ИСТИНА

Three-dimensional topological insulators (TIs) are characterized by spin-polarized Dirac-cone surface states that are protected from backscattering by time-reversal symmetry. Within the past few years, TIs have attracted a lot of interest due to their unique electronic structure with spin-polarized topological surface states (TSSs), which may pave the way for these materials to have a great potential in multiple applications. Magnetic doping is expected to open a band gap at the Dirac point of topological insulators by breaking time-reversal symmetry and to enable novel topological phases. We study the effect of Fe impurities deposited on the surface of the topological insulators Bi2Se3 and Bi2Te3 by means of core-level and ARPES. We found that the topological surface state is tolerant against magnetic adsorbates. This indicates that topological insulators such as Bi2Se3 can be interfaced with a ferromagnet without losing the topological surface state and its unique quasirelativistic dispersion and Dirac point. In order to exhaust possible preparation conditions, room- and low temperature deposition have been compared. They lead to very different behavior, i.e., an opposite doping trend and different chemical environments, but they agree in the dispersion of the topological surface state and its gapless nature. The robustness is the precondition for the exploration and the successful functionalization of interfaces between topological insulators and ferromagnets. This will involve growing a perpendicularly magnetized ferromagnetic film on top of a topological insulator and monitoring the effect of the exchange coupling on the topological surface state underneath [1]. We characterize the atomically precise interface between the 3d transition metal Fe and the TI Bi2Te3 at different stages of its formation. Using photoelectron diffraction and holography, we show that after deposition of up to 3 monolayers Fe on Bi2Te3 at room temperature, the Fe atoms are ordered at the interface despite the surface disorder revealed by our scanning-tunneling microscopy images. We find that Fe occupies two different sites: a hollow adatom deeply relaxed into the Bi2Te3 quintuple layers and an interstitial atom between the third (Te) and fourth (Bi) atomic layers. We further show that upon deposition of Fe up to a thickness of 20 nm, the Fe atoms penetrate deeper into the bulk forming a 2-5 nm FeTe interface layer. In addition, excessive Bi is pushed down into the bulk of Bi2Te3 leading to the formation of septuple layers of Bi3Te4 within a distance of ~25 nm from the interface. Controlling the magnetic properties of the complex interface structures revealed by our work might turn out to be of critical importance when optimizing the efficiency of spin injection in TI-based devices. Epitaxial (Bi1-xMnx)2Se3 is a prototypical magnetic topological insulator with a pronounced surface band gap. We show that this gap is neither due to ferromagnetic order in the bulk or at the surface nor to the local magnetic moment of the Mn, making the system unsuitable for realizing the novel phases. We further show that Mn doping does not affect the inverted bulk band gap and the system remains topologically nontrivial. We suggest that strong resonant scattering processes cause the gap at the Dirac point and support this by the observation of in-gap states using resonant photoemission. Our findings establish a mechanism for gap opening in topological surface states which challenges the currently known conditions for topological protection [2]. We have studied quantum phase transition in the (Bi1−xInx)2Se3 series of compounds as a function of In composition at room temperature. ARPES data reveal that backscattering channels open up well-before the bulk band gap closes. This is evidenced by the progressive opening of a surface gap at the Dirac node of the TSS with increasing In doping. Our calculations based on the one step-model of photoemission in prototypical TIs reveal that bulk-surface coupling strongly influences the orbital nature of the surface state wave function, and thus the spin polarization of the TSS due to interaction with surface resonances (SR) appearing at the bulk-conduction band edges. The same coupling mechanism is expected to be even more pronounced across the quantum-phase transition.