物構研シンポジウム'09「放射光・中性子・ミュオンを用いた表面・ 界面科学の最前線」
エポカルつくば，2009年11月17日～18日

強相関酸化物ヘテロ界面の相競合と新規物性
澤 彰仁1，山田 浩之1，
Ping-Hua Xiang 1,2，Ping-Hua
11産業技術総合研究所
22JST-CREST
JST-CREST

強相関遷移金属酸化物
超伝導

強磁性(Nd,Ce)2CuO4(La,Sr)MnO3

強誘電＋反強磁性（マルチフェロイック）
BiFeO3

Wang et al., Science 299, 1719 (2003)

磁場制御

電場制御

1.2Resistance [Ω]MR ratio ρ(H)/ρ(0)

10.8
0.6
0.4
0.2
010115K

125K

1087100.5
11.5
Magnetic field [T]Tomioka et al.

210108106105
104103102

T=20 K, μ0H=0 T50Light

107

109

106135K

109

Pr1-x CaxMnO 3(x=0.3)

10Resistance(Ω)(Nd,Sm)1/2Sr1/2MnO3

光制御

0200 400 600 800 1000

Applied voltage [V]Asamitsu et al., Nature 388, 50 (1997)

101100-2030K↑
Laser4 pulse
02040Time (μs)

60

Fiebig et al., Scinence 280, 1925 (1998)

遷移金属酸化物界面の特異な電子状態
LaAlO3-SrTiO3 Interface (band insulators)
Polar catastrophe
Ohtomo and Hwang,
Nature 427,423 (2004)
Nakagawa et al.,Nature Mater. 5, 204 (2006)

two-dimensional electron gas?

Thiel et al.,Science 313,1942 (2006)

Reyren et al.,Science 317,1196 (2007)

Brinkman et al.,Nature Mater. 6, 493 (2007)

強相関酸化物界面の特異な電子状態
LaMnO3-SrMnO3 Interface (Mott insulators)
Short-Period
Uniform Valence
Alloy-like metal
LaMnO3

LaMnO3

Koida et al.,
PRB 66, 144418 (2002)
Salvador et al., APL 75, 2638 (1999)
Yamada et al., APL 89, 052506 (2006)
Bhattacharya et al., PRL 99, 196404 (2007)

SrMnO3Ferro Metal

SrMnO3

SrMnO3 LaMnO3[(LMO)m/(SMO)m]nLong-Period
Nonuniform ValenceInterface Ferro.
FerrocAFFerro

AFFerroCharge transfer at interfaces

Electronic reconstruction at LMO-SMO interface
Phase diagram of La1-xSrxMnO3

Mn３+ (t2g3eg１)

LaMnO3eg

CantedA-AFI

t2g

Charge transfer
Mn4+ (t2g3eg0)eg

SrMnO3

t2g

G-AFI

Mn
La
Sr

LaMnO3Ferromagnetic
metal

Urushibara et al.,
PRB (1994)

SrMnO3

Interface has a doping level intermediate
between LMO and SMO?
(~0.5)

Recent study
H. Nakao (PF-KEK) et al., JPSJ 78, 024602 (2009)
Resonant X-ray Scattering
LMO 2uc+SMO2uc SLabrupt
uniform

Interface roughness changes
interfacial electronic states

Valence modulation may be very small.

Intrinsic electronic states at LMO-SMO interface ??

Strategy for high-quality superlattice synthesis(1) Substrate-choiceinterface(La,Sr)(Al,Ta)O3

SrMnO3(SMO)

RHEED Oscillation

SrTiO33.905ÅSMO

RHEED Intensity

LaMnO3(LMO)(2) Improvement of Samples

LSAT
3.87Å

STO：tensile-strain
LSAT: lattice-matching

LMO

1μm
0200

400

Time (sec.)

600800Feedback
Characterization with XRDCharacterization with Synchrotron XRD
BL4c , Photon Factory, KEK, Tsukuba, 2009 (Prof. Nakao)
RTBad superlattice• Sharp superlattice reflection• Laue Fringes until h=1.5
9 Modulation in superstructure
Good as films, but structural imperfection

Good superlattice
Clear Laue Fringes
perfect structureImpact of Improvement on Properties
Bad L2S2= Ferro Metal
La0.5Sr0.5MnO3Good L2S2 = Insulator

L4S4, L5S5= Ferro Insulator

M, 1/ρ, Tc

Bad > Good
Bad- Good= Extrinsic FM(interface rougness etc.)

Short-Period LMO-SMO SL
LaO
2LaMnO3 MnO
LaO
MnO2
SrO
SrMnO3 MnO2
SrO×?

Mn Valence
Under-doped
Nearly 3+interface

LaMnO3
SrMnO3
Alloy-like(Ferro. Metal)

Over-doped
Nearly 4+
Insulator? Even in the short periodicity, electronic state is not uniform.
? Charge transfer alone cannot explain the properties.

Additional mechanism?

H-Induced Insulator-to-Metal Transition
Good L2S2

Alloy LSMO/LSAT

H//film

Sr

AFI

FM

z Metallic for H >7T :strong coupling withspin/orbital degrees of freedomz No large MR effect:

L2S2: Magnetic-Field Dependence
H-annealing effect

Phase Separation

History of H-application
affects Msat and MR effect.

Hann210110ρ (Ωcm)

01010-1

10-2Hann
1TShort-range
FM domain Orbital-Order (AFI)

zfc

3T
7T
9T

T. Kimura et al.,PRL 83 (1999)

Magnetorelaxor

T=10 K-3

Cr-doped Nd0.5Ca0.5MnO3M (μB/Mn)

10 2
1Hann
1T
3T
7T

0-1-2-10-505

Magnetic Field (T)

10Phase competition at LMO-SMO interface

Phase diagram of perovskite Mn-oxides

Tomioka and Tokura, PRB 70, 014432 (2004)

bi-critical point: phase competition=one-electron bandwidth

Lattice distortion induces an electronic phase variationLayer-Thickness Dependence (asymmetric)
LMO 2uc < SMO 3uc
Antiferromagnetic Insulator(TN~250K)
Reduced MR (Insulator for H<9T)

L2S2（TN~200K)
LMO 3uc > SMO 2uc
Ferromagnetic metal(TC=240K, M<3μB)

Ground-State Phase Diagram

2u.c .

complicated ground-states

AFI
L2S3

L5S5
L4S4FILongPeriod
LMO

4u.c.
SrMnO3

Short-Period

Insulatorbulk-like properties

L2S2
2u.c.

L3S2

FM
6u.c.

M-I boundary

LaMnO3 layer
Insulator (L2S2) H Metal
Nearly-metal CMRElectron-doped CaMnO3Metal-insulator transition with lightly electron doping
Ca1-xCexMnO3Mn4+ (t2g3eg0)

eg
Ce4+t2g

cG-type AF Metal

G-type AF Insulator

C-type AF Insulator

Caspi et al., PRB 69, 104402 (2004)
Tomioka et al., unpublishedCa1-xCexMnO3 (CCMO): x = 0 – 0.08
a (CCMO x=0.05) ~ 0.374 nm
Substrate: (100)-oriented
NAO: NdAlO3 (a = 0.3751 nm,nm +0.3%)

1 μm x 1 μm

2101

10010-1CMO

10-110-210-3x=0050

100

1500T
5T

200

Temperature (K)250300

CCMO5x=0.05

1010-2

Resistivity (Ωcm)

10Resistivity (Ωcm)

Resistivity (Ωcm)

Xiang et al., APL 94, 062109 (2009)

0T
3T
5T
9T-3050100150200

Temperature (K)25030010210110010-110-210-3

CCMO8x=0.08
0T
5T050100150200

250

Temperature (K)

300

NAO

Resistivity (Ωcm)

KrF LasereRHEEDCaMnO3102101

100

CCMO8CMO
CMO@5T
CCMO8CCMO8@5T

CMO-110-2

10-3

10Ca1-xCexMnO3(x=0.08)050

100150200

250

Temperature (K)300

[CMO(x=0)×m/CMO(x=0) )×n]
G-AFI

G-AFI
C-AFI
G-AFI
C-AFI

interface

C-AFI

Charge transfer +Competition of different spin structuresat interfaces
Phase separation?

+

まとめ
強相関酸化物ヘテロ界面・電荷移動 ・・・・ 限定的（？）
キャリア濃度は界面でシャープに変化
界面1ユニットセル程度の領域だけでキャリア濃度が変化
・電子相競合強相関電子系の効果
・・・・ 軌道（格子）、スピンの協調と競合

界面電子相・新機能
今後
・界面の局所構造・電子状態（価数、スピン状態など）の詳細な評価
シンクロトロンX線実験，透過電子顕微鏡観察
・界面電子相競合状態の外場（電場、光など）による制御 ・・・・ 機能化