Spectroscopic evidence of intra-unit-cell charge redistribution in charge-neutral magnetic topological insulator Sb-doped MnBi6Te10

TL;DR

This study uses spectroscopic techniques to reveal intra-unit-cell charge redistribution and spontaneous polarization in Sb-doped MnBi6Te10, highlighting challenges in tuning the material for quantum anomalous Hall effect applications.

Contribution

It provides direct spectroscopic evidence of intra-unit-cell charge redistribution and polarization in Sb-doped MnBi6Te10, advancing understanding of its electronic structure and doping challenges.

Findings

Observation of transient surface photovoltage effects

Detection of band structure splitting indicating charge redistribution

Evidence of static doping deviations from charge neutrality

Abstract

The magnetic topological insulator MnBi$_{6}$Te$_{10}$ has emerged as a promising candidate for realizing the quantum anomalous Hall effect (QAHE), owing to its ability to retain ferromagnetism through precise control of anti-site defects. The next important task for realizing the QAHE is to tune the chemical potential into the energy gap formed by the broken time-reversal symmetry. Here we reveal an intra-unit-cell charge redistribution even when the overall doping suggests a near-charge-neutral condition. By performing time- and angle-resolved photoemission spectroscopy (trARPES) on the optimally 18% Sb-doped MnBi$_{6}$Te$_{10}$, we observe transient surface photovoltage (SPV) effects on both the MnBi$_{2}$Te$_{4}$ and single-Bi$_{2}$Te$_{3}$ terminations. Furthermore, we observe a time-dependent splitting of the band structure indicating multiple SPV shifts with different magnitudes. This observation suggests that adjacent plateaus with nominally the same terminating layer exhibit a strong intra-unit-cell charge redistribution, resulting in spontaneous electrical polarization. This is consistent with static micro-ARPES measurements revealing significant doping deviations from the charge-neutral configuration. Our findings underscore the challenges of engineering the family of Mn-Bi-Te materials to realize QAHE purely through chemical doping. Achieving the desired topological quantum phase requires both a uniform carrier doping and a ferromagnetic ground state. Furthermore, the light-induced polarization within each unit cell of ferromagnetic Mn(Bi$_{0.82}$Sb$_{0.18}$)$_{6}$Te$_{10}$ may open new possibilities for optoelectronic and spintronics.