Real-Space Imaging of the Band Topology of Transition Metal Dichalcogenides

The topological properties of Bloch bands are intimately tied to the structure of their electronic wavefunctions within the unit cell of a crystal. Here, we show that scanning tunneling microscopy and spectroscopy (STM/S) measurements on the prototypical transition metal dichalcogenide (TMD) semiconductor WSe2 can be used to unambiguously determine the location of the Wannier center of the valence band. Using site-specific substitutional doping, we first determine the position of the atomic sites within STM images, establishing that the maximum electronic density of states at the K-point lies between the atoms. In contrast, the maximum density of states at the point is at the atomic sites. This signifies that WSe2 is a topologically obstructed atomic insulator that cannot be adiabatically transformed to a trivial atomic limit, providing the first direct experimental evidence of this phase.