The advent of intrinsic magnetic topological insulators provides a pathway toward low-dimensional topological orders at realistic cryogenic temperatures. These materials are represented by MnBi₂Te₄ and its derived superlattices MnBi_2nTe_3n+1. However, it has been controversial whether these materials exhibit the key ingredient for magnetic topological phases: an energy gap due to the time-reversal symmetry breaking. In this talk, I will present a new technique, layer-encoded frequency-domain angle-resolved photoemission spectroscopy (LEFD-ARPES), which allows us to decipher the layer origins of various electronic states. By encoding layer indices with intralayer phonon frequencies, we measure the strengths of coupling with layer-specific phonons. This experiment reveals that the topological surface states on the magnetic termination of antiferromagnetic MnBi₄Te₇ are partially relocated to the nonmagnetic layers, reconciling the mystery of vanishing broken-symmetry gaps [1]. Instead, a pair of quasi-two-dimensional electronic states with an enormous Rashba splitting is characterized by LEFD-ARPES to live on the magnetic surface layer. Time-resolved ARPES on MnBi₂Te₄ unveils that these quasi-two-dimensional states mediate a surface-only Ruderman-Kittel-Kasuya-Yasuda (RKKY) interaction [2]. The surface RKKY interaction distinguishes the surface magnetism from the bulk counterpart, which may explain the mysterious residual magnetism in even-layer MnBi₂Te₄ flakes. Finally, I will elucidate how a delicate interplay of the Mn migration and Mn vacancies in MnBi₆Te₁₀ enables us to achieve a tunable magnetic ground state, realizing the long-wanted broken-symmetry gap on the topological surface state. 

 [1] Lee et al. Nature Phys. 19, 950-955 (2023).

[2] Nguyen et al. Submitted (2023).

[3] Yan et al. Nano Lett. 22, 9815-9822 (2022).

 Acknowledgement: Our static ARPES studies are supported by NSF DMR-2145373. Time-resolved ARPES studies are supported by DOE DE-SC0022960. Molecular beam epitaxy and surface morphology characterization are partially supported by NSF DMR-2011854. Transmission electron microscopy is partially supported by DOE DE-SC0022984. Bulk crystal synthesis is supported by NSF through the Penn State 2D Crystal Consortium-Materials Innovation Platform under NSF cooperative agreement DMR-2039351, as well as by DOE DE-SC0019068.