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Charge-transfer energy in iridates: A hard x-ray photoelectron spectroscopy study

D. Takegami, D. Kasinathan, K. K. Wolff, S. G. Altendorf, C. F. Chang, K. Hoefer, A. Melendez-Sans, Y. Utsumi, F. Meneghin, T. D. Ha, C. H. Yen, K. Chen, C. Y. Kuo, Y. F. Liao, K. D. Tsuei, R. Morrow, S. Wurmehl, B. Büchner, B. E. Prasad, M. Jansen, A. C. Komarek, P. Hansmann, and L. H. Tjeng
Phys. Rev. B 102, 045119 – Published 13 July 2020
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  • INTRODUCTION
  • METHODS
  • RESULTS
  • ANALYSIS AND DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    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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    • Received 10 February 2020
    • Revised 22 May 2020
    • Accepted 30 June 2020

    DOI:https://doi.org/10.1103/PhysRevB.102.045119

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by the Max Planck Society.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Electronic structure
    1. Physical Systems
    Iridates
    1. Techniques
    Hard x-ray photoelectron spectroscopyTight-binding model
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    D. Takegami1, D. Kasinathan1, K. K. Wolff1, S. G. Altendorf1, C. F. Chang1, K. Hoefer1, A. Melendez-Sans1, Y. Utsumi1,*, F. Meneghin1,2, T. D. Ha1,3, C. H. Yen1,4, K. Chen5, C. Y. Kuo1,6, Y. F. Liao6, K. D. Tsuei6, R. Morrow7, S. Wurmehl7, B. Büchner7,8, B. E. Prasad1, M. Jansen9, A. C. Komarek1, P. Hansmann1,10, and L. H. Tjeng1

    • 1Max Planck Institute for Chemical Physics of Solids, Nöthnitzer Straße 40, 01187 Dresden, Germany
    • 2Politecnico di Milano, Piazza Leonardo da Vinci, 32, 20133 Milano, Italy
    • 3Department of Electrophysics, National Chiao Tung University, 1001 Ta Hsueh Road, 30010 Hsinchu, Taiwan
    • 4Department of Physics, National Tsing Hua University, 101 Kuang Fu Road, 30013 Hsinchu, Taiwan
    • 5Institute of Physics II, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
    • 6National Synchrotron Radiation Research Center (NSRRC), 101 Hsin-Ann Road, 30076 Hsinchu, Taiwan
    • 7Leibniz Institute for Solid State and Materials Research IFW Dresden, Helmholtzstraße 20, 01069 Dresden, Germany
    • 8Institut für Festkörperphysik, Technische Universität Dresden, 01062 Dresden, Germany
    • 9Max Planck Institute for Solid State Research, Heisenbergstraße 1, 70569 Stuttgart, Germany
    • 10Department of Physics, University of Erlangen–Nuremberg, 91058 Erlangen, Germany

    • *Present Address: Institute of Physics, Bijenička 46, 10000 Zagreb, Croatia.

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    Issue

    Vol. 102, Iss. 4 — 15 July 2020

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    Images

    • Figure 1
      Figure 1

      Crystal structure of the double perovskite AA′BIrO6. Red spheres correspond to the position of oxygen atoms and the dark green spheres to the A and A′ sites. Cyan and dark blue octahedra correspond to the IrO6 and BO6 octahedra, respectively.

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    • Figure 2
      Figure 2

      Valence band HAXPES spectra (hν=6.5 keV) of the double perovskite compounds containing formally Ir4+ (left panel) and Ir5+ (right panel) ions.

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    • Figure 3
      Figure 3

      Calculated density of states (DOS) and the Ir 5d, O 2p, and transition metal 3d partial density of states (PDOS) of the double perovskite compounds with Ir4+ (left panel) and Ir5+ (right panel).

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    • Figure 4
      Figure 4

      Calculated valence band spectra of the double perovskite compounds with Ir4+ (left panel) and Ir5+ (right panel). The spectra were obtained by multiplying the calculated PDOS of the occupied states by their respective photoionization cross sections at 6.5 keV photon energy, followed by a broadening to account for experimental conditions, and then their summation. Finally, a rigid shift is applied to simulate the gap.

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    • Figure 5
      Figure 5

      (a), (b) Comparison of the DOS and PDOS obtained from the ab initio calculations and the tight-binding calculations for La2MgIrO6 (left panel), chosen as a representative compound for the double perovskites with Ir4+, and Ba2YIrO6 (right panel), as representant for the double perovskites with Ir5+. O 2pπ (σ) corresponds to the contribution of the O 2p orbitals perpendicular (parallel) to the Ir-O direction. (c), (d) Results of the tight-binding calculations using the parameters obtained from the downfolding of the ab initio calculations. (e), (f) Results after fine tuning of the parameters to match the experimental spectra [43, 44]. (g), (h) Results of the tight-binding calculations performed with the fine tuned parameters but with pdπ=pdσ=0, i.e., with the hybridization between Ir 5d and O 2p switched off.

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    • Figure 6
      Figure 6

      Comparison of the experimental HAXPES spectra (top) with the simulated Ir 5d valence band spectrum based on the optimized tight-binding calculations (bottom) for La2MgIrO6 (left panel) and Ba2YIrO6 (right panel).

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    • Figure 7
      Figure 7

      Valence band HAXPES spectra (hν=6.5keV) of the double perovskite compounds La2MgIrO6, Bi2NaIrO6, and Sr2YIrO6 taken with photoelectrons collected in the direction parallel (black, “Horizontal”) and perpendicular (red, “Vertical”) to the electrical field vector of the photon beam. The calculated Bi 6s partial DOS is indicated for the Bi2NaIrO6 compound.

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