Materials Overview of Iridates
Tomohiro TakayamaMax Planck Institute for Solid State Research
Part I... Perovskite related structures-Metal-insulator transition in iridates-
Energy Landscape why 5d oxides?
Energy (K)
101
102
103
104
105
3d TM(Fe, Co, Ni, Cu..)
4f RE(Ce, Pr, Nd)
CoulombU
CoulombU
Crystalfield
Spin-orbitcoupling(SOC)
Spin-orbitcoupling
Crystalfield
5d TM(Re, Os, Ir, Pt)
Spin-orbitcoupling
Crystalfield
CoulombU
Traditional playground for correlated electron physics
Interplay between U, t, and within close energy scale
transfer ttransfer t
Iridium oxide
Sc Ti V Cr Mn Fe Co Ni Cu Zn
Y Zr Nb Mo Tc Ru Rh Pd Ag Cd
Lu Hf Ta W Re Os Ir Pt Au Hg77
Ir4+, 5d5 (Ir3+, Ir5+, Ir6+ also possible (but difficult))
Strong neutron absorberL-edge E(L2) ~ 12.8 keV, E(L3) ~11.2 keV
ICSD-database... Number of reported compounds
Cu-O: ~ 8,000, Mn-O: ~8,500Co-O: ~ 4,700, Ru-O: ~ 1,600
Ir-O: ~ 480!
Rh3+ t2g6
Iridates... From quantum spin liquid to Dirac/Weyl
Spin-orbit Mott
Kitaev Spin liquid
Weyl semimetalTopological Mott Ins.
Topological Ins.Dirac electron
3D quantum spin liquid
Sr2IrO4
Na2IrO3
SrIrO3(111) perovskiteNa4Ir3O8
A2Ir2O7
Outline
Part I... Perovskite and related structures- Metal-insulator transition in iridates-
I. Ruddlesden Popper seriesFrom Jeff = 1/2 Mott to spin-orbit induced semimetal
II. Artificial Ruddlesden PopperSuperlattice of [(SrIrO3)m, SrTiO3]
III. Hexagonal perovskite iridates& (111) oriented superlattice
IV. Various perovskite-related iridates
Part II... Iridates with edge-shared IrO6(various spin-liquid candidates)2D, 3D honeycomb, Hyperkagome iridates
Perovskite ABO3 structure
A ion (larger cation)... 12 coordination
B ion (smaller cation)... 6 coordination
Goldschmidts Tolerance factor
A
BO6
Corner-shared BO6 network
2
t < 1... BO6 too larget > 1... A too large for the void
t 1.04Cubic pv
0.95
Hexagonal pvDistorted pvTetra, ortho, monocli
~0.7
ilmenite
Sr-Ir-O Ba-Ir-OCa-Ir-O
Layered perovskite...Ruddlesden Popper series
1) Cut perovskite at (100) plane
2) Shift by (a/2, a/2)
3) Insert AO layer
General formula: AO(ABO3)n If A = A, An+1BnO3n+1
(ABO3)nn: number of layers
(c.f. cut along (110) A2B2O7-type layered perovskite)
Ruddlesden Popper iridate Srm+1IrmO3m+1
m = 1 2 3
High-pressure
Sr2IrO4
Sr3Ir2O7
Sr4Ir3O10
SrIrO3
1) Larger coordination number for A-cation2) Less tetragonal distortion
Outline
I. Ruddlesden Popper seriesFrom Jeff = 1/2 Mott to spin-orbit semimetal
II. Artificial Ruddlesden PopperSuperlattice of [(SrIrO3)m, SrTiO3]
III. Hexagonal perovskite iridates& (111) oriented superlattice
IV. Various perovskite-related iridates
Unexpected magnetic insulator 5d Sr2IrO4
Sr2CoO4 Ferromagnetic metalSr2RhO4 Paramagnetic metalSr2IrO4 AF insulator
IrO2
Sr
IrO6 rotation ~ 11
Mott insulator despite widely extended 5d-orbital of Ir?
K2NiF4-type structure
Canted AF
Insulating
G. Cao et al., PRB 57 (1998) 11039
B. J. Kim et al., Science (09)
Sr2IrO4 as a Jeff = 1/2 Spin-orbital Mott insulator
Ir4+ 5d5
10 Dq~ 3 eV
S = 1/2 picture
xy
yz, zx
tetra
( )21,21,21,3
12/1 mm zxiyzxyJeff +=
eg
t2g
11
11
022
=
+=
+=
zx
yz
xy eff
Leff = -L= -1Complex phase i : recovery of orbital motion
Sr2IrO4: Spin-orbital Mott insulator
B. J. Kim et al., PRL 101 (2008) 076402
Need too large U
Jeff = 1/2 picture
Jeff = 1/2
Jeff = 3/2
SO ~ 0.5 eV
Contrasted RXS scattering Jeff = 1/2 state
Why Jeff= 1/2 magnets interesting?
1. Unique magnetic couplingComplex phase gives rise to interference effectPossible route for Kitaev spin liquid
2. Weak Mott insulatorVicinity to metal-insulator transitionCharge fluctuation, long range hopping can be sizable
Magnetic coupling between Jeff = 1/2 (direct or via oxygen)
3. Ideal platform for synchrotron experimentsL-edge of 5d elements in hard x-ray regionRXS, RIXS and XMCD etc.
Jackeli & KhaliullinJeff = 1/2
Isotropic Heisenberg AF expected for Sr2IrO4G. Jackeli and G. Khaliullin, PRL 102, 017205 (2009)
Ir O Ir : 180(corner - sharing IrO6)
Heisenberg + pseudodipolar interaction
IrO
x
y
Sr2IrO4
IrO6 rotation about c-axis
DM interaction
Local axis rotated in accord with IrO6
IsotropicHeisenberg
c
Heisenberg AF despite with strong SOC??
Local axis
Ir
2D isospin-correlation survives well above TN
T > TN 2D rod
T < TNBragg point
h
Magnetic correlation above TNContour plot in (h,0, l) plane
Diffuse scattering observed by RXS
2D spin correlation survives above TN
S. Fujiyama et al., PRL 108, 247212 (2012)
In-plane
T < TN
T > TN
Even 20 K above TN,a ~ 100a0 (a0: Ir-Ir length)
Sr2IrO4
Magnetic scattering above TNS. Fujiyama
2D Heisenberg antiferromagnetism with J ~ 0.1 eV
2D Heisenberg
2D Ising
2D XY(BKT)
)/25.1exp(276.0 TJaa =M. Makivic and HQ DingPRB 43, 3562 (1991)
ab-plane:
c-axis: critical divergence
}/){( NN TTTc
T-dependence: Heisenberg
S = 1/2 Heisenberg AF, J ~ 0.1 eV similar to La2CuO4
TN looks determined by inter-layer coupling
Magnetic correlation length by RXS
Sr2IrO4
J ~ 0.1 eV
S. Fujiyama et al., PRL 108, 247212 (2012)
More evidences for Heisenberg coupling
Magnon dispersion studied by RIXS Analysis of high-temperature susceptibility
T. Takayama et al., unpublishedJ. Kim et al., PRL 108, 177003 (2012)
Magnon Excitation Survives far above TNT = 20 K
T = 250 K
T = 400 K
q = (, 0)
AF correlation survives far above TNin IrO2 planes
RIXS @ SPring-8 BL11XU
Ei = 11.214 keV (L3), Q = (h, k, 32.5)
K. Ishii
Sr2IrO4 TN ~ 230 K
Ba2IrO4... K2NiF4-type without IrO6 rotation
Ba2IrO4
Stabilized under high-pressure ~ 6 GPa
No IrO6 rotation...No DM couplingJeff = 1/2 Mott like Sr2IrO4, TN ~ 243 K
H. Okabe et al., PRB 83, 155118 (2011)
Jeff = 1/2 characterBasal plane AF order
S. Bossegia et al.,PRL 110, 117207 (2013)
Moment along (110)
Distinct behavior under high-pressure
Sr2IrO4 remains insulating Ba2IrO4 becomes metallic
D. A. Zocco et al., J. Phys.: Condes. Matter 26, 255603 (2014). D. Orii et al., JKPS 63, 349 (2012)
No Superconductivity observed...
Due to the presence or absence of IrO6 rotation?
Bi-layer iridate Sr3Ir2O7
Double IrO2 layer -> increased bandwidth
S. Fujiyama et al., PRB 86, 174414 (2012)
shows an anomaly at magnetic order (~280 K)No weak-ferromagnetic moment along a-axis
Band gap almost diminishes
S. J. Moon et al., PRL 101, 226402 (2008)
Distinct magnetic structure and excitation spectra
Magnetic structure Magnetic excitation spectra
Jeff = 1/2 robust
Collinear AFSpin // c-axis
Due toPseudodipolar term?
J. W. Kim et al., PRL 109, 037204 (2012)
Large magnon gap ~ 90 meV
No single ion anisotropy expected in Jeff = 1/2
J. Kim et al., PRL 109, 157402 (2012)
Pseudodipolar term (~ = JH/U)enhanced at the border of MIT?
Bandwidth control in Ruddlesden Popper Series Srm+1IrmO3m+1
m = 1 2 3
SrIrO3
Insulator to metal by increasing number of IrO2 plane
Metallic iridate realized at the end member SrIrO3
Is SrIrO3 a half-filled Jeff = 1/2 metal?
Perovskite SrIrO3
~ 3 10-4 emu/mol, ~ 2.2 mJ/molK2
Large Wilson ratio RW ~ 10
Close to magnetism??
Sr2IrO4 SrIrO3?
GdFeO3-typeIrO6 rotation & tilt
Orthorhombic
Polycrystalline sampleprepared under 5 GPa
2ac 2ac 2ac
poor metal
(ac: cubic unit cell)
SrIrO3: not half-filled metal, but semi-metal!
SrIrO3
T-dependent RH
Large Nernst ambipolar effect
Jeff = 1/2 half-filled picture n ~ 11022 cm-3>
>
Estimate from RH n ~ 51019 cm-3
non-linear xy two carrier
RH, S < 0 larger mobility
for electrons
Is Jeff = 1/2 picture valid in SrIrO3?
Peaks at same energy (~ 0.7 eV) Jeff =3/2 to 1/2 intra-atomic d-d excitation seen in SrIrO3
2p
5d
Ir L-edge
Elementary excitations studied by RIXS
Ir L3-edge ~ 11.2 keV
RIXS can detectMagnon, d-d excitation, charge gap
Sr2IrO4
J. Kim et al., PRL 108, 177003 (2012)
T = 20 K
Semi-metal by band crossing and spin-orbit coupling
GGA+SOC for SrIrO3
Band crossing around EFdue to BZ folding
Band splitting at thecrossing points by SOC
GGA for SrIrO3
semi-metal
Semi-metal protected by the Dirac node
1/2
3/2
M. Zeb and H. Y. Kee, PRB 86, 085149 (2012)
Dirac dispersion & magnetotransport
Large longitudinal MR not related with Lorentz force
Non-quadratic dependence
B1.5?
ARPES
Dirac-dispersion in electron bandIdentified.
Y. F. Nie at al., PRL 114, 016401 (2015)
T. Takayama et al., unpublished
Carrier doping onto Jeff = 1/2 Mott Sr2IrO4Chemical doping (controversial...) Ionic liquid gating
No metallic state realized
C. Lu et al., PRB 91, 104401 (2015)
J. Ravichandran et al., arXiv:1312.7015
A. De la Torre et al., arXiv:1506.0061
X. Chen et al., arXiv:1506.0757
O. B. Korneta et al., PRB 82, 115117 (2010)
Fermi arc & d-wave gap opening?
Surface electron-doping by K-deposition
Y. K. Kim et al., Science 345, 187 (2014)
SC gap? by STM
Y. J. Yan et al., arXiv:1506.06557
Metal-insulator transition in (Sr1-xLax)3Ir2O7ARPESBulk MIT occurs by La doping
1st order phase boundary
T. Hogan et al., PRL 114, 257203 (2015) Electron pocket observed
x = 0.065
x = 0.066
J. He et al., Sci. Rep. 5, 8533 (2015)
A. de la Torre et al., PRL 113, 256402 (2014)
Outline
I. Ruddlesden Popper seriesFrom Jeff = 1/2 Mott to spin-orbit semimetal
II. Artificial Ruddlesden PopperSuperlattice of [(SrIrO3)m, SrTiO3]
III. Hexagonal perovskite iridates& (111) oriented superlattice
IV. Various perovskite-related iridates
How spin-orbital Mott evolves into a semi-metal?
Jeff = 1/2 Mott Semi-metal(from half-filled) (almost band ins.)
Intermediate phase: Sr4Ir3O10 Resistivity (poly) Magnetic susceptibility (poly)
4SrCO3+3IrO2 (precursor)
1100 , 5GPa for 1h
Only polycrystal availableNot clean (inter-growth??)
More sophisticated way totrace MIT?
How spin-orbital Mott evolves into a semi-metal?
SrIrO3SrTiO3
Artificial Ruddlesden Popper series!
n = 1 2 m = 1 m = 2
Bulk crystalRP series Srn+1IrnO3n+1
limited materials
Super-lattice
on-demand sequence[(SrIrO3)m, SrTiO3](001)
Jeff = 1/2 Mott Semi-metal(from half-filled) (almost band ins.)
J. Matsuno
J. Matsuno et al., PRL 114, 247209 (2015)
Superlattice reproduced (semi-)metal - insulator evolution
[(SrIrO3)m, SrTiO3](001)(m: number of SrIrO3 layers)
Metal-insulator change with inserting SrTiO3 layers
m
J. Matsuno et al., PRL 114, 247209 (2015)
Bulk
Magnetic insulator to semi-metal
m = ... Semi-metal as like bulk
Large in-plane moment appears in m = 1 and 2m = 1 strongly insulating above Tc... Mott insulator?
J. Matsuno et al., PRL 114, 247209 (2015)
MIT appears at m = 3 accompanying appearance of magnetism
Up-up-down-down in-plane moments in Sr2IrO4
Sr2IrO4AF and metamagnetism
Cantedmoment
H = 0
1st IrO2layer
2nd IrO2layer
3rd IrO2layer
4th IrO2layer
Up-up-down-down configurationof in-plane momentGround state: AF magnetism
In-plane moment appearsdue to IrO6 rotation & DM interaction
Metamagnetic transition by in-plane field
M ~ 0.07 B/Ir
Weak ferromagnetism in m = 1 superlattice
Cantedmoment
1st IrO2layer
TiO2layer
2nd IrO2layer
TiO2layer
SrIrO3/SrTiO3/SrIrO3/SrTiO3Weak Ferromagnetism
m = 1 ground state: weak ferro
m = 1 essentially reproduces the nature of Sr2IrO4
Ir-O-Ti-O-Ir coupling along c-axisFM irrespective of Ir-O-Ti FM or AF
parallel canting moment
M ~ 0.02 B/Ir smaller than Sr2IrO4 smaller rotation &
Electronic structure of superlattices
Single layer
Bilayer
J. Matsuno et al., PRL 114, 247209 (2015)
Semi-metal
Large FS
Magnetic Insulatorwith large gap
Marginally insulating
Outline
I. Ruddlesden Popper seriesFrom Jeff = 1/2 Mott to spin-orbit semimetal
II. Artificial Ruddlesden PopperSuperlattice of [(SrIrO3)m, SrTiO3]
III. Hexagonal perovskite iridates& (111) oriented superlattice
IV. Various perovskite-related iridates
Another form of AIrO3...hexagonal perovskite
Tolerance factor
6M-SrIrO3
ambient pressure phase
t ~ 0.99
IrO6 connected by corner and face sharing
G. Cao et al., PRB 76, 1004002 (2007)
Reported as non-Fermi-liquid correlated metal...
How to see cubic perovskiteSee perovskite along (111) direction
AO3AO3AO3AO3
B
B
B
B
A: large ion size, make close-packed layer with O2-(111)
B: occupy octahedral void between AO3 layers
Stacking of AO3 layers
AO2-
(AO3 layer)
(B layer)
- I II III I II III I II III I II III -
Layer I
iii i ii iii iii iiiii iii i i
Layer II
(ccp)
B cation
Layer III
Corner shared BO6
How to see hexagonal perovskite
When tolerance factor t > 1 (large A and/or small B),B-O bonds cannot connect each other within AO3 lattice.
Stacking of AO3 layers
AO2-
Layer ILayer II
B cation
(AO3) -I II I II I II I II -
(B) iii iii iii iii iii iii iii
2H BaNiO3-type
(hcp)
Combination of two stacking pattern
(AO3) -I II I II III II III I III - I-
(B) iii iii iii i i i ii ii ii
Unit cell
(AO3) -I II III I III II I II III - I-
(B) iii i ii ii i iii iii i ii
Unit cell
Variation of hexagonal perovskite
9H 6H2H (3c)
Face-sharing BO6... Repulsion between cations larger BO6High-pressure leads to more corner-shared network
High-pressure
Cubic perovskite
J. M. Longo, Mat. Res. Bull. 3, 687 (1968)
Magnetic insulator BaIrO3
Ba
Ir3O12
9M structure
Ambient pressure phaseIr3O12 unit connect along c-axis
150
100
50
0
-50
M (e
mu/m
ol)
300250200150100500
Temperature (K)
25
20
15
10
5
0
(
m
cm)
H = 1 T
9M-type BaIrO3
Phase transition Tc ~ 180 K
Resistivity increases below TcWeak ferromagnetic below Tc
Also G. Cao et al., Solid state com. 113, 657- (2000)
CDW(SDW), Canted AF or FM??
XMCD
/ = 2.8(2)
Sizable orbital momentSpin-orbital Mott?
Non-linear I-V
T < Tc, non-linear conduction... Charge density wave?
G. Cao et al., Solid state com. 113, 657- (2000)
No CDW q-vector, Magnetic Bragg peak identified to date
SR
Clear magnetic order below Tc ~ 180 K
M. L. Brooks et al., PRB 71, 220411(R) (2005)
M. A. Languna-Marco et al.,PRL 105, 216407 (2010)
Ferromagnetic metal 5M-BaIrO35M BaIrO3
Stabilized under narrow pressure range around 4 GPa
J. -G. Cheng et al., PRB 80, 104430 (2009)
Weak moment ~ 0.007 B/IrSmall anomaly in C/T
-(IrO6)-(Ir2O9)-(Ir2O9)- sequence
6M Sr(Ba)IrO3 : semimetal
Monoclinic SrIrO3
BaIrO36M-type
RH < 0
S > 0
n ~ 51020 cm-3
RH < 0
S > 0
... n ~ 41021 cm-3
Negative Hall and Positive Seebeck represent two-types of carriers
synthesizedP > 5 GPa
ambientpressure phase
Metal-insulator transition in hexagonal iridates
Ambient pressure phase
9M BaIrO3
Magnetic insulatorThe nature of transition unclear(Mott, SDW, etc.)
Ir3O12
C2/m 6M Sr(Ba)IrO3, C2/c
-(Ir3O12)-(Ir3O12)-(Ir3O12)-
High pressure phase
> 5 GPa
-(IrO6)-(Ir2O9)-(IrO6)-
Semi-metal
5M BaIrO3 C2/m
Intermediate phase
3~4 GPa
-(IrO6)-(Ir2O9)-(Ir2O9)-
Ferromagnetic metal
Electronic structure of 6M-SrIrO3
A. Yaresko
Ir1 at single IrO6
Ir2 at Ir2O9
Dirac cones above and below EF
A. Yaresko
Honeycomb physics involved?
Hopping between Ir2 is week
Layered quasi-2D?
Hopping between Ir1 and Ir2 (via O 2p) is stronger.
Ir1 and Ir2 formhoneycomb-like lattice in ab-plane
Topological Ins. by (111) bi-layer
Bi-layer perovskite along (111) regarded as buckled honeycombLayer potential, next-nearest hopping can open a gap.
bi-layer SrIrO3... Topological insulator
Fabrication of (111) superlattice
D. Hirai
(111) Oriented thin-film on STO... Difficulty due to polar surface (SrO3)4- & Ti4+
SrIrO3... 6M thin-film (incl. face-share) obtainedCaIrO3... Relaxed due to lattice mismatch
Ca0.5Sr0.5IrO3 (CSIO) gives the best results
Superlattice [CSIO2m, STO2]k
D. Hirai et al., APL Materials 3, 041508 (2015)m = 2m = 1
Metal-insulator transition in (111) superlattice
Like (001) superlattice, MIT takes place by thickness change
(m = 1, 2: too high to measure xy)
For small m, the superlattice is magnetic insulator.Not TI, correlation effect must be considered.
D. Hirai et al., APL Materials 3, 041508 (2015)
Outline of Lectures
I. Ruddlesden Popper seriesFrom Jeff = 1/2 Mott to Dirac semimetal
II. Artificial Ruddlesden PopperSuperlattice of [(SrIrO3)m, SrTiO3]
III. Hexagonal perovskite iridates& (111) oriented superlattice
IV. Various perovskite-related iridates
Other perovskite related structures
Perovskite can accommodate multiple A or B ions
Y. Shimakawa et al., J. Phys. Condens. Matter 26, 473203 (2014)
Double perovskite A2BBO6 ... Frustration?
A: Sr2+, Ba2+
B4+, B4+
A: Ln3+
B4+, B2+
e.g. Sr2CeIrO6, Ba2CeIrO6
e.g. La2MgIrO6, La2ZnIrO6
IrO6
Ba2Ce4+IrO6
FCC lattice
TN ~ 17 KW ~ -177 K
Frustrated?
M. Wakeshima et al., J. Mater. Chem. 10, 419 (2000)
Distorted, P21/n
f ~ 10
Also,K2IrCl6 etc.
Double perovskite... Ir-O-O-Ir Kitaev?
La2BIrO6 (B = Mg, Zn)
Less frustrated...
W ~ -3 K
W ~ -24 KTN ~ 12 K
Tc ~ 7.5 K
Ir-O-O-Ir Kitaev interaction dominant?
A. A. Aczel et al., arXiv:1507.0792
G. X. Cao et al., PRB 87, 155136 (2013)
A-type AF
Ordered hexagonal PV... Spin liquid candidate
Spin-liquid behavior in Ba3IrTi2O9
Ir-O-O-Ir bond... Kitaev exchange in triangular lattice?
6M-type stacking
Ir occupies face-sharing octahedra~7% of site-mixing at Ti(3)~37% site mixing between Ir(1) and Ti(2)
T. Dey et al., PRB 86, 140405 (2012)
W ~ -133 K
Ordered hexagonal PV... Jeff = 1/2 singlet?
Spin singlet formation in Ba3BiIr2O9Bi4+
Ir4+
Ir4+ makesIr2O9 dimers
Sudden drop of at T* ~ 74 K
Spin gap opening in dimers
Ir-Ir distance increasesbelow T*??
W. Miller at al, JACS 134, 3265 (2012)
A-site ordered perovskite
AA3B4O12A:124 coordination
Cubic Im-3 (No. 204) Z=2
X Y Z
A 0 0 0
A 0.5 0 0
B 0.250 0.250 0.250
O ~0.18 ~0.31 0
2a0 x 2a0 x 2a0
a8 CaCu3M4O12 M4+RE3+Cu3M4O12 M3.75+
Slide from M. Isobe
Overview of ACu3B4O12 B :3d
ACu2+3V4O12
ACu2+3Mn4O12
ACu2+3Fe4O12
A = Na+, Ca2+, Y3+ Pauli paramagnetic metal
A = Ca2+
Ferromagnetic metal A= Bi3+, R3+Ferrimagnetic semiconductor
A = Ca2+ ,Sr2+ A= Bi3+, R3+
charge disproportionation or charge transfer
ACu2+3Ti4O12 A = Ca2+ Magnetic insulator
ACu2+3Cr4O12 A = Ca2+,Sr2+
Partial charge transfer AF metal A= R3+Pauli paramagnetic metal
ACu2+3Co4O12A = Ca2+ Pauli para metal A= La3+ low-spin insulator
(ACu3+3Co4O12)
AF insulator Ferri semiconductor
charge disproportionation charge disproportionation charge disproportionation charge disproportionation FerriFerriFerriFerri metal metal metal metal A= Bi3+
Slide from M. Isobe
New heavy fermion oxide? CaCu3Ir4O12
~ 120 mJ/Ir-molK2
Curie-Weiss Cu2+ 3d seems localizedBut deviates below T* ~ 80 K.Below T*, shows pronounced decrease
Hybridization of Cu 3d& Ir 5d ?
Upturn of C/T at low temp.
J. G. Cheng et al., PRL 111, 176403 (2013)
Post-perovskite... High-pressure phase of PV
From SPring-8 webpage
Post-perovskite... High-pressure phase of perovskite
MgSiO3
MgSiO3... Perovskite to post-perovskite around 120 GPa, 2500 KS. Murakami et al., Science 304, 855 (2004)
For CaIrO3, this transition occurs around 4 GPa, 1200 K
Along a, edge-shareAlong c, corner-share
CaIrO3Mg2SiO4
Post-perovskite CaIrO3... Kitaev & Heisenberg
Ca1-xNaxIrO3 ...MIT takes place
Canted AF
Along a, edge-shareAlong c, corner-share
K. Ohgushi et al., PRB 74, 241104 (2006)
AF coupling along cFM alignment along a
Coexistence of Heisenberg& Kitaev coupling?
K. Ohgushi et al., PRL 110, 217212 (2013)
Summary
Structural flexibility (controlled by ionic size or pressure)... Systematic evolution of crystal and electronic structure provide opportunity to trace metal-insulator transition.
The metallic iridates at the one end are Dirac semi-metals?
Possibility of various new materials in the ordered structure
Perovskite iridates
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