Spectroscopic evidence of intra-unit-cell charge redistribution in a charge-neutral magnetic topological insulator

The magnetic topological insulator MnBi₆Te₁₀ has emerged as a promising candidate for realizing the quantum anomalous Hall effect (QAHE), thanks to its ability to retain ferromagnetism through precise control of anti-site defects. Angle-resolved photoemission spectroscopy (ARPES) reveals a significant gap at the Dirac point in the topological surface state, suggesting the potential for more exotic behaviors if the Fermi level is tuned into this gap. By doping with 18% Sb, we achieved the charge-neutral point, as confirmed by Hall measurements. However, micro-ARPES, capable of resolving the electronic structure of individual domains with different top layers, reveals a more complex scenario. Despite overall charge neutrality, ARPES spectra from the MnBi₂Te₄ and single-Bi₂Te₃ terminations show consistently electron-doped bands. Additionally, time-resolved ARPES experiments detected surface photovoltage effects in these terminations, indicating significant band bending in the near-surface region. These observations suggest intra-unit-cell charge redistribution in optimally doped MnBi₆Te₁₀, resulting in spontaneous electrical polarization. Our findings underscore the challenges of engineering Mn-Bi-Te family materials to realize QAHE purely through chemical doping. Thus, achieving desired material properties requires a careful balance between electric and magnetic ground states.