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

The topological properties of Bloch bands are tied to the structure of their electronic wavefunctions within the unit cell of a crystal. Here we show that scanning tunnelling microscopy and spectroscopy measurements on the prototypical transition metal dichalcogenide semiconductor WSe2 can be used to determine the location of the Wannier centre of the valence band. Using site-specific substitutional doping, we determine the position of the atomic sites within real-space scanning tunnelling microscopy images, and establish that the maximum electronic density of states at the corner of the Brillouin zone lies between the atoms. By contrast, the maximum density of states at the Brillouin zone centre is at the atomic sites. This signifies that WSe2 is a topologically obstructed atomic insulator, which cannot be adiabatically transformed into a trivial atomic limit, constituting direct experimental evidence of this phase of matter.