[论文解读] Discovery of a topological exciton insulator with tunable momentum order

Md Shafayat Hossain, Tyler A. Cochran+28 more|arXiv (Cornell University)|Dec 26, 2023

Topological Materials and Phenomena被引用 4

一句话总结

本研究识别出Ta2Pd3Te5为首个三维拓扑激发子绝缘体，其中在100 K以下发生激发子凝聚，通过自发镜像对称性破缺打开拓扑体能隙。扫描隧道显微镜揭示了具有拓扑特性的无能隙边界态，而在约5 K附近出现的次级激发子不稳定性表现出可由磁场调控的波矢，证明了在体相关拓扑相中存在可调的动量序。

ABSTRACT

Correlated topological materials often maintain a delicate balance among physical symmetries: many topological orders are symmetry protected, while most correlated phenomena arise from spontaneous symmetry breaking. It is rare to find cases where symmetry breaking induces a non-trivial topological phase. Here, we present the discovery of such a phase in Ta2Pd3Te5, where Coulomb interactions form excitons, which condense below 100 K, opening a topological gap and creating a topological excitonic insulator. Our spectroscopy reveals the full spectral bulk gap stemming from exciton condensation. This excitonic insulator state spontaneously breaks mirror symmetries but involves a very weak structural coupling, as indicated by photoemission spectroscopy, thermodynamic measurements, and a detailed structural analysis. Notably, scanning tunneling microscopy uncovers gapless boundary modes in the bulk insulating phase. Their magnetic field response, together with theoretical modeling, suggests a topological origin. These observations establish Ta2Pd3Te5 as the first confirmed topological excitonic insulator in a three-dimensional crystal. This allows to access the associated physics through bulk-sensitive techniques. Furthermore, we uncover another surprising aspect of the topological excitonic insulator, a secondary excitonic instability near 5 K that breaks the translational symmetry. The wavevector of this state shows an unprecedented magnetic field tunability. Thus, we unveil a unique sequence of topological exciton condensations in a bulk crystal, offering new opportunities to study critical behavior and excitations.

研究动机与目标

• 识别并表征一种在三维晶体中由激发子凝聚诱导拓扑序的关联拓扑相。
• 解析强关联电子系统中对称性破缺与拓扑保护之间的相互作用。
• 探索通过激发子不稳定性在体材料中出现可调动量序的机制。
• 建立一个利用体敏感光谱技术研究拓扑激发子相的平台。

提出的方法

• 角分辨光电子能谱（ARPES）用于探测电子能带结构和体态谱隙。
• 扫描隧道显微镜（STM）用于成像表面态并检测无能隙边界态。
• 热力学与结构分析用于评估晶格耦合强度和对称性破缺程度。
• 磁场依赖性测量用于调控次级激发子不稳定性波矢。
• 理论建模用于确认观测到的边界态的拓扑起源。
• 库仑相互作用驱动的激发子形成与凝聚被确定为体态能隙打开的机制。

实验结果

研究问题

• RQ1 能否在体三维晶体中通过激发子凝聚引起的自发对称性破缺诱导出拓扑相？
• RQ2 Ta2Pd3Te5绝缘相中的边界态具有何种性质，其是否具有拓扑起源？
• RQ3 次级激发子不稳定性波矢对外部磁场的响应特性如何？
• RQ4 尽管存在对称性破缺，该激发子绝缘体相中的结构耦合在多大程度上仍较微弱？
• RQ5 如ARPES和STM等体敏感技术能否可靠探测激发子绝缘体的拓扑特性？

主要发现

• 在Ta2Pd3Te5中观察到低于100 K的完整谱体能隙，其直接归因于激发子凝聚。
• 通过扫描隧道显微镜检测到无能隙边界态，其磁场响应与拓扑起源一致。
• 在约5 K附近的次级激发子不稳定性表现出可由磁场调控的波矢，表明存在一种新型动量序。
• 镜像对称性破缺伴随极小的结构畸变，该结论得到光电子能谱与热力学测量的证实。
• 理论建模支持存在具有非平庸Z2不变量的拓扑激发子绝缘体相。
• Ta2Pd3Te5被确立为首个确认的三维拓扑激发子绝缘体，实现了对拓扑激发子物理的体态可及性。