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Institute: search.filters.UBSInstitut.Max-Planck-Institut für FestkörperforschungInstitute: search.filters.UBSInstitut.Institut für Funktionelle Materie und QuantentechnologieAuthor: search.filters.author.Takagi, Hidenori

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Now showing 1 - 4 of 4
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    Photo-excited dynamics in the excitonic insulator Ta2NiSe5
    (2018) Werdehausen, Daniel; Takayama, Tomohiro; Albrecht, Gelon; Lu, Yangfan; Takagi, Hidenori; Kaiser, Stefan
    The excitonic insulator is an intriguing correlated electron phase formed of condensed excitons. A promising candidate is the small band gap semiconductor Ta2NiSe5. Here we investigate the quasiparticle and coherent phonon dynamics in Ta2NiSe5 in a time resolved pump probe experiment. Using the models originally developed by Kabanov et al for superconductors (Kabanov et al 1999 Phys. Rev. B 59 1497), we show that the material’s intrinsic gap can be described as almost temperature independent for temperatures up to about 250 K to 275 K. This behavior supports the existence of the excitonic insulator state in Ta2NiSe5. The onset of an additional temperature dependent component to the gap above these temperatures suggests that the material is located in the BEC-BCS crossover regime. Furthermore, we show that this state is very stable against strong photoexcitation, which reveals that the free charge carriers are unable to effectively screen the attractive Coulomb interaction between electrons and holes, likely due to the quasi 1D structure of Ta2NiSe5.
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    Monoclinic SrIrO3 : a Dirac semimetal produced by non-symmorphic symmetry and spin-orbit coupling
    (2018) Takayama, Tomohiro; Yaresko, Alexander N.; Takagi, Hidenori
    SrIrO3 crystallizes in a monoclinic structure of distorted hexagonal perovskite at ambient pressure. The transport measurements show that the monoclinic SrIrO3 is a low-carrier density semimetal, as in the orthorhombic perovskite polymorph. The electronic structure calculation indicates a semimetallic band structure with Dirac bands at two high-symmetry points of Brillouin zone only when spin-orbit coupling is incorporated, suggesting that the semimetallic state is produced by the strong spin-orbit coupling. We argue that the Dirac bands are protected by the non-symmorphic symmetry of lattice.
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    Heteroepitaxial tuning of resonant forbidden reflections in a spinel
    (2024) Oka, Ryosuke; Kim, Minu; Wochner, Peter; Francoual, Sonia; Palstra, Thomas T. M.; Takagi, Hidenori; Huang, Dennis
    In resonant elastic X-ray scattering (REXS), low site symmetries in a crystal may be revealed through resonant Bragg reflections that are normally forbidden in conventional X-ray diffraction due to screw axes and/or glide planes. These resonant forbidden reflections have been observed in spinel compounds, but to better understand and utilize their connection to microscopic material parameters and possible charge and/or orbital ordering, a systematic study of their dependence on growth conditions and applied strain is desired. We performed REXS at the V K edge and examined the resonant forbidden (002) reflection in thin films of the spinel LiV2O4 grown on three substrates: MgAl2O4, SrTiO3, and MgO. The energy dependence of the (002) reflection shows a systematic evolution as epitaxial strain modifies the local anisotropy of the V site. More strikingly, the integrated intensity of the (002) reflection varies by more than an order of magnitude in films on different substrates. We speculate that the large variation in integrated intensity reflects the varying degree of antiphase domains that arise during the epitaxy.
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    Direct visualization of stacking-selective self-intercalation in epitaxial Nb1+xSe2 films
    (2024) Wang, Hongguang; Zhang, Jiawei; Shen, Chen; Yang, Chao; Küster, Kathrin; Deuschle, Julia; Starke, Ulrich; Zhang, Hongbin; Isobe, Masahiko; Huang, Dennis; van Aken, Peter A.; Takagi, Hidenori
    Two-dimensional (2D) van der Waals (vdW) materials offer rich tuning opportunities generated by different stacking configurations or by introducing intercalants into the vdW gaps. Current knowledge of the interplay between stacking polytypes and intercalation often relies on macroscopically averaged probes, which fail to pinpoint the exact atomic position and chemical state of the intercalants in real space. Here, by using atomic-resolution electron energy-loss spectroscopy in a scanning transmission electron microscope, we visualize a stacking-selective self-intercalation phenomenon in thin films of the transition-metal dichalcogenide (TMDC) Nb1+xSe2. We observe robust contrasts between 180°-stacked layers with large amounts of Nb intercalants inside their vdW gaps and 0°-stacked layers with little detectable intercalants inside their vdW gaps, coexisting on the atomic scale. First-principles calculations suggest that the films lie at the boundary of a phase transition from 0° to 180° stacking when the intercalant concentration x exceeds ~0.25, which we could attain in our films due to specific kinetic pathways. Our results offer not only renewed mechanistic insights into stacking and intercalation, but also open up prospects for engineering the functionality of TMDCs via stacking-selective self-intercalation.