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RIXS study of honeycomb iridates

IXS 2013, Aug. 15, 2013, Stanford. RIXS study of honeycomb iridates. Young-June Kim University of Toronto. Acknowledgements. Toronto. Samples. Yogesh Singh (IISER Mohali ) Philip Gegenwart (Gottingen) S.-W. Cheong (Rutgers) G. Cao (U. Kentucky) K. H. Kim (Seoul National U).

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RIXS study of honeycomb iridates

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  1. IXS 2013, Aug. 15, 2013, Stanford RIXS study of honeycomb iridates Young-June Kim University of Toronto

  2. Acknowledgements Toronto Samples • Yogesh Singh (IISER Mohali) • Philip Gegenwart (Gottingen) • S.-W. Cheong (Rutgers) • G. Cao (U. Kentucky) • K. H. Kim (Seoul National U) H. Gretarsson J. P. Clancy Theory Argonne+Brookhaven Jeroen van den Brink (Dresden) LiviuHozoi (Dresden) VamshiKatukuri (Dresden) Yong Baek Kim (Toronto) Hae Young Kee (Toronto) ArunParamekanti (Toronto) Heungsik Kim (Seoul) Jaejun Yu (Seoul) Jungho Kim Diego Casa Mary Upton Ayman Said Thomas Gog John Hill Xuerong Liu Emil Bozin

  3. Outline • RIXS and iridates - an overview • Why are we studying Iridates? • RIXS • Materials • Quantum compass model and honeycomb lattice iridates: Na2IrO3 • Kitaev interaction • Orbital excitations • Magnetic excitations + Phonons • Future directions

  4. Theoretical motivation Mott Insulator ? U/t Topological (Band) Insulator Metal l/t Adapted from Pesin & Balents, Nature Physics 6, 376 (2010)

  5. Why iridates? l U

  6. Physics of Ir4+ (5d5) in cubic CEF Ir4+ (5d5) eg CEF jeff = 1/2 t2g jeff = 3/2 SO (leff= “-1”) B.J. Kim et al, PRL (2008)

  7. Iridate materials A2Ir2O7 A2IrO3 Srn+1IrnO3n+1 Candidate for Topological Insulator, Weyl Semi-Metal, Metallic Spin Liquid: Yanagishima et al. JPSJ (2001) Yang et al, PRB (2010), Wan et al, PRB (2011), Witczak-Krempa et al, PRB (2012) Nakatsuji et al, PRL (2006) Candidate for Topological Insulator, Kitaev-Heisenberg Model: Shitade et al, PRL (2009), Chaloupka et al, PRL (2010), Choi et al, PRL (2012) Comin et al., PRL (2012) Gretarsson et al, PRL (2013) Jeff=1/2 spin orbital Mott insulator, possiblehigh-temperaturesuperconductivity BJ Kim et al, PRL (2008) BJ Kim et al, Science (2009) J. Kim et al, PRL (2012) Wang et al, PRL (2011)

  8. Why RIXS? • Neutron scattering is very difficult • Large neutron absorption cross-section of Ir • Large single crystal sample unavailable ($$) • Emergence of Resonant Inelastic X-ray Scattering (RIXS) as a viable scattering technique for elementary excitations (Ament et al. RMP 83, 705 (2011) • Magnons in La2CuO4 (Braicovich et al. PRL 2009) • Paramagnons in cuprates (Le Tacon et al. Nat. Phys. 2011) • Triplons in spin ladders Sr14Cu24O41 (Schlappa et al. PRL 2009) • Orbitonsin spin chain Sr2CuO3 (Schlappa et al. Nature 2012) • Looking for problems to study with hard x-ray L3 edge RIXS  Iridates

  9. Iridate RIXS: The beginning ~ 1 eV Energy resolution

  10. Progress in energy resolution Insulating cuprates: Cu K-edge 0.4 eV 1 eV 0.1 eV Kim et al. PRL 2002 Ellis et al. PRB 2008 Hill et al. PRL 1998

  11. MERIX spectrometer Si(844) Analyzer Diego Casa Ayman Said Yuri Shvydko

  12. MERIX: workhorse • Sr2IrO4Jungho Kim et al. PRL 108, 177003 (2012). • Sr3Ir2O7Jungho Kim et al. PRL 109, 157402 (2012). • Sr3IrCuO6Xuerong Liu et al., PRL 109, 157401 (2012). • Na2IrO3Gretarsson et al., PRL 110, 076402 (2013); PRB 87, 220407 (2013). • Na4Ir3O8Xuerong Liu et al. • Eu2Ir2O7L. Hozoi et al., submitted • Sr2(Ir,Rh)O4, J. P. Clancy, NEXT TALK

  13. Ei dependence

  14. Outline • RIXS and iridates - an overview • Why are we studying Iridates? • RIXS • Materials • Quantum compass model and honeycomb lattice iridates: Na2IrO3 • Kitaev interaction • Spin-orbital excitations • Magnetic excitations • Future directions H. Gretarsson et al. PRL 110, 076402 (2013) H. Gretarsson et al. PRB 87, 220407 (R) (2013)

  15. Kitaev’s compass model Kitaev, Ann. Phys. 2006 Nussinov and van den Brink arXiv:1303.5922 • Exactly solvable model • Spin liquid ground state (frustration) • Topological quantum computing Interactions are bond dependent 2. Honeycomb lattice

  16. Bond-dependent interaction Kugel and Khomskii, Sov. Phys. JETP 37, 725 (1973) • Orbital character is needed for bond-dependent interaction • Isospins (jeff=1/2) Jackeli and Khaliullin PRL 2009

  17. A2IrO3 (A=Na, Li) • Honeycomb lattice of Ir4+ • Heisenberg-Kitaev model? Li2IrO3powder Na2IrO3crystal ~2x2 mm2 Singh, Gegenwart

  18. However… F. Ye et al., PRB 85 180403 (2012) Ir4+ (5d5) eg Need to look at CEF excitations  RIXS CEF jeff = 1/2 t2g jeff = 3/2 (leff= 1)

  19. RIXS spectra

  20. Momentum dependence

  21. RIXS spectra Quatum Chemical calculation: van den Brink group

  22. However… Chaloupka, Jackeli, and Khaliullin PRL 2010 • X. Liu, et al., PRB (2011) Experimental structure • Phase diagram of NN Heisenberg-Kitaev model • No zig-zag phase! • Still a lot of confusion about the Hamiltonian • Large 2nd and 3rd nearest neighbor interaction • AntiferromagneticKitaev term

  23. More RIXS data

  24. Resonance behavior Na2IrO3

  25. Elastic + shoulder

  26. Temperature dependence Ca2+5xY2−5xCu5O10 W. S. Lee et al. arXiv:1301.4267

  27. Momentum dependence

  28. Magnetic dispersion Chaloupka et al., PRL 110, 097204 (2013) • Need a large energy scale • Kitaev term? • AF or FM?

  29. Magnetic excitation (neutron) S. K. Choi, R. Coldea, et al., PRL 108, 127204 (2012)

  30. Conclusions • RIXS is a powerful experimental probe for studying magnetism of iridates • Na2IrO3 • Jeff=1/2 description works well • Magnetic excitation around 30-40 meV – Kitaev energy scale? • Phonons • Future directions • Pressure • Thin Films • Time-resolved

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