Discovery of a weak topological phase in a transition-metal pnictide

Topological insulators (TIs) can host a variety of exotic phenomena, such as spin-polarized topological surface states (TSSs), topological magnetoelectric effects, and topological superconductivity. Early on, it was realized that TIs can be grouped into two families: strong TIs that host TSSs on all crystal surfaces, and weak TIs, which host TSSs only on some surfaces. Initially, it was believed that unlike for strong TIs, TSSs in weak TIs would be susceptible to localization by disorder. However, subsequent theoretical studies found that weak TIs should be more robust against disorder than originally assumed and that they can harbour unusual physical properties, such as 1D conduction channels at step edges and in screw dislocations, which are protected from localization by topology. Yet, despite multiple theoretical proposals and experimental reports, “smoking-gun” evidence for the existence of a weak topological phase - such as momentum resolved measurement of a TSS dispersion - is still elusive. Here, by performing angle-resolved photoemission measurements and ab-initio calculations for the surface states of two crystal surfaces, we can conclusively confirm the existence of a weak topological phase in a transition-metal pnictide.