Weak topological insulator state in quasi-one-dimensional bismuth iodide observed by surface-selective nano-ARPES

Recently, various materials have been confirmed to be topologically non-trivial by angle-resolved photoemission spectroscopy (ARPES) [1]. Despite intensive research on topological materials, a weak topological insulator (WTI) has so far remained hypothetical, since topological surface states (TSS) emerge only on the side surfaces, which are usually undetectable by conventional ARPES. We have overcome this difficulty by employing newly developed nano-ARPES. Its high spatial resolution allows us to perform surface selective measurements, which is necessary for the discovery of a WTI state in real 3D crystals.
In this contribution, we provide first experimental evidence for a WTI state in quasi-one-dimensional β-Bi₄I₄. The crystal has naturally cleavable top and side surfaces both stacked via van-der-Waals forces, and TSSs emerge only on the side surface as the WTI phase [2,3]. Our nano-ARPES results reveal quasi-1D Dirac-like surface state emerge only on the side surface whlie the top surface is topologically dark, which is the hall mark of the WTI state in β-Bi₄I₄ [4].
[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).
[2] G. Autès et al., Nat. Mater. 15, 154 (2015).
[3] C.-C. Liu et al., Phys. Rev. Lett. 116, 66801 (2016).
[4] R. Noguchi et al., arXiv: 1802.03860 (2018).