放射光を用いた強相関薄膜実験に期待すること

Department of Physics
Tohoku University
Sumio Ishihara

物講研シンポジウム
放射光・中性子・ミュオンを用いた表面・界面科学の最前線
つくばエポカル Nov. 17-18, 2009

Outline
Introduction
Calculation of (SrMnO3)n/(LaMnO3)n superlattice & SHG
A perspective
Collaborators
T. Satoh (Tokyo), K. Miyano (Tokyo)
Acknowledgements
H. Nakao (KEK)
Reference
T. Satoh, K. Miyano, Y. Ogimoto, H. Tamaru, and SI,
Phys. Rev. B 72, 224403 (2005).

Introduction

Correlated electron systems

相関電子系の物性研究

High Tc superconductivity
Colossal magneto resistanceMultiferroics
Heavy fermion state
Quantum Hall effectetc[1] Strong electron correlation
Competition between itineracy / localization[2] Multi degrees of freedom& their coupling/separation
charge, spin, orbital, lattice

COFM

X-ray diffraction/spectroscopy[1] X-ray detects both the k-space and real-space nature of electron

r-spacek-space

x-ray
diffraction

ARPES
RIXS

EXAFS
(PDF)

RXS（element specific）

RIXS（core hole effect、local excitation)

X-ray diffraction/spectroscopy[２] X-ray accesses to charge/spin/orbital degrees

ChargeX-ray diffraction
Resonant x-ray scattering
Resonant inelastic x-ray scattering

spin
Magnetic x-ray scattering
Magnetic compton scattering
MCD

orbitalResonant x-ray scattering

Correlated electron systems
Superconductivity
CMR

Metal-Insulator
Transition

Spin liquidchargeMultiferroics

electroncorrelation
spin

StripeorbitalSurface/Interface in thin film/superlatticeNovel phenomena, Colossal response, New functions(which do not appear in bulk)

Correlated electron superlattice(SrTiO3)m-(LaTiO3)n superlattice

A. Ohtomo, D. A. Muller, J. L. Grazul & H. Y. Hwang,
NATURE 419, 378(2002)

Band insulator (d0) / Mott insulator (d1) → metallic interface
EELS

Carrier density from Hall effect

SrTiO3LaTiO3

SrTiO3insulator + insulator = metal

Correlated electron superlattice(SrMnO3)m-(LaMnO3)n superlattice
Mott insulator (d4)Mott insulator (d3)3z2−r22x −yxy2yzzx

d(3x2-r2)/d(3y2-r2) orbital orderwith Jahn-Teller distortion
A-type Antiferromagnetic orderG-type AF orderColossal
Magneto Resistance
巨大磁気抵抗効果
Colossal Magneto Resistance (CMR)
La1-x Srx MnO3

Pr1-x Cax MnO3

(Tokura group)Phase Diagram
La1-xSrxMnO3

Nd1-xSrxMnO3(Tokura group)

電荷スピン
軌道

絶縁体
A-型反強磁性
3x2-r2/ 3y2-r2型
軌道秩序

金属
強磁性
軌道液体

電荷秩序
CE-型反強磁性
3x2-r2/ 3y2-r2型
軌道秩序

金属
A-型反強磁性x2-y2型
軌道秩序

絶縁体
C-型反強磁性
3z2-r2型
軌道秩序

Theoretical Phase Diagram
S. Okamoto, SI, S. Maekawa,Phys. Rev. B 61, 14647 (‘00).

Hartree-Fock approx.

0.14

SrMnO3

LaMnO3
0.12
0.10

TOO
FMT/t00.08

TOO

0.06

TN0.04

A-AF

0.02
0.00
0.0

ExperimentTC

A-AFFM
0.20.4x0.60.8

1.0Pr1-xSrxMnO3(Tokura Group)
Staggered orbital(Super-exchange)

Uniform orbital(Double exchange)Correlated electron superlattice(SrMnO3)m-(LaMnO3)n superlattice
Mott insulator (d4)Mott insulator (d3)3z2−r22x −yxy2yzzxd(3x2-r2)/d(3y2-r2) orbital orderwith Jahn-Teller distortion
A-type Antiferromagnetic orderG-type AF order

Correlated electron superlattice(SrMnO3)m-(LaMnO3)n super-lattice
P. A. Salvador et al. APL 75, 2638, (‘99)
n/(n+m)=0.26, m=1-15, STO substrate
J. Verbeeck et al. APL 79, 2037, (‘01)
n=8, m=4, STO substrate
T. Koida et al. PRB 66, 144418, (‘02)
n=m, m=1-32, STO[100] substrate
and more
Theory:C. Lin, A. MiIlis, PRB 78, 184405(’08) DMFT
S. Dong, et al. PRB 78, 201102(R) (‘08) MC

C. Adamo et al. PRB 79, 045125, (‘09)
n=2m, m=1-16, STO substrate

LaMnO3
SrMnO3
LaMnO3
SrMnO3
STO

Ferromagnetic metallic behavior with decreasing m and n

Correlated electron superlattice(SrMnO3)m-(LaMnO3)n superlattice
H. Nakao et al. JPSJ 78, 024602, (‘09)
n=m, m=1-16. STO substratesample quality is important
Insulating behavior for all mgood sample
bad sample

Correlated electron superlattice(SrMnO3)m-(LaMnO3)n superlattice
H. Nakao et al. JPSJ 78, 024602, (‘09)
n=m, m=1-16. STO substrate

Intermediate valence of Mn3+ and Mn4+(consistent with present calculation ?)A. Sawa (previous talk)H. Yamada (CMRC meeting)
LSAT substrate

AFM-Insulator → FM-Insulator

Resonant
x-rayscattering@ Mn KSHG and electronic structurein (LaMnO3)n/(SrMnO3)n interface

SH Experiments
T. Satoh, K. Miyano, Y. Ogimoto, H. Tamaru & SI
PRB (’05)

Maker fringe pattern[001] epitaxial growth

ωLSAT

LSAT
LaMnO3
SrMnO3

2ω

LSAT

LSAT

LaMnO3

SrMnO3

SrMnO3
LaMnO33-4 units

SH Experiments
SH spectra v.s. photon energy
insulator
10 times larger
than χ(2) in BaTiO3LSAT
LaMnO3
SrMnO3LSATLSAT

LaMnO3

SrMnO3LSAT
SrMnO3
LaMnO3

Model(LMnO3)n-(SMnO3)n super lattice
Tight binding model for O 2p & Mn 3d electrons
Extended d-p model

L (La): trivalent cation
S (Sr): divalent cation

LMnO3

SMnO3zMn
OL3+L, S: point charge
MnO2 LOO : bonding orbital
Mn : eg orbitals + t2g localized spin

S2+SO

MnO2layers

ModelExtended d-p modeleg

U, U’, Itεd(l)tεd(l+1)

εp(l)

t2g
Mn

Jt2g
O

z=l

Mn
z=l+1

(Mn 3d electron)(O 2p electron)(Mn 3d –O 2p transfer)(Jahn-Teller coupling)

JHMethod
Hartree-Fock approximationMean fields
Order parameters (at each layer)d electron number

Madelung potential
at each layer/site

p electron number
Magnetic order
Orbital order

Madelung potential(Ewald method)

HF bands

Electronic structure
electron density
H. Nakao et al. JPSJ 78, 024602, (‘09)

2.0p-electron

1.5

1.0

N(d)
N(px)
N(py)
N(pz)

d-electron

0.5

0.0-8

orbitalintermediate
density-6-4-202468

site

LMnO3

SMnO3

0.0-0.2

Tx Anti-ferro-0.4-0.6-0.8-1.0-1.2-8Tz
TxTz Ferro-6-4-20site

2468

Electronic structure
1.2M(Fz)
M(Fx)
M(AFz)
M(AFx)

1.0
0.8

Ferro

eg spin(in-plane component)

0.6

spin cant

0.4
0.2

t2g spin(in-plane component)

Anti-ferro

0.0-8-6-4-202468site

LMnO3

2.5
2.0

SMnO3

Anti-ferro

1.5
1.0

Spin cant ~ Phase separation (?)(c.f. Sawa’s talk)

Mt2(Fz)Mt2(Fx)
Mt2(AFz)
Mt2(AFx)

Ferro

0.5
0.0-8-6-4

in-plane FM-20site

2468

in-plane AFMElectronic structure
DOS 10

total
SMO
LMO
interfaceDOS(eV-1)86

Mn3d
linear optical absorption

EF4

1.4

total
SMO
LMO
interface

1.2
20-5010

Energy (eV)25EFO2p20

DOS(eV-1)

515

total
SMO
LMO
interfaceImχ1(ω)

1.0
0.8
0.6
0.4
0.2
0.0

1002468

Energy (eV)5insulator
0-50

Energy (eV)510

SHG spectra
One-body (Hartree-Fock) scheme

ωlnω

m2ω

SHG spectra
1.0

Theory0.8

SL 2+χ |

0.6|χ

(2)

zzz

|2|χ

aSaL 2|χ |0.4

Experiment

0.2aLLS 2|χ + χ |aS 2|χ |0.0123

Second-harmonic photon energy 2ω/JT. Satoh, K. Miyano, Y. Ogimoto, H. Tamaru, and SI
Phys. Rev. B 72, 224403 (2005).

4

SHG spectra
Charge transfer excitation over interface

Mn3d (eg)2

Mn3d (eg)1EFO2p6O2p6

Mn3d (eg)1

LMnO3Interface

4

HF energy level SMnO3

Mn3d(eg)1ed
epxyepz
ed+U-3I

32O2p (LMO) / Mn3d (eg) （SMO) → O2p (LMO) / Mn3d (eg) （SMO)60

51O2p (SMO) / Mn3d (eg) （LMO) → O2p (SMO) / Mn3d (eg) （LMO)
61521O2p60-1-8-6-4-20site

2468

A perspective(理論側からの期待)More direct observation
Observation of electronic/lattice structure only at/around interface
X-ray diffraction
Crystal truncation rods (CTR) measurements
Grazing incident x-ray scattering (GIXD)

X-ray absorption spectroscopy
tuned @ absorption edge in deeper layer
low angle of incident x-rayPhotoemission
resonant
penetration depthCu L-edge

J. Chakhalian, et al. Science 318, 1114 (‘07)

More direct observation(SrMnO3)n-(LaMnO3)2n super latticen=4, 5, STO substrate
Resonant soft X-ray scattering

S. Smadici, et al. PRL 99, 169404 (2007)@ O K-edge@ Cu L-edge

Structure factor
Magnetic scattering(superlattice forbidden reflection)

a perspective
Electronic structure (almost) only at interface
Breaking of space inversion symmetry+
Sometime breaking oftime reversal symmetry(FM )crystalline symmetry(orbital )Interference of dipole-quadrupole transition at pre-edgeLinear/circular polarization analyses

Intrinsic properties of correlated electron interface
Novel Function attributed to interface(GMR, SC, SHG etc. )Differences from (band insulator)/(band insulator) interface
from carrier doping effect in Mott insulator
electrondensity

holedensity

Summary
SHG and electronic structurein (RMnO3)n-(AMnO3)n superlattice
Insulating interface：
Large SHG:

Almost discontinuous
New charge transfer channel across interface

T. Satoh, K. Miyano, Y. Ogimoto, H. Tamaru, and SI,Phys. Rev. B 72, 224403 (2005)

A perspective from a theoretical side （理論家からの期待）
More direct observation of electronic state @ interface
breaking of(space inversion)+ (time reversal / crystal symmetries)
Interference of dipole+quadrupole transitions
Intrinsic properties in correlated electron interface