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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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acknowledgements
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

outline
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
theoretical motivation
Theoretical motivation

Mott

Insulator

?

U/t

Topological

(Band) Insulator

Metal

l/t

Adapted from Pesin & Balents, Nature Physics 6, 376 (2010)

physics of ir 4 5d 5 in cubic cef
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)

iridate materials
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)

why rixs
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
iridate rixs the beginning
Iridate RIXS: The beginning

~ 1 eV

Energy resolution

progress in energy resolution
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

merix spectrometer
MERIX spectrometer

Si(844) Analyzer

Diego Casa

Ayman Said

Yuri Shvydko

merix workhorse
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
outline1
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)

kitaev s compass model
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

bond dependent interaction
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

a 2 iro 3 a na li
A2IrO3 (A=Na, Li)
  • Honeycomb lattice of Ir4+
  • Heisenberg-Kitaev model?

Li2IrO3powder

Na2IrO3crystal

~2x2 mm2

Singh, Gegenwart

however
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)

rixs spectra1
RIXS spectra

Quatum Chemical calculation: van den Brink group

however1
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
temperature dependence
Temperature dependence

Ca2+5xY2−5xCu5O10

W. S. Lee et al.

arXiv:1301.4267

magnetic dispersion
Magnetic dispersion

Chaloupka et al., PRL 110, 097204 (2013)

  • Need a large energy scale
  • Kitaev term?
  • AF or FM?
magnetic excitation neutron
Magnetic excitation (neutron)

S. K. Choi, R. Coldea, et al., PRL 108, 127204 (2012)

conclusions
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