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End date: 2023 × Start date: 2023 × Has files: Yes × Subject: Electronic structure × WGL Institute: IFWD ×

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Now showing 1 - 3 of 3
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    Two-dimensional ferromagnetic extension of a topological insulator
    (College Park, MD : APS, 2023) Kagerer, P.; Fornari, C. I.; Buchberger, S.; Tschirner, T.; Veyrat, L.; Kamp, M.; Tcakaev, A. V.; Zabolotnyy, V.; Morelhão, S. L.; Geldiyev, B.; Müller, S.; Fedorov, A.; Rienks, E.; Gargiani, P.; Valvidares, M.; Folkers, L. C.; Isaeva, A.; Büchner, B.; Hinkov, V.; Claessen, R.; Bentmann, H.; Reinert, F.
    Inducing a magnetic gap at the Dirac point of the topological surface state (TSS) in a three-dimensional (3D) topological insulator (TI) is a route to dissipationless charge and spin currents. Ideally, magnetic order is present only at the surface, as through proximity of a ferromagnetic (FM) layer. However, experimental evidence of such a proximity-induced Dirac mass gap is missing, likely due to an insufficient overlap of TSS and the FM subsystem. Here, we take a different approach, namely ferromagnetic extension (FME), using a thin film of the 3D TI Bi2Te3, interfaced with a monolayer of the lattice-matched van der Waals ferromagnet MnBi2Te4. Robust 2D ferromagnetism with out-of-plane anisotropy and a critical temperature of Tc≈15 K is demonstrated by x-ray magnetic dichroism and electrical transport measurements. Using angle-resolved photoelectron spectroscopy, we observe the opening of a sizable magnetic gap in the 2D FM phase, while the surface remains gapless in the paramagnetic phase above Tc. Ferromagnetic extension paves the way to explore the interplay of strictly 2D magnetism and topological surface states, providing perspectives for realizing robust quantum anomalous Hall and chiral Majorana states.
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    Influence of multi-walled carbon nanotubes in polytetrafluoroethylene on the parameters of electronic structure and absorption of ultra-high-frequency radiation
    (Berlin ; Heidelberg : Springer, 2022) Galstian, I.Y.; Tsapko, Y.A.; Makarenko, O.V.; Yampolskiy, A.L.; Tarusin, Y.V.; Len, E.G.
    Using the methods of angular correlation of annihilation radiation (ACAR), attenuation of electromagnetic radiation in 1.5–2.2 GHz frequency range, and optical ellipsometry, it was shown that in composites of polytetrafluoroethylene (PTFE) + multi-walled carbon nanotubes (MWCNTs), a 2% decrease in the probability of annihilation of positrons in free volumes in PTFE leads to changes in other parameters of electronic structure of composites by 8–29%. Polytetrafluoroethylene is transparent to electromagnetic radiation, but after the addition of 10 wt.% or more of MWCNTs, the composites demonstrate 200–410-fold decrease in the electromagnetic radiation intensity when the radiation passes through a specimen with a thickness of ≈2 mm. It was found that the average radius of the free volumes and the probability of annihilation of positrons are determined by the defect and electronic structures of the polymer matrix only. The Fermi angle and the probability of positrons annihilation with free electrons are determined by the analogous structures of MWCNTs only. Since the electronic characteristics of the atoms and defects in the polymer matrix (at least outside the interphase) do not change, the changes in the other ACAR parameters are mainly due to changes in the imperfect MWCNTs’ atomic and electronic structures. The average radius of free volumes reaches its maximum value in the composite with 10 wt.% MWCNTs. It was found that in a specimen with 10 wt.% MWCNTs, the highest density of free electrons is observed due to charge transfer from free volumes to MWCNTs, and the highest electron density is observed on defects. A disorder of MWCNTs and their branched conductive network can form the ‘tails’ of electronic density of states in a band gap. Thus, composite with 10 wt.% MWCNTs has the highest absorption coefficient for electromagnetic radiation.
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    Lifshitz transition in titanium carbide driven by a graphene overlayer
    (College Park, MD : APS, 2023) Krivenkov, M.; Marchenko, D.; Golias, E.; Sajedi, M.; Frolov, A.S.; Sánchez-Barriga, J.; Fedorov, A.; Yashina, L.V.; Rader, O.; Varykhalov, A.
    Two-dimensional (2D) Dirac materials are electronically and structurally very sensitive to proximity effects. We demonstrate, however, the opposite effect: that the deposition of a monolayer 2D material could exercise a substantial influence on the substrate electronic structure. Here we investigate TiC(111) and show that a graphene overlayer produces a proximity effect, changing the Fermi surface topology of the TiC from six electron pockets to one hole pocket on the depth of several atomic layers inside the substrate. In addition, the graphene electronic structure undergoes an extreme modification as well. While the Dirac cone remains gapless, it experiences an energy shift of 1.0 eV beyond what was recently achieved for the Lifshitz transition of overdoped graphene. Due to this shift, the antibonding π∗ band at the M¯ point becomes occupied and observable by photoemission.