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 have investigated the molecular orientation and electronic structures of nonplanar vanadyl phthalocyanine (VOPc) on the Si( 111)-(7 × 7) and Ag(111) surfaces by X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and X-ray magnetic circular dichroism. The VOPc molecule adsorbs on Ag(111) in a parallel orientation to the surface with an oxygen-up configuration. A strong interaction between the N and C atoms of VOPc and surface Ag atoms is observed at the interface, although no marked change in the electronic state is observed for the V atom, similarly to the case for VOPc in a multilayer. On the other hand, the chemical interaction of the O atom of VOPc with the surface Si atoms favors the oxygen-down configuration. This chemical interaction causes the cleavage of the VO π bond and facilitates electron charge transfer to the V−N−C molecular orbitals. Such intermediation of the oxygen atom between the V atom and Si surface suppresses the direct interaction between them, and the spin magnetic moment of V remains the same as that of bulk VOPc molecules.

Vanadium phthalocyanine (VPc) monolayers and multilayers were synthesized on Ag(111), and the electronic and magnetic states of an unachieved VPc with a divalent state of V were investigated. The VPc monolayer was fabricated by directly depositing the V atoms on a metal-free phthalocyanine (H 2 Pc) monolayer under ultrahigh-vacuum conditions. The VPc multilayer was synthesized by repeated VPc monolayer deposition and subsequent sample annealing at approximately 450 K. The N 1s X-ray photoelectron spectra (XPS) of these samples showed a remarkable reduction in the peak assigned to H-bonded N atoms, concomitant with the appearance of a new peak attributed to V-bonded N atoms close to the peak of iminic N. Additionally, the oxidation state of V estimated from the V 2p XPS peak position corresponded to 1.6 and 2.4 in the monolayer and multilayer samples, respectively. These results clearly imply that VPc monolayers and multilayers were successfully obtained. The main ground-state electronic configuration of the V center was found to be 2 E g by angle-dependent V L-edge X-ray absorption spectroscopy. Furthermore, X-ray magnetic circular dichroism (XMCD) measurements suggest that this 2 E g state was mixed with the 2 A 1g state by spin−orbit coupling in the ground state. Data revealed that VPc shows a paramagnetic state on the Ag surface and in an H 2 Pc film but an antiferromagnetic state in the multilayer. Partial electron charge transfer was also observed from the Ag surface to the V center at the VPc/Ag(111) interface, leading to a significant decrease in XMCD signals in the monolayer.

Electronic states of the clean Ge͑001͒ surface are studied by angle-resolved photoemission spectroscopy and standing wave observation with scanning tunneling microscopy. The bottom of the surface conduction band is 0.3 eV above the top of the valence band, and thermally filled with increasing temperature above 470 K. Its dispersion in the direction perpendicular to the dimer axis is much larger than that in the Ge dimer-axis direction. The valence band top at ⌫ is a bulk state, and surface resonances exist near this point. These features are consistent with the band calculation based on the density functional theory for the c͑4 ϫ 2͒ surface.