Materials Overview of Iridates - Sciencesconf.org oxide Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc...

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




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





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


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?


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


Semi-metal

Large FS

Magnetic Insulatorwith large gap

Marginally insulating

Outline





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




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

Materials Overview of Iridates - Sciencesconf.org oxide Sc Ti V Cr Mn Fe Co Ni Cu Zn Y Zr Nb Mo Tc...

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