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Further to the structure of the intrinsic magnetic topological insulators MnBi2Te4⋅n(Bi2Te3) with n0 overwhelmingly dominates by the cooperative atomic displacements in the quintuple building blocks., This work was performed within the research program “Development of preparation technology of multifunctional convertors based on nanosized structures” and was supported by TÜBITAK- ANAS Project No. 120N296. M.M.O. acknowledges the support by Spanish Ministerio de Ciencia e Innovación (Grant No. PID2019-103910GB-100). E.V.C. acknowledges support from Saint Petersburg State University (Project ID No. 90383050).

A tunable gap in the topological surface state is of great interest for novel spintronic devices and applications in quantum computing. Here, we study the surface electronic structure and magnetic properties of the Gd-doped topological insulator TlBi0.9Gd0.1Se2. Utilizing superconducting quantum interference device magnetometry, we show paramagnetic behavior down to 2 K. Combining spin- and angle-resolved photoemission spectroscopy with different polarizations of light, we demonstrate that the topological surface state is characterized by the Dirac cone with a helical spin structure and confirm its localization within the bulk band gap. By using different light sources in photoemission spectroscopy, various Dirac-point gap values were observed: 50 meV for hν=18eV and 20 meV for hν=6.3eV. Here, we discuss the gap observation by the angle-resolved photoemission spectroscopy method as a consequence of the scattering processes. Simulating the corresponding spectral function, we demonstrate that the asymmetric energy-distribution curve of the surface state leads to an overestimation of the corresponding gap value. We speculate that 20 meV in our case is a trustworthy value and attribute this gap to be originated by scattering both on magnetic and charge impurities provided by Gd atoms and surface defects. Given the complexity and importance of scattering processes in the topological surface state together with our observations of distinctive photoemission asymmetry, we believe our results are important for research of the massive Dirac fermions in novel quantum materials., This work was supported by St. Petersburg State University Project (ID No. 51126254), by the Russian Science Foundation (Grant No. 18-12-00062), by the Ministry of Science and Higher Education of the Russian Federation (Grant No 2020-1902-01-058), and by the Science Development Foundation under the President of the Republic of Azerbaijan (Grant No. EIF-BGM-4-RFTF-1/2017-1/04/1-M-02). The studies were also carried out at the resource centers of St. Petersburg State University “Physical Methods for Surface Investigation” and “Diagnosis of Functional Materials for Medicine, Pharmacology, and Nanoelectronics.” In addition, the work was supported by the German-Russian Interdisciplinary Science Center (G-RISC) funded by the German Federal Foreign Office via the German Academic Exchange Service (DAAD) and Russian-German Laboratory at BESSY II (Helmholtz Zentrum, Berlin). We thank the Hiroshima Synchrotron Radiation Center (Proposal No. 18BG026), Helmholtz-Zentrum Berlin für Materialien und Energie for the allocation of synchrotron radiation beam times, and the N-BARD, Hiroshima University for supplying liquid helium. A.K. was financially supported by KAKENHI (Grants No. 17H06138, No. 17H06152, and No. 18H03683).

Despite immense advances in the field of topological materials, the antiferromagnetic topological insulator (AFMTI) state, predicted in 2010, has been resisting experimental observation up to now. Here, using density functional theory and Monte Carlo method we predict and by means of structural, transport, magnetic, and angle-resolved photoemission spectroscopy measurements confirm for the first time realization of the AFMTI phase, that is hosted by the van der Waals layered compound MnBi$_2$Te$_4$. An interlayer AFM ordering makes MnBi$_2$Te$_4$ invariant with respect to the combination of the time-reversal ($\Theta$) and primitive-lattice translation ($T_{1/2}$) symmetries, $S=\Theta T_{1/2}$, which gives rise to the $Z_2$ topological classification of AFM insulators, $Z_2$ being equal to 1 for this material. The $S$-breaking (0001) surface of MnBi$_2$Te$_4$ features a giant bandgap in the topological surface state thus representing an ideal platform for the observation of such long-sought phenomena as the quantized magnetoelectric coupling and intrinsic axion insulator state.