Low-dimensional topological materials investigated by ARPES

Ryo Noguchi^1*

¹Center for Condensed Matter Sciences, National Taiwan University, Taipei, Taiwan

* Presenter:Ryo Noguchi, email:rnoguchi@ntu.edu.tw

Topological materials, which exhibit distinctive electronic structures such as Dirac and Weyl dispersions, have attracted significant interest due to their potential applications in spintronics and quantum computation. Angle-resolved photoemission spectroscopy (ARPES) has played a pivotal role in advancing this field by enabling the direct observation of electronic band structures. Recently, materials of lower dimensionality have emerged as new platforms for exploring diverse topological quantum phenomena. In this talk, I will present our ARPES studies on a quasi-one-dimensional topological material.
In this study, we demonstrate that a robust weak topological insulator (WTI) phase can be realised in the bismuth halide system Bi₄X₄ (X = Br, I) by controlling the chemical composition and stacking sequence. Although β-Bi₄I₄ exhibits a WTI phase, the observation of quantum transport phenomena is difficult due to its small band gap (<100 meV) [1]. Conversely, Bi₄Br₄ exhibits a larger gap (~300 meV), yet it stabilises a higher-order topological insulator phase due to its bilayer stacking [2]. In order to identify another WTI system, our focus was on the mixed compound Bi₄Br₂I₂, in which three-layer stacking stabilises the weak topology, while Br-I mixing enlarges the band gap. We reveal the robustness of the WTI phase using ARPES measurements.
[1] R. Noguchi et al., Nature 566, 518–522 (2019).
[2] R. Noguchi et al., Nat. Mater. 20, 473 (2021).
[3] R. Noguchi et al., Phys Rev. Lett. 133, 086602 (2024).

Keywords: Topological insulator, ARPES, Quasi-one-dimensional material