by Angle-resolved Photoemission Spectroscopy

Single-layer ZrTe₅ was known as a two-dimensional topological insulator, while ZrTe₅ bulk crystal consisting of many ZrTe₅ layers has been argued as either a three-dimensional strong topological insulator or a weak topological insulator. Using high-resolution photon-energy dependent angle-resolved photoemission spectroscopy, we succeeded in observing a quasi-one-dimensional and
linearly dispersive energy band near the Fermi level on the surface that is perpendicular to the two-dimensional ZrTe₅ layers. The observed quasi-one-dimensional band comes from the one-dimensional topological edge state of the ZrTe5 layers. The weak dispersion of the quasi-one-dimensional band along the normal direction of ZrTe₅ layer indicates that the inter-edge coupling between the ZrTe₅ layers are not strong enough to form a three-dimensional strong topological insulator. Our findings provided the comprehensive information about the quasi-one-dimensional edge state and unambiguously indicated that ZrTe₅ bulk crystal is a three-dimensional weak topological insulator.