the interplay between magnetic order and non-trivial surface state. Here in this work, we study the surface structure and electronic property of EuIn2As2 single crystal by using scanning tunneling microscopy/spectroscopy (STM/S) and non-contact atomic force microscopy (NC-AFM). Considering the strength of bonds, the easy cleavage plane of the crystals possibly lies between In-In layers or between Eu-As layers. The STM topographies show that the cleaved surface is dominated by a striped pattern. And the dominated step height is an integer multiple of c/2, which implies that only one kind of cleavage plane is preferred. Atomic-resolved surface topographies show that the striped pattern is the 1\times 2 surface reconstruction with 50% coverage. Hence an In-terminated surface which will be 100% coverage is ruled out. The spatial evolution of STS near vacancies on the striped pattern shows a hole-doping feature. All of these results reveal that the striped pattern is the 1\times 2 surface reconstruction of the Eu terminated surface with 50% coverage. Using the STS, we measure the local densities of states on the striped surface at various temperatures. We find that there is an asymmetric valley-peak feature in the density of states near the Fermi energy at 4 K, which is gradually weakened with increasing temperature, and disappears above the antiferromagnetic Néel temperature, indicating that the asymmetric valley-peak feature is closely related to the antiferromagnetic order. Besides, a maze-like pattern is observed occasionally near some step edges. The STM topographies show atoms both on bright and dark stripes of the maze-like pattern, which form a whole hexagonal lattice. And the NC-AFM images show that the maze-like pattern is about 1 Å higher than the Eu terminated striped pattern. Based on these results, the maze-like pattern can be explained as the buckled Eu surface with 100% coverage. These results provide important information for understanding the surface electronic band structure and topological nature of EuIn2As2.

     

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