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End date: 2020 × Start date: 2020 × DDC: 530 × Subject: Electronic structure ×

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Now showing 1 - 4 of 4
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    Electronic correlations and magnetic interactions in infinite-layer NdNiO2
    (Woodbury, NY : Inst., 2020) Katukuri, Vamshi M.; Bogdanov, Nikolay A.; Weser, Oskar; Van den Brink, Jeroen; Alavi, Ali
    The large antiferromagnetic exchange coupling in the parent high-Tc cuprate superconductors is believed to play a crucial role in pairing the superconducting carriers. The recent observation of superconductivity in hole-doped infinite-layer (IL-) NdNiO2 brings to the fore the relevance of magnetic coupling in high-Tc superconductors, particularly because no magnetic ordering is observed in the undoped IL-NdNiO2, unlike in parent copper oxides. Here, we investigate the electronic structure and the nature of magnetic exchange in IL-NdNiO2 using state-of-the-art many-body quantum chemistry methods. From a systematic comparison of the electronic and magnetic properties with isostructural cuprate IL-CaCuO2, we find that the on-site dynamical correlations are significantly stronger in IL-NdNiO2 compared to the cuprate analog. These dynamical correlations play a critical role in the magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest-neighbor isotropic J of 77 meV between the Ni1+ ions within the ab plane. While we find many similarities in the electronic structure between the nickelate and the cuprate, the role of electronic correlations is profoundly different in the two. We further discuss the implications of our findings in understanding the origin of superconductivity in nickelates. © 2020 authors.
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    Autocorrected off-axis holography of two-dimensional materials
    (College Park, ML : American Physical Society, 2020) Kern, Felix; Linck, Martin; Wolf, Daniel; Alem, Nasim; Arora, Himani; Gemming, Sibylle; Erbe, Artur; Zettl, Alex; Büchner, Bernd; Lubk, Axel
    The reduced dimensionality in two-dimensional materials leads to a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high-resolution microscopy techniques are indispensable. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the two-dimensional material. We demonstrate these features with the example of h-BN, for which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.
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    Charge-transfer energy in iridates: A hard x-ray photoelectron spectroscopy study
    (College Park, ML : American Physical Society, 2020) Takegami, D.; Kasinathan, D.; Wolff, K.K.; Altendorf, S.G.; Chang, C.F.; Hoefer, K.; Melendez-Sans, A.; Utsumi, Y.; Meneghin, F.; Ha, T.D.; Yen, C.H.; Chen, K.; Kuo, C.Y.; Liao, Y.F.; Tsuei, K.D.; Morrow, R.; Wurmehl, S.; Büchner, B.; Prasad, B.E.; Jansen, M.; Komarek, A.C.; Hansmann, P.; Tjeng, L.H.
    We have investigated the electronic structure of iridates in the double perovskite crystal structure containing either Ir4+ or Ir5+ using hard x-ray photoelectron spectroscopy. The experimental valence band spectra can be well reproduced using tight-binding calculations including only the Ir 5d, O 2p, and O 2s orbitals with parameters based on the downfolding of the density-functional band structure results. We found that, regardless of the A and B cations, the A2BIrO6 iridates have essentially zero O 2p to Ir 5d charge-transfer energies. Hence double perovskite iridates turn out to be extremely covalent systems with the consequence being that the magnetic exchange interactions become very long ranged, thereby hampering the materialization of the long-sought Kitaev physics. Nevertheless, it still would be possible to realize a spin-liquid system using the iridates with a proper tuning of the various competing exchange interactions.
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    V4 tetrahedral units in AV4X8 lacunar spinels: Near degeneracy, charge fluctuations, and configurational mixing within a valence space of up to 21 d orbitals
    (2020) Hozoi, L.; Eldeeb, M.S.; Rößler, U.K.
    All properties of a given molecule or solid are determined by the way valence electrons are distributed over single-particle energy levels. For multiple, closely spaced single-particle levels, different occupation patterns may provide many-electron quantum states that are close in energy, interact, and admix. We address such near-degeneracy electron correlation effects for V4 vanadium tetrahedral units as encountered in the lacunar spinel GaV4S8, explicitly taking into account up to 21 vanadium valence orbitals, and find effective orbital occupation numbers much different as compared to the picture previously laid out on the basis of mean-field calculations. In light of these results, a modified theoretical frame seems necessary to explain the peculiar magnetic properties of lacunar spinels and of related compounds.