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 the QAHE has been limited to low temperatures. Here we have succeeded in generating a massive Dirac cone in a MnBi2Se4/Bi2Se3 heterostructure, which was fabricated by self-assembling a MnBi2Se4 layer on top of the Bi2Se3 surface as a result of the codeposition of Mn and Se. Our experimental results, supported by relativistic ab initio calculations, demonstrate that the fabricated MnBi2Se4/Bi2Se3 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 “topotronics” devices.

We report the temperature-dependent three-dimensional angle-resolved photoemission spectra of the Kondo semiconductor SmB6. We found a difference in the temperature dependence of the peaks at the X and Γ points, due to hybridization between the Sm 5d conduction band and the nearly localized Sm 4f state. The peak intensity at the X point has the same temperature dependence as the valence transition below 120 K, while that at the Γ point is consistent with the magnetic excitation at Q = (0.5, 0.5, 0.5) below 30 K. This suggests that the hybridization with the valence transition mainly occurs near the X point, and the initial state of the magnetic excitation is located near the Γ point.PACS numbers: 71.27.+a, 79.60.-i Materials with strong electron correlation have exotic physical properties that cannot be predicted from firstprinciple band calculations. One example may be seen in a semiconductor with a very small energy gap, which appears in rare-earth compounds such as the Kondo semiconductor/insulator (KI)1 . At high temperatures, KI behaves like a dense Kondo metal, while an energy gap with activation energy of several 10 meV appears at low temperature. The energy gap is believed to originate from hybridization between the nearly localized 4f state near the Fermi level (E F ) and the conduction band (c-f hybridization).Numerous studies have investigated the energy gap of KI, using optical conductivity 2,3 , point contact spectroscopy 4 , angle-integrated photoemission spectroscopy 5,6 , and other methods. However, the momentum dependence of the c-f hybridization gap, as well as the relation of the electronic structure to other physical properties, has yet to be studied. Because the cf hybridization occurs at a specific momentum vector, the most direct method of observing the band dispersion of the c-f hybridization gap is three-dimensional angle-resolved photoemission spectroscopy (3D-ARPES) using a tunable photon source from synchrotron radiation. Thus, we applied the 3D-ARPES method to observe the c-f hybridization gap creation of a typical KI, SmB 6 . SmB 6 is a valence-fluctuation material in between Sm 2+ (4f 6 ) and Sm 3+ (4f 5 ) ions 7 . The electrical resistivity (ρ) decreases on cooling, like a metal, above the temperature of 100 K, but then reveals a semiconductorlike character with activation energy of 15 meV 8 . There are two characteristic temperatures on SmB 6 ; one is valence transition below 120 K, and the other is magnetic excitation below 30 K. The mean valence changes from 2.57 at 120 K to 2.50 at 40 K on cooling 9 . Coincidentally, the lattice constant, which normally shrinks above 120 K on cooling, anomalously expands from 120 K to a few tens K indicating the valence change from Sm 3+ to Sm 2+10 . On the other hand, the magnetic excitation at the scattering vector of Q = (0.5, 0.5, 0.5), observed by inelastic neutron scattering (INS), rapidly increases below 30 K 11 . Then the mean valence slightly recovers from 2.50 to 2.52 below 30 K, and the lattice constant shrinks again. The reason ...