Trigger of the ubiquitous surface band bending in 3D topological insulators

TL;DR

This paper reveals that ultraviolet illumination during ARPES experiments triggers surface band bending in 3D topological insulators, clarifying discrepancies between different measurement techniques and providing a comprehensive understanding of the band structure evolution.

Contribution

It identifies illumination as the key factor causing band bending in TIs, revisiting previous assumptions and explaining experimental discrepancies between ARPES and magneto-transport studies.

Findings

Illumination causes energetic shifts in surface electronic bands.

Band bending occurs rapidly under UV exposure and then flattens over time.

The study predicts the evolution of TI band structures during ARPES measurements.

Abstract

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.