Topological hinge state in a 1D stacking material

Due to a higher order bulk boundary correspondence, a higher-order topological insulator hosts one-dimensional helical edge states around the hinges of the crystal. However, spectroscopic evidence for topological hinge states has been so far limited to semimetallic materials [1], where the measured edge conductivity is blurred by bulk carries. In this contribution, we show evidence for topological hinge states in a semiconducting quasi-one-dimensional material. Importantly, the crystal has naturally cleavable top and side surfaces which are stacked via van-der-Waals forces, and therefore the electronic structure around the hinge can be investigated in a cleaved surface or in an exfoliated thin flake. Our experiments by high-resolution angle-resolved photoemission spectroscopy reveal quasi-1D states inside the bulk band gap. Moreover, the corresponding signals with high conductivity are detected at the edges of the crystal by microwave impedance microscopy. These observations, therefore, indicate the existence of topological hinge states in a 1D stacking material.
[1] F. Schindler et al., Nat. Phys. 14, 918 (2018).