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Register a new userDelicate Ferromagnetism in MnBi$_6$Te$_{10}$
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Engineering magnetic orders in topological insulators is critical to the realization of topological quantum phenomena such as the axion insulator state and the quantum anomalous Hall insulator state. Here we establish MnBi$_6$Te$_{10}$ as a tunable topological material platform where ferromagnetism and antiferromagnetism can be selectively obtained. We conduct a comprehensive measurement of ferromagnetic MnBi$_6$Te$_{10}$ bulk crystals via laser-based angle-resolved photoemission spectroscopy, and compare the results with those from their antiferromagnetic counterparts. For ferromagnetic MnBi$_6$Te$_{10}$, we observe a magnetically driven broken-symmetry gap of 15 meV at the topological surface state on the MnBi$_2$Te$_4$ termination, which disappears when the temperature is raised above the Curie temperature. In contrast, antiferromagnetic MnBi$_6$Te$_{10}$ exhibits gapless topological surface states on all terminations. We consider disorder in the form of Mn migration from MnBi$_2$Te$_4$ layers to the neighboring Bi$_2$Te$_3$ layers as a possible driving force for the delicate ferromagnetism. Our spectroscopic study establishes MnBi$_6$Te$_{10}$ as the first bulk MnBi$_{2n}$Te$_{3n+1}$ compound to host tunable topological orders due to its highly variable electronic and magnetic structures.
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A striking feature of time reversal symmetry (TRS) protected topological insulators (TIs) is that they are characterized by a half integer quantum Hall effect on the boundary when the surface states are gapped by time reversal breaking perturbations. While time reversal symmetry (TRS) protected TIs have become increasingly under control, magnetic analogs are still largely unexplored territories with novel rich structures. In particular, topological magnetic insulators can also host a quantized axion term in the presence of lattice symmetries. Since these symmetries are naturally broken on the boundary, the surface states can develop a gap without external manipulation. In this work, we combine theoretical analysis, density functional calculations and experimental evidence to reveal intrinsic axion insulating behavior in MnBi6Te10. The quantized axion term arises from the simplest possible mechanism in the antiferromagnetic regime where it is protected by inversion symmetry and a fractional translation symmetry. The anticipated gapping of the Dirac surface state at the edge is subsequently experimentally established using Angle Resolved Spectroscopy. As a result, this system provides the magnetic analogue of the simplest TRS protected TI with a single, gapped Dirac cone at the surface.
Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$, the $n=$~1 and 2 members of a modular (Bi$_2$Te$_3$)$_n$(MnBi$_2$Te$_4$) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi$_2$Te$_3$-terminated surfaces but remains preserved for MnBi$_2$Te$_4$-terminated surfaces. Our results firmly establish the topologically non-trivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination.
The search for materials to support the Quantum Anomalous Hall Effect (QAHE) have recently centered on intrinsic magnetic topological insulators (MTIs) including MnBi$_2$Te$_4$ or heterostructures made up of MnBi$_2$Te$_4$ and Bi$_2$Te$_3$. While MnBi$_2$Te$_4$ is itself a MTI, most recent ARPES experiments indicate that the surface states on this material lack the mass gap that is expected from the magnetism-induced time-reversal symmetry breaking (TRSB), with the absence of this mass gap likely due to surface magnetic disorder. Here, utilizing small-spot ARPES scanned across the surfaces of MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$, we show the presence of large mass gaps (~ 100 meV scale) on both of these materials when the MnBi$_2$Te$_4$ surfaces are buried below one layer of Bi$_2$Te$_3$ that apparently protects the magnetic order, but not when the MnBi$_2$Te$_4$ surfaces are exposed at the surface or are buried below two Bi$_2$Te$_3$ layers. This makes both MnBi$_4$Te$_7$ and MnBi$_6$Te$_{10}$ excellent candidates for supporting the QAHE, especially if bulk devices can be fabricated with a single continuous Bi$_2$Te$_3$ layer at the surface.
Surface magnetism and its correlation with the electronic structure are critical to understand the gapless topological surface state in the intrinsic magnetic topological insulator MnBi$_2$Te$_4$. Here, using static and time resolved angle-resolved photoemission spectroscopy (ARPES), we find a significant ARPES intensity change together with a gap opening on a Rashba-like conduction band. Comparison with a model simulation strongly indicates that the surface magnetism on cleaved MnBi$_2$Te$_4$ is the same as its bulk state. The coexistence of surface ferromagnetism and a gapless TSS uncovers the novel complexity of MnBi$_2$Te$_4$ that may be responsible for the low quantum anomalous Hall temperature of exfoliated MnBi$_2$Te$_4$.
Using scanning tunneling microscopy and spectroscopy, we visualized the native defects in antiferromagnetic topological insulator $mathrm{MnBi_2Te_4}$. Two native defects $mathrm{Mn_{Bi}}$ and $mathrm{Bi_{Te}}$ antisites can be well resolved in the topographic images. $mathrm{Mn_{Bi}}$ tend to suppress the density of states at conduction band edge. Spectroscopy imaging reveals a localized peak-like local density of state at $sim80$~meV below the Fermi energy. A careful inspection of topographic and spectroscopic images, combined with density functional theory calculation, suggests this results from $mathrm{Bi_{Mn}}$ antisites at Mn sites. The random distribution of $mathrm{Mn_{Bi}}$ and $mathrm{Bi_{Mn}}$ antisites results in spatial fluctuation of local density of states near the Fermi level in $mathrm{MnBi_2Te_4}$.
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