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Reconstructing the Wave Function of Magnetic Topological Insulators MnBi2Te4 and MnBi4Te7 Using Spin-Resolved Photoemission

Xue Han1,2, Jason Qu1,2, Hengxin Tan3, Zicheng Tao4, Noah M. Meyer1,2, Patrick S. Kirchmann1, Yanfeng Guo4,5, Binghai Yan3, Zhi-Xun Shen1,2,* et al.

Jonathan A. Sobota1,†

  • *Contact author: zxshen@stanford.edu
  • †Contact author: sobota@slac.stanford.edu
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Phys. Rev. X 15, 031022 – Published 18 July, 2025

DOI: https://doi.org/10.1103/lfxc-lsc1

Abstract

Despite their importance for exotic quantum effects, the surface electronic structure of magnetic topological insulators MnBi2Te4 and MnBi4Te7 remains poorly understood. Using high-efficiency spin- and angle-resolved photoemission spectroscopy, we directly image the spin-polarization and orbital character of the surface states in both compounds and map our observations onto a model wave function to describe the complex spin-orbital texture, which solidifies our understanding of the surface band structure by establishing the single-band nature of the most prominent states. Most importantly, our analysis reveals a new mechanism for reducing the magnetic gap of the topological surface states based on the orbital composition of the wave function.

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Popular Summary

It is an ongoing challenge to explain quantum effects in materials with complex electronic structure. Modeling the electron wave function is beneficial as it aids in the microscopic understanding of the internal structure of their quantum states. A particularly interesting family of magnetic topological insulators, MnBi2nTe3n+1, hosts unusual quantum phenomena linked to the structure of these wave functions. However, a puzzle remains: These exotic effects appear only at extremely low temperatures, much lower than expected based on their magnetic properties. We introduce a new way to reconstruct the electron wave function to better understand its role in the material’s unusual behavior.

We focus on two compounds, MnBi2Te4 and MnBi4Te7. By shining laser light onto their surfaces, we eject electrons and analyze their momentum, energy, and spin. By rotating the polarization of the light and tracking how the electrons’ spin orientation responds, we uncover how the spin is linked to the orbital states in the material. Using a theoretical model, we then extract detailed numerical values describing the wave functions. Our measurements show that the total angular momentum has a larger orbital contribution than expected from theory calculations. This reduces the net magnetic moment of the electrons, which reduces their ability to be influenced by magnetic fields.

These results show how spin- and orbital-resolved photoemission experiments can directly reveal the internal structure of electron wave functions. Our findings provide crucial insights on understanding exotic topological phenomena and guide future efforts to discover and optimize magnetic topological insulators.

Article Text

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Phys. Rev. X 15, 031022– Published 18 July, 2025
  • Received 18 September 2024
  • Revised 27 February 2025
  • Accepted 26 June 2025
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