Magnetically doped topological insulators enable the quantum anomalous Hall effect (QAHE) which provides quantized edge states for lossless charge transport applications [1][2][3][4][5][6][7][8][9]. The edge states are hosted by a magnetic energy gap at the Dirac point[2] but all attempts to observe it directly have been unsuccessful. The size of this gap is considered the clue to overcoming the present limitations of the QAHE, which so far occurs only at temperatures one to two orders of magnitude below its principle limit set by the ferromagnetic Curie temperature T C [8,9]. Here, we use low temperature photoelectron spectroscopy to unambiguously reveal the magnetic gap of Mn-doped Bi 2 Te 3 films which is present only below T C . Surprisingly, the gap turns out to be ∼ 90 meV wide, which not only exceeds k B T at room temperature but is also 5 times larger than predicted by density functional theory [10]. By an exhaustive multiscale structure characterization we show that this enhancement is due to a remarkable structure modification induced by Mn doping. Instead of a disordered impurity system, it forms an alternating sequence of septuple and quintuple layer blocks, where Mn is predominantly incorporated in the center of the septuple layers. This self-organized heterostructure substantially enhances the wave-function overlap and the size of the magnetic gap at the Dirac point, as recently predicted [11]. Mn-doped Bi 2 Se 3 forms a similar heterostructure, however, only a large, albeit nonmagnetic gap is formed. We explain both differences based on the higher spin-orbit interaction in Bi 2 Te 3 with the most important consequence of a magnetic anisotropy perpendicular to the films, whereas for Bi 2 Se 3 the spin-orbit interaction it is too weak to overcome the dipole-dipole interaction. Our findings provide crucial insights for pushing the lossless transport properties of topological insulators towards room-temperature applications.We thank B. Henne, F. Wilhelm, and A. Rogalev for support of the XANES and EX-AFS measurements at ID 12 and BM23 beam lines of the ESRF, V. Holý for advices on the structure model, W. Grafeneder for the TEM sample preparation and G. Bihlmayer and A. Ernst for helpful discussions. S.A.K and J.M. are grateful for support from CEDAMNF (CZ.02.1.01/0.0/0.0/15 003/0000358) of Czech ministry MSMT.

Ferromagnetic topological insulators exhibit the quantum anomalous Hall effect, which is potentially useful for high‐precision metrology, edge channel spintronics, and topological qubits.  The stable 2+ state of Mn enables intrinsic magnetic topological insulators. MnBi2Te4 is, however, antiferromagnetic with 25 K Néel temperature and is strongly n‐doped. In this work, p‐type MnSb2Te4, previously considered topologically trivial, is shown to be a ferromagnetic topological insulator for a few percent Mn excess. i) Ferromagnetic hysteresis with record Curie temperature of 45–50 K, ii) out‐of‐plane magnetic anisotropy, iii) a 2D Dirac cone with the Dirac point close to the Fermi level, iv) out‐of‐plane spin polarization as revealed by photoelectron spectroscopy, and v) a magnetically induced bandgap closing at the Curie temperature, demonstrated by scanning tunneling spectroscopy (STS), are shown. Moreover, a critical exponent of the magnetization β ≈ 1 is found, indicating the vicinity of a quantum critical point. Ab initio calculations reveal that Mn–Sb site exchange provides the ferromagnetic interlayer coupling and the slight excess of Mn nearly doubles the Curie temperature. Remaining deviations from the ferromagnetic order open the inverted bulk bandgap and render MnSb2Te4 a robust topological insulator and new benchmark for magnetic topological insulators.

Using spin-and angle-resolved spectroscopy and relativistic many-body calculations, we investigate the evolution of the electronic structure of (Bi1−xInx)2Se3 bulk single crystals around the critical point of the trivial to topological insulator quantum-phase transition. By increasing x, we observe how a surface gap opens at the Dirac point of the initially gapless topological surface state of Bi2Se3, leading to the existence of massive fermions. The surface gap monotonically increases for a wide range of x values across the topological and trivial sides of the quantum-phase transition. By means of photon-energy dependent measurements, we demonstrate that the gapped surface state survives the inversion of the bulk bands which occurs at a critical point near x = 0.055. The surface state exhibits a non-zero in-plane spin polarization which decays exponentially with increasing x, and that persists on both the topological and trivial insulator phases. Its out-of-plane spin polarization remains zero demonstrating the absence of a hedgehog spin texture expected from broken time-reversal symmetry. Our calculations reveal qualitative agreement with the experimental results all across the quantum-phase transition upon the systematic variation of the spin-orbit coupling strength. A non-time reversal symmetry breaking mechanism of bulk-mediated scattering processes that increase with decreasing spin-orbit coupling strength is proposed as explanation.