Observation of topological bulk-boundary correspondence in a strong-coupling charge density wave state*

The interplay between topology and charge order is at the frontier of condensed matter research. However, there is no direct experimental evidence for a topological bulk-boundary correspondence within a fully gapped charge-ordered state yet. Here, using scanning tunneling microscopy, we directly visualize the topological bulk-boundary correspondence in a topological charge density wave (CDW) material. Below the CDW transition temperature, tunneling spectra on an atomically resolved lattice reveals a large insulating gap (over 500 meV) whose △/k_BT_c≈20, suggesting a strong coupling nature of the CDW. Remarkably, in an atomically sharp monolayer step edge, we find an in-gap gapless helical state. Both the insulating gap and the gapless edge state disappear above the CDW transition temperature. The presence of the edge state within the insulating CDW gap indicates the topological bulk-boundary correspondence, pointing to the realization of a topological CDW state.