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Author: Janson, Oleg ×

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    Observation of orbital order in the van der Waals material 1T−TiSe2
    (College Park, MD : APS, 2022) Peng, Yingying; Guo, Xuefei; Xiao, Qian; Li, Qizhi; Strempfer, Jörg; Choi, Yongseong; Yan, Dong; Luo, Huixia; Huang, Yuqing; Jia, Shuang; Janson, Oleg; Abbamonte, Peter; van den Brink, Jeroen; van Wezel, Jasper
    Besides magnetic and charge order, regular arrangements of orbital occupation constitute a fundamental order parameter of condensed matter physics. Even though orbital order is difficult to identify directly in experiments, its presence was firmly established in a number of strongly correlated, three-dimensional Mott insulators. Here, reporting resonant x-ray-scattering experiments on the layered van der Waals compound 1T-TiSe2, we establish that the known charge density wave in this weakly correlated, quasi-two-dimensional material corresponds to an orbital ordered phase. Our experimental scattering results are consistent with first-principles calculations that bring to the fore a generic mechanism of close interplay between charge redistribution, lattice displacements, and orbital order. It demonstrates the essential role that orbital degrees of freedom play in TiSe2, and their importance throughout the family of correlated van der Waals materials.
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    Phase diagram of nickelate superconductors calculated by dynamical vertex approximation
    (Lausanne : Frontiers Media, 2022) Held, Karsten; Si, Liang; Worm, Paul; Janson, Oleg; Arita, Ryotaro; Zhong, Zhicheng; Tomczak, Jan M.; Kitatani, Motoharu
    We review the electronic structure of nickelate superconductors with and without effects of electronic correlations. As a minimal model, we identify the one-band Hubbard model for the Ni 3dx2−y2 orbital plus a pocket around the A-momentum. The latter, however, merely acts as a decoupled electron reservoir. This reservoir makes a careful translation from nominal Sr-doping to the doping of the one-band Hubbard model mandatory. Our dynamical mean-field theory calculations, in part already supported by the experiment, indicate that the Γ pocket, Nd 4f orbitals, oxygen 2p, and the other Ni 3d orbitals are not relevant in the superconducting doping regime. The physics is completely different if topotactic hydrogen is present or the oxygen reduction is incomplete. Then, a two-band physics hosted by the Ni 3dx2−y2 and 3d3z2−r2orbitals emerges. Based on our minimal modeling, we calculated the superconducting Tc vs. Sr-doping x phase diagram prior to the experiment using the dynamical vertex approximation. For such a notoriously difficult to determine quantity as Tc, the agreement with the experiment is astonishingly good. The prediction that Tc is enhanced with pressure or compressive strain has been confirmed experimentally as well. This supports that the one-band Hubbard model plus an electron reservoir is the appropriate minimal model.
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    Different types of spin currents in the comprehensive materials database of nonmagnetic spin Hall effect
    (London : Nature Publ. Group, 2021) Zhang, Yang; Xu, Qiunan; Koepernik, Klaus; Rezaev, Roman; Janson, Oleg; Železný, Jakub; Jungwirth, Tomáš; Felser, Claudia; van den Brink, Jeroen; Sun, Yan
    Spin Hall effect (SHE) has its special position in spintronics. To gain new insight into SHE and to identify materials with substantial spin Hall conductivity (SHC), we performed high-precision high-throughput ab initio calculations of the intrinsic SHC for over 20,000 nonmagnetic crystals. The calculations revealed a strong relationship between the magnitude of the SHC and the crystalline symmetry, where a large SHC is typically associated with mirror symmetry-protected nodal line band structures. This database includes 11 materials with an SHC comparable to or even larger than that of Pt. Materials with different types of spin currents were additionally identified. Furthermore, we found that different types of spin current can be obtained by rotating applied electrical fields. This improves our understanding and is expected to facilitate the design of new types of spin-orbitronic devices.
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    Nickelate superconductors - a renaissance of the one-band Hubbard model
    (Berlin : Springer Nature, 2020) Kitatani, Motoharu; Si, Liang; Janson, Oleg; Arita, Ryotaro; Zhong, Zhicheng; Held, Karsten
    The recently discovered nickelate superconductors appear, at first glance, to be even more complicated multi-orbital systems than cuprates. To identify the simplest model describing the nickelates, we analyse the multi-orbital system and find that it is instead the nickelates which can be described by a one-band Hubbard model, albeit with an additional electron reservoir and only around the superconducting regime. Our calculations of the critical temperature TC are in good agreement with experiment, and show that optimal doping is slightly below 20% Sr-doping. Even more promising than 3d nickelates are 4d palladates.