角分辨光电子能谱在探索和研究拓扑材料中新型量子态方面的应用

Taking advantages of recent advances in band theory and sample synthesis, the research on the topological materials gets a rapid development in recent years. The electronic structures of these materials are protected by certain symmetries and thus are robust upon the perturbations. Owing to this property, we can expect their promising potential applications in functional devices in the near future. In this project, by combining angle-resolved photoemission spectroscopy (ARPES) with transport measurements and first-principles calculations, we plan to carry out researches into two-dimensional topological insulator (LaSbTe), extremely large magnetoresistance (XMR) in non-magnetic semimetals (NbAs2 and W2As3), and novel topological semimetals with exotic fermions (InBi, Co3Sn2S2, and Ti3Al). In order to find the bulk evidence, i.e., the realization of a global band gap, of two-dimensional topological insulator under easily accessible experimental conditions, we will utilize hydrogenation and other surface decoration to manipulate the band structures of LaSbTe, which is predicted to be an idea two-dimensional topological insulator. By mapping out the three-dimensional electronic structures of NbAs2 and W2As3, which exhibit quadratic XMR behavior, we can determine whether their Fermi surfaces are dominated by the open-orbit topologies or not. Based on the ARPES results, further magnetic and electric transport measurements will be performed to study whether the open-orbit Fermi surface topology can explain the quadratic XMR in these materials or not. To realize Dirac fermions in a simple and stable structured material, and time-reversal-breaking Weyl fermions in a magnetic system, we plan to evaporate potassium on the cleaved surfaces of InBi and Co3Sn2S2 crystals, respectively, after which the up-shift of Fermi levels and the massless fermions can be observed in ARPES experiments. We will carry out a comprehensive investigation on the electronic structure of Ti3Al to search for the novel topological nodal-surface fermions, which behave as two-dimensional surfaces in the momentum space, beyond the zero-dimensional Dirac/Weyl fermions and one-dimensional nodal-line fermions.

近年来由于理论计算和材料制备的进步，拓扑材料的研究进展迅速，其电子结构受对称性保护而性质稳定，使其在功能器件的应用中具备较大潜力。本项目我们将基于角分辨光电子能谱，结合输运测量与第一性原理计算对二维拓扑绝缘体（LaSbTe）、非磁半金属的极大磁阻效应（NbAs2和W2As3）和新型拓扑半金属材料（InBi、Co3Sn2S2和Ti3Al）开展研究。在LaSbTe中计划利用氢化作用和其他表面修饰调控其能带结构，寻找二维拓扑绝缘体的体态证据，即实现全局能隙。通过测量NbAs2和W2As3的三维电子结构，确定其是否具有开放轨道，结合输运性质研究其极大磁阻是否起源于开放轨道费米面。计划对InBi和Co3Sn2S2进行原位蒸钾使其费米能级上移，分别实现简单、稳定结构中的狄拉克费米子和破缺时间反演对称性的磁性外尔费米子。详细研究Ti3Al的三维电子结构，探索并实现其中具有二维动量曲面的新型节点面费米子。

项目负责人基于本项目计划书的内容，并结合拓扑量子材料领域发展的前沿问题，对非磁半金属NbAs2和W2As3中的极大磁阻效应、拓扑非平庸kagome金属CsV3Sb5、反铁磁拓扑半金属NdBi、磁性外尔半金属PrAlSi和SmAlSi、以及拓扑超导体候选材料CoSb/SrTiO3界面开展了系统的电子结构研究。我们发现：NbAs2和W2As3中存在的开放轨道费米面是理解其中极大磁阻效应的关键因素；CsV3Sb5中破缺时间反演对称性的电荷密度波很可能起源于费米面嵌套机制，电荷密度波的形成为其超导电性提供了有利的发展平台；NdBi中存在与磁性相关的新奇表面态费米弧；PrAlSi和SmAlSi中的4f电子与巡游电子的耦合很弱；CoSb纳米条带是典型的朝永-拉廷格液体。

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