Micro-metric electronic patterning of a topological band structure using a photon beam

The only states crossing E$_F$ in ideal, 3D TIs are topological surface states. Single crystals of Bi$_2$Se$_3$ and Bi$_2$Te$_3$ are too defective to exhibit bulk-insulating behaviour, and ARPES shows topologically trivial 2DEGs at E$_F$ in the surface region due to downward band bending. Ternary \& quaternary alloys of Bi$\big /$Te$\big /$Se$\big /$Sb hold promise for obtaining bulk-insulating crystals. Here we report ARPES data from quaternary, bulk-insulating, Bi-based TIs. Shortly after cleavage in UHV, downward band bending pulls the bulk conduction band below E$_F$, once again frustrating the ``topological only'' ambition for the Fermi surface. However, there is light at the end of the tunnel: we show that a super-band-gap photon beam generates a surface photovoltage sufficient to flatten the bands, thereby recovering the ideal, ``topological only'' situation. In our bulk-insulating quaternary TIs, this effect is local in nature, and permits the writing of arbitrary, micron-sized patterns in the topological energy landscape at the surface.