Center for Correlated Electron Systems


Electronic structures of quasi-one-dimensional topological insulators Bi4X4 (X=Br, I) investigated by ARPES

February 25, 2021l Hit 131
Date : March 2, 2021 15:00 ~ 16:00
Speaker : Dr. Ryo Noguchi (Institute for Solid State Physics, University of Tokyo)
Location : Zoom (

- Zoom (

                      * ID: 822 8586 2512 / PW: 107314


- Abstract : Topological insulators (TIs) host gapless surface/hinge states in their bulk band gap, depending on the nontrivial topology of the bulk band structures. In three dimensions, the conventional Z2 TI is classified as either strong or weak, while higher-order TIs (HOTIs) have also been predicted beyond the framework of Z2 TIs. Although the strong topological insulator has been experimentally confirmed rapidly after the theoretical prediction [1], the weak topological insulator (WTI) and HOTI have been experimentally elusive, due to the difficulty in detecting the surface states in a WTI and the hinge states in a HOTI.

           In this talk, I will provide experimental evidence for a WTI state in β-Bi4I4 and a HOTI state in Bi4Br4 by utilizing high-resolution laser-based (spin-resolved) angle-resolved photoemission spectroscopy (ARPES) and synchrotron-based nano-ARPES [2,3]. Importantly, the crystals have naturally cleavable top and side surfaces both stacked by van-der-Waals forces, which is desirable for the experimental detection of the WTI state and the HOTI state. As a definitive signature of these states, we fined quasi-1D Dirac states in their bulk band gap. The quasi-1D topological state we have verified in this study will bring the new capability of designing and engineering functional materials benefiting from highly directional spin-currents protected against backscattering.


[1] M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010).

[2] R. Noguchi et al., Nature 566, 518 (2019).         

[3] R. Noguchi et al., Nat. Mater. (2021).



Host : Prof. Changyoung Kim