@article{027aa82d0bf54bfbb74deed911345c62,
title = "Optical bulk-boundary dichotomy in a quantum spin Hall insulator",
abstract = "The bulk-boundary correspondence is a critical concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z2 topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carriers beyond transport and electronic spectroscopy techniques. Here, we utilize mid-infrared absorption micro-spectroscopy and pump–probe micro-spectroscopy to elucidate the bulk-boundary optical responses of Bi4Br4, a recently discovered room-temperature quantum spin Hall insulator. Benefiting from the low energy of infrared photons and the high spatial resolution, we unambiguously resolve a strong absorption from the boundary states while the bulk absorption is suppressed by its insulating gap. Moreover, the boundary absorption exhibits strong polarization anisotropy, consistent with the one-dimensional nature of the topological boundary states. Our infrared pump–probe microscopy further measures a substantially increased carrier lifetime for the boundary states, which reaches one nanosecond scale. The nanosecond lifetime is about one to two orders longer than that of most topological materials and can be attributed to the linear dispersion nature of the helical boundary states. Our findings demonstrate the optical bulk-boundary dichotomy in a topological material and provide a proof-of-principal methodology for studying topological optoelectronics.",
keywords = "BiBr, Edge states, Mid-infrared absorption micro-spectroscopy, Pump–probe micro-spectroscopy, Quantum spin Hall effect, Topological insulator",
author = "Junfeng Han and Pengcheng Mao and Hailong Chen and Yin, {Jia Xin} and Maoyuan Wang and Dongyun Chen and Yongkai Li and Jingchuan Zheng and Xu Zhang and Dashuai Ma and Qiong Ma and Yu, {Zhi Ming} and Jinjian Zhou and Liu, {Cheng Cheng} and Yeliang Wang and Shuang Jia and Yuxiang Weng and Hasan, {M. Zahid} and Wende Xiao and Yugui Yao",
note = "Funding Information: This work was supported by the National Natural Science Foundation of China (11734003, 62275016, and 12274029), the National Key Research and Development Program of China (2020YFA0308800), Beijing Natural Science Foundation (Z210006) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB30000000). We are grateful to the Instrument Analysis Center of Xi{\textquoteright}an Jiaotong University for assistance with infrared absorption measurement and Analysis & Testing Center of Beijing Institute of Technology for assistance with SEM and XRD analyses. We acknowledge fruitful discussions with Xiang Li, Junxi Duan, Qingsheng Wang, Gang Wang, Jie Ma, and Yuanchang Li. Funding Information: This work was supported by the National Natural Science Foundation of China (11734003, 62275016, 12274029, and 92163206), the National Key Research and Development Program of China (2020YFA0308800), Beijing Natural Science Foundation (Z210006 and Z190006) and the Strategic Priority Research Program of Chinese Academy of Sciences (XDB30000000). We are grateful to the Instrument Analysis Center of Xi'an Jiaotong University for assistance with infrared absorption measurement and Analysis & Testing Center of Beijing Institute of Technology for assistance with SEM and XRD analyses. We acknowledge fruitful discussions with Xiang Li, Junxi Duan, Qingsheng Wang, Gang Wang, Jie Ma, and Yuanchang Li. Yugui Yao, Junfeng Han, and Wende Xiao supervised this project; Pengcheng Mao, Hailong Chen, and Junfeng Han carried out infrared absorption measurements; Dongyun Chen, Yongkai Li, Jingchuan Zheng, Xu Zhang, Yeliang Wang, Shuang Jia, Yuxiang Weng, and Wende Xiao synthesized and characterized materials; Maoyuan Wang, Dashuai Ma, Zhiming Yu, Jinjian Zhou, Cheng-Cheng Liu, and Yugui Yao performed first-principle calculations; Junfeng Han, Jiaxin Yin, Qiong Ma, M. Zahid Hasan, Maoyuan Wang, Wende Xiao, and Yugui Yao wrote the paper; all authors discussed and analyzed the data. Publisher Copyright: {\textcopyright} 2023",
year = "2023",
month = feb,
day = "26",
doi = "10.1016/j.scib.2023.01.038",
language = "English (US)",
volume = "68",
pages = "417--423",
journal = "Science Bulletin",
issn = "2095-9273",
publisher = "Springer Science + Business Media",
number = "4",
}