  The main scientific activity in the field of topological insulators (TIs)
consists of determining their electronic structure by means of
magneto-transport and electron spectroscopy with a view to devices based on
topological transport. There is however a caveat in this approach. There are
systematic experimental discrepancies on the electronic structure of the most
pristine surfaces of TI single crystals as determined by Shubnikov de Haas
(SdH) oscillations and by Angle Resolved PhotoElectron Spectroscopy (ARPES). We
identify intense ultraviolet illumination -that is inherent to an ARPES
experiment- as the source for these experimental differences. We explicitly
show that illumination is the key parameter, or in other words the trigger, for
energetic shifts of electronic bands near the surface of a TI crystal. This
finding revisits the common belief that surface decoration is the principal
cause of surface band bending and explains why band bending is not a prime
issue in the illumination-free magneto-transport studies. Our study further
clarifies the role of illumination on the electronic band structure of TIs by
revealing its dual effect: downward band bending on very small timescales
followed by band flattening at large timescales. Our results therefore allow us
to present and predict the complete evolution of the band structure of TIs in a
typical ARPES experiment. By virtue of our findings, we pinpoint two
alternatives of how to approach flat band conditions by means of photon-based
techniques and we suggest a microscopic mechanism that can explain the
underlying phenomena.
