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We study the surface crystalline and electronic structures of the antiferromagnetic topological insulator MnBi2Te4 using scanning tunneling microscopy/spectroscopy (STM/S), micro(μ)-laser angle-resolved photoemission spectroscopy (ARPES), and density functional theory calculations. Our STM images reveal native point defects at the surface that we identify as BiTe antisites and MnBi substitutions. Bulk X-ray diffraction further evidences the presence of the Mn-Bi intermixing. Overall, our characterizations suggest that the defects concentration is nonuniform within crystals and differs from sample to sample. Consistently, the ARPES and STS experiments reveal that the Dirac point gap of the topological surface state is different for different samples and sample cleavages, respectively. Our calculations show that the antiparallel alignment of the MnBi moments with respect to those of the Mn layer can indeed cause a strong reduction of the Dirac point gap size. The present study provides important insights into a highly debated issue of the MnBi2Te4 Dirac point gap.

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator MnBi 2 Te 4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies. We have distinguished both large (60–70 meV) and reduced ( < 20 meV ) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ( T N = 24.5 K ). We propose that the gap above T N remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the “large gap” sample and apparently significantly reduced effective magnetic moment for the “reduced gap” sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.

Magnetic topological insulators are narrow-gap semiconductor materials that combine non-trivial band topology and magnetic order 1 . Unlike their nonmagnetic counterparts, magnetic topological insulators may have some of the surfaces gapped, which enables a number of exotic phenomena that have potential applications in spintronics 1 , such as the quantum anomalous Hall effect 2 and chiral Majorana fermions 3 . So far, magnetic topological insulators have only been created by means of doping nonmagnetic topological insulators with 3 d transition-metal elements; however, such an approach leads to strongly inhomogeneous magnetic 4 and electronic 5 properties of these materials, restricting the observation of important effects to very low temperatures 2 , 3 . An intrinsic magnetic topological insulator—a stoichiometric well ordered magnetic compound—could be an ideal solution to these problems, but no such material has been observed so far. Here we predict by ab initio calculations and further confirm using various experimental techniques the realization of an antiferromagnetic topological insulator in the layered van der Waals compound MnBi 2 Te 4 . The antiferromagnetic ordering  that MnBi 2 Te 4  shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to a ℤ 2 topological classification; ℤ 2  = 1 for MnBi 2 Te 4 , confirming its topologically nontrivial nature. Our experiments indicate that the symmetry-breaking (0001) surface of MnBi 2 Te 4 exhibits a large bandgap in the topological surface state. We expect this property to eventually enable the observation of a number of fundamental phenomena, among them quantized magnetoelectric coupling 6 – 8 and axion electrodynamics 9 , 10 . Other exotic phenomena could become accessible at much higher temperatures than those reached so far, such as the quantum anomalous Hall effect 2 and chiral Majorana fermions 3 . An intrinsic antiferromagnetic topological insulator, MnBi 2 Te 4 , is theoretically predicted and then realized experimentally, with implications for the study of exotic quantum phenomena.

Further to the structure of the intrinsic magnetic topological insulators MnBi2Te4 n(Bi2Te3) with n<4, where index n is the number of quintuple Te-Bi-Te-Bi-Te building blocks inserted between the neighboring septuple Te-Bi-Te-Mn-Te-Bi-Te building blocks, the structure of the members with n=4, 5 and 6 was studied using X-ray powder diffraction. The unit cell parameters and atomic positions were calculated. The obtained and available structural data were summarized to show that the crystal structure of all members of MnBi2Te4 n(Bi2Te3) follows the cubic close packing principle, independently of the space group of the given member. Confocal Raman spectroscopy was then applied. Comparative analysis of the number, frequency, symmetry, and broadening of the vibration modes responsible for the lines in the Raman spectra of the systems with n=1,. . . ,6, as well as MnBi2Te4 (n=0) and Bi2Te3 (n=infinity) has shown that lattice dynamics of MnBi2Te4 n(Bi2Te3) with n>0 overwhelmingly dominates by the cooperative atomic displacements in the quintuple building blocks.

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator$${\\hbox {MnBi}}_2 {\\hbox {Te}}_4$$MnBi 2 Te 4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies. We have distinguished both large (60–70 meV) and reduced ($$<20~ \\hbox {meV}$$< 20 meV ) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ($$T_{\\mathrm{N}} = 24.5~ \\hbox {K}$$T N = 24.5 K ). We propose that the gap above$$T_{\\mathrm{N}}$$T N remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the “large gap” sample and apparently significantly reduced effective magnetic moment for the “reduced gap” sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9-35 K), light polarizations and photon energies. We have distinguished both large (60-70 meV) and reduced ( ) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ( ). We propose that the gap above remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the \"large gap\" sample and apparently significantly reduced effective magnetic moment for the \"reduced gap\" sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.

Modification of the gap at the Dirac point (DP) in axion antiferromagnetic topological insulator $${\\hbox {MnBi}}_2 {\\hbox {Te}}_4$$ MnBi2Te4 and its electronic and spin structure have been studied by angle- and spin-resolved photoemission spectroscopy (ARPES) under laser excitation at various temperatures (9–35 K), light polarizations and photon energies. We have distinguished both large (60–70 meV) and reduced ( $$<20~ \\hbox {meV}$$ <20meV) gaps at the DP in the ARPES dispersions, which remain open above the Neél temperature ( $$T_{\\mathrm{N}} = 24.5~ \\hbox {K}$$ TN=24.5K). We propose that the gap above $$T_{\\mathrm{N}}$$ TN remains open due to a short-range magnetic field generated by chiral spin fluctuations. Spin-resolved ARPES, XMCD and circular dichroism ARPES measurements show a surface ferromagnetic ordering for the “large gap” sample and apparently significantly reduced effective magnetic moment for the “reduced gap” sample. These observations can be explained by a shift of the Dirac cone (DC) state localization towards the second Mn layer due to structural disturbance and surface relaxation effects, where DC state is influenced by compensated opposite magnetic moments. As we have shown by means of ab-initio calculations surface structural modification can result in a significant modulation of the DP gap.

The domain structure of a MnSb\\(_2\\)Te\\(_4\\) single crystal with a Curie temperature \\(T_C \\approx 45~K\\) was studied using the high-resolution Bitter decoration technique. Magnetotransport measurements confirm a soft ferromagnetic ordering with a coercive field of \\( \\sim 100\\) Oe. We revealed the formation of a hierarchical domain structure characterized by two distinct spatial scales. These results indicate the existence of two magnetically weakly coupled subsystems -- surface and bulk. The observed sub-domain structure can be attributed to the formation of a ferromagnetic well due to an inhomogeneous distribution of \\(\\mathrm{Mn_{Sb}}\\) antisite defects, with an additional contribution from symmetry breaking in the near-surface layer.

The results of a comprehensive study of MnSb\\(_2\\)Te\\(_4\\) single crystals are presented. The structure, Raman spectra, low-temperature transport, Hall effect, magnetization, and magnetic susceptibility are studied. It was established that the crystals are ferromagnetic, with a Curie temperature ranging from 22 to 45\\,K for different samples. Hall and magnetization measurements demonstrated that the system is a soft ferromagnet, which is of interest for practical applications.

Magnetic topological insulators (TIs) herald a wealth of applications in spin-based technologies, relying on the novel quantum phenomena provided by their topological properties. Particularly promising is the (MnBi\\(_2\\)Te\\(_4\\))(Bi\\(_2\\)Te\\(_3\\))\\(_n\\) layered family of established intrinsic magnetic TIs that can flexibly realize various magnetic orders and topological states. High tunability of this material platform is enabled by manganese-pnictogen intermixing, whose amounts and distribution patterns are controlled by synthetic conditions. Positive implication of the strong intermixing in MnSb\\(_2\\)Te\\(_4\\) is the interlayer exchange coupling switching from antiferromagnetic to ferromagnetic, and the increasing magnetic critical temperature. On the other side, intermixing also implies atomic disorder which may be detrimental for applications. Here, we employ nuclear magnetic resonance and muon spin spectroscopy, sensitive local probe techniques, to scrutinize the impact of the intermixing on the magnetic properties of (MnBi\\(_2\\)Te\\(_4\\))(Bi\\(_2\\)Te\\(_3\\))\\(_n\\) and MnSb\\(_2\\)Te\\(_4\\). Our measurements not only confirm the opposite alignment between the Mn magnetic moments on native sites and antisites in the ground state of MnSb\\(_2\\)Te\\(_4\\), but for the first time directly show the same alignment in (MnBi\\(_2\\)Te\\(_4\\))(Bi\\(_2\\)Te\\(_3\\))\\(_n\\) with n = 0, 1 and 2. Moreover, for all compounds, we find the static magnetic moment of the Mn antisite sublattice to disappear well below the intrinsic magnetic transition temperature, leaving a homogeneous magnetic structure undisturbed by the intermixing. Our findings provide a microscopic understanding of the crucial role played by Mn-Bi intermixing in (MnBi\\(_2\\)Te\\(_4\\))(Bi\\(_2\\)Te\\(_3\\))\\(_n\\) and offer pathways to optimizing the magnetic gap in its surface states.