Inducing magnetism into topological insulators is intriguing for utilizing exotic phenomena such as the quantum anomalous Hall effect (QAHE) for technological applications. While most studies have focused on doping magnetic impurities to open a gap at the surface-state Dirac point, many undesirable effects have been reported to appear in some cases that makes it difficult to determine whether the gap opening is due to the time-reversal symmetry breaking or not. Furthermore, the realization of

more »

... e QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi 2 Se 4 /Bi 2 Se 3 heterostructure which was fabricated by self-assembling a MnBi 2 Se 4 layer on top of the Bi 2 Se 3 surface as a result of the co-deposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi 2 Se 4 /Bi 2 Se 3 heterostructure shows ferromagnetism up to room temperature and a clear Dirac-cone gap opening of ∼100 meV without any other significant changes in the rest of the band structure. It can be considered as a result of the direct interaction of the surface Dirac cone and the magnetic layer rather than a magnetic proximity effect. This spontaneously formed self-assembled heterostructure with a massive Dirac spectrum, characterized by a nontrivial Chern number C = −1, has a potential to realize the QAHE at significantly higher temperatures than reported up to now and can serve as a platform for developing future âĂIJtopotronicsâĂİ devices. Keywords Topological insulators, Magnetism, Massive Dirac cone, Quantum anomalous Hall effect Classification of materials and phases based on the "topological properties" of the system has become one of the most extensively studied research fields in physics and was the topic for the Nobel prize in physics in 2016. 1 Topological insulators (TIs) are insulating materials that have metallic surface states, whose electron spins are locked to their momentum. 2,3 In the simplest case, the surface states are helical Dirac fermions and the Dirac point is robust due to the protection by time-reversal symmetry. When the time-reversal symmetry is broken in TIs, novel quantum phenomena have been predicted to occur including the formation of magnetic monopoles, 4 the quantum anomalous Hall effect (QAHE), 5 and the topological magnetoelectric effect. 6 In terms of the electronic structure, a magnetic TI is expected to host a massive Dirac cone with a band gap. Many researches have been performed to induce magnetism in TIs by magnetic impurity doping when growing thin films. 7,8 Although it seemed that this was the most efficient way with the successful observation of the QAHE in Cr or V-doped (Bi,Sb) 2 Te 3 thin films, 8-10 the precise quantization of the Hall resistance at zero field is still only limited to low temperature (10-100 mK). The temperature for the realization of the QAHE (T QAHE ) depends on the Curie temperature as well as the size of the Dirac cone gap of the system. Up to now, the main obstacle in enhancing T QAHE is an inhomogeneous distribution of magnetic atoms over the TI film that results in strong fluctuations of the magnetic energy gap. 11,12 Modulation doping was shown to increase T QAHE and the QAHE was observed at 2 K for Cr-doped (Bi, Sb) 2 Te 3 thin films when the magnetic-doped layer was placed 1 nm below the surface. 13 A recent theoretical work suggests that this can be enhanced by V and I co-doping of Sb 2 Te 3 . 14