Topological Electronic Structure and Its Temperature Evolution in Antiferromagnetic Topological Insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$

The intrinsic magnetic topological insulator ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ exhibits rich topological effects such as quantum anomalous Hall effect and axion electrodynamics. Here, by combining the use of synchrotron and laser light sources, we carry out comprehensive and high-resolution angle-resolved photoemission spectroscopy studies on ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and clearly identify its topological electronic structure. In contrast to theoretical predictions and previous studies, we observe topological surface states with diminished gap forming a characteristic Dirac cone. We argue that the topological surface states are mediated by multidomains of different magnetization orientations. In addition, the temperature evolution of the energy bands clearly reveals their interplay with the magnetic phase transition by showing interesting differences between the bulk and surface states, respectively. The investigation of the detailed electronic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and its temperature evolution provides important insight into not only the exotic properties of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, but also the generic understanding of the interplay between magnetism and topological electronic structure in magnetic topological quantum materials.

Popular Summary

Magnetic topological insulators represent a novel state of quantum materials with unique blends of topological properties and magnetism. They are potentially useful for engineering a variety of exotic topological magnetoelectric effects, such as the quantum anomalous Hall effect (in which a quantized Hall effect occurs without an external magnetic field) and the axion insulator phase (in which the insulating surface states give rise to a half-quantized anomalous Hall conductivity). Recently, researchers proposed the first intrinsic magnetic topological insulator, ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, in which several remarkable breakthroughs, including the quantized anomalous Hall effect and axion insulator phase, have been experimentally realized. In order to understand the exotic properties of this material and explore its full potential, we systematically study the subtle electronic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ using angle-resolved photoemission spectroscopy.

Combining experiments with first-principles calculations, we successfully identify both the bulk states and the topological surface states that unexpectedly show a diminished energy gap. Interestingly, we observe a clear difference between the interplay of the bulk and surface states with the magnetic phase transition: While the bulk states show clear energy splitting when the antiferromagnetic order forms, the topological surface states show a negligible energy gap even under the bulk antiferromagnetic order.

Our results confirm the topological nature of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ and the band structure that is modulated by magnetic ordering, which not only provides important insights into the generic understanding of the interplay between magnetism and topological electronic structure but also paves the way for the design and realization of novel phenomena and applications.

Figure 1

Basic properties and characterization of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ single crystals. (a) Schematic of the layered crystal structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. (b) Image of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ single crystal after cleavage and x-ray diffraction patterns along different directions. (c) Angle scan of x-ray diffraction along $c$ axis. (d) Magnetization (with magnetic field applied along $c$ axis) and resistance as functions of temperature. (e) Scanning tunneling microscopy mapping of surface topography obtained with $I=250\mathrm{mA}$ and $V=1\mathrm{V}$. (f) X-ray photoemission spectroscopy showing characteristic Mn, Bi, and Te core levels. (g) Constant energy contour near 270 meV below the Fermi energy over multiple BZ; data were collected under 100 eV photon energy at 18 K.

Figure 2

Photon-energy-dependent band structure measurements on ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. (a) Band dispersion near $\overline{\mathrm{\Gamma }}$ measured with different photon energies. The topological surface state (TSS) as marked is observed at photon energies lower than 16 eV. (b) Photon energy dependence of (i) energy distribution curve (EDC) near $\overline{\mathrm{\Gamma }}$, (ii) extracted conduction band minimum (CBM) and valance band maximum (VBM), and (iii) bulk band gap. (c)(i) Calculated bulk band dispersion along $\mathrm{\Gamma }K$ ($k_z=0$ plane). (c)(ii)–(c)(iv) Band dispersion obtained at low-photon energies showing characteristic TSSs. The insets are the momentum distribution curves (MDCs) integrated near the Dirac point. Data were collected at 14 K.

Figure 3

Electronic structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$ measured with 6.994 eV laser. (a),(b) Constant energy contours at different binding energies. (c) Band dispersion along the $\overline{\mathrm{\Gamma }}\overline{K}$ direction with a clear Dirac cone formed by TSSs. (d) Enlarged plot of the dispersion near the Dirac point (DP). (e) Second derivative of ARPES intensity map in (c). The dashed curves are guides to eye for band dispersions. (f) Stacking plot of MDCs. The bulk conduction bands and TSSs with diminished gap are clearly observed. Data were collected at 7.5 K.

Figure 4

Temperature evolution of the band structure of ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$. (a),(b) Band dispersions along $\overline{\mathrm{\Gamma }}\overline{K}$ (a) and corresponding MDCs (b) at selected temperatures. (c) Side-by-side comparison of the second derivative of ARPES intensity maps at 7.5 and 30 K. (d) Temperature evolution of EDCs at $\overline{\mathrm{\Gamma }}$. (e) The energy position of $\mathrm{CB}{1}_a$ and $\mathrm{CB}{1}_b$ bands together with the energy difference between them as functions of the temperature showing a band splitting below 27 K. Data were taken with 6.994 eV laser.