物構研シンポ 澤 091117 r -...
TRANSCRIPT
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強相関酸化物ヘテロ界面の相競合と新規物性
強相関酸化物ヘテロ界面の強相関酸化物ヘテロ界面の相競合と新規物性相競合と新規物性
澤 彰仁1,山田 浩之1,Ping-Hua Xiang 1,21産業技術総合研究所
2JST-CREST
澤澤 彰仁彰仁11,山田,山田 浩之浩之11,,PingPing--Hua XiangHua Xiang 1,21,2
11産業技術総合研究所産業技術総合研究所22JSTJST--CRESTCREST
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
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強相関遷移金属酸化物強相関遷移金属酸化物超伝導超伝導 強磁性強磁性
(Nd,Ce)2CuO4 (La,Sr)MnO3 BiFeO3
強誘電+反強磁性強誘電+反強磁性(マルチフェロイック)(マルチフェロイック)
Wang et al., Science 299, 1719 (2003)
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2
MR
rat
io ρ
(H)/
ρ(0)
Magnetic field [T]
115K 125K 135K
(Nd,Sm)1/2
Sr1/2
MnO3
磁場制御 光制御電場制御
4
Time (μs)
Res
ista
nce(
Ω)
↑Laser pulse
30K
Light
-20 0 20 40 60100101102
103104105106107108109
105
106
107
108
109
1010
0 200 400 600 800 1000
Pr1-x
CaxMnO
3
(x=0.3)R
esis
tanc
e [Ω
]
Applied voltage [V]
T=20 K, μ0H=0 T
Asamitsu et al., Nature 388, 50 (1997) Fiebig et al., Scinence 280, 1925 (1998)Tomioka et al.
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遷移金属酸化物界面の特異な電子状態遷移金属酸化物界面の特異な電子状態
LaAlOLaAlO33--SrTiOSrTiO33 Interface (band insulators)Interface (band insulators)
Polar catastrophePolar catastrophe
Ohtomo and Hwang,Nature 427,423 (2004)
Nakagawa et al.,Nature Mater. 5, 204 (2006)
Thiel et al., Science 313,1942 (2006)
Reyren et al., Science 317,1196 (2007)
Brinkman et al., Nature Mater. 6, 493 (2007)
twotwo--dimensional electron gas?dimensional electron gas?
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強相関酸化物界面の特異な電子状態強相関酸化物界面の特異な電子状態
LaMnOLaMnO33--SrMnOSrMnO33 Interface (Mott insulators)Interface (Mott insulators)
[(LMO)m/(SMO)m]n
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)
Short-PeriodUniform ValenceAlloy-like metal
Long-PeriodNonuniform Valence
Interface Ferro.
Ferro Metal
cAF
Ferro
AF
Ferro
Ferro
SrM
nO3
LaM
nO3LaMnO3
SrMnO3
LaMnO3
SrMnO3
Charge transfer at interfaces Charge transfer at interfaces
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CantedA-AFI
G-AFI
LaMnOLaMnO33
SrMnOSrMnO33
MnLa Sr
LaMnOLaMnO33 SrMnOSrMnO33
Phase diagram of LaPhase diagram of La11--xxSrSrxxMnOMnO33
Ferromagnetic metal
Interface has a doping level intermediate Interface has a doping level intermediate between LMO and SMO?between LMO and SMO?
(~0.5)(~0.5)
Electronic reconstruction at LMOElectronic reconstruction at LMO--SMO interfaceSMO interface
Urushibara et al., PRB (1994)Mn4+ (t2g3eg0)
t2g
eg
Mn3+ (t2g3eg1)
t2g
eg
Charge transferCharge transfer
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H. Nakao (PF-KEK) et al., JPSJ 78, 024602 (2009)
Resonant XResonant X--ray Scatteringray Scattering
LMO 2uc+SMO2uc SL
abruptuniform
Valence modulation may be very small.Interface roughness changes interfacial electronic states
Recent studyRecent study
Intrinsic electronic states at LMOIntrinsic electronic states at LMO--SMO interface ??SMO interface ??
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RH
EED
Int
ensi
ty
8006004002000Time (sec.)
LMO
SMO
(1) Substrate-choice (2) Improvement of Samples
LSAT: lattice-matching
STO:tensile-strain
LSAT3.87Å
(La,Sr)(Al,Ta)O3
interface
SrTiO33.905Å
LaMnO3(LMO)
SrMnO3(SMO)
Characterization with XRD
1μm
RHEED Oscillation
FeedbackFeedback
Strategy for highStrategy for high--quality superlattice synthesisquality superlattice synthesis
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Characterization with Synchrotron XRDCharacterization with Synchrotron XRD
BL4c , Photon Factory, KEK, Tsukuba, 2009 (Prof. Nakao)
Clear Laue Fringes
perfect structure
Bad superlattice
Good superlattice
RT• Sharp superlattice reflection • Laue Fringes until h=1.5
Modulation in superstructure
Good as films, but structural imperfection
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Impact of Improvement on Properties Impact of Improvement on Properties
Bad L2S2La0.5Sr0.5MnO3
= Ferro Metal
Good L2S2 = Insulator
L4S4, L5S5= Ferro InsulatorM, 1/ρ, Tc
Bad > Good
Bad- Good = Extrinsic FM
(interface rougness etc.)
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LaMnO3
InsulatorAlloy-like(Ferro. Metal)
SrMnO3
LaOMnO2
MnO2
SrO
LaO
MnO2SrO
interface
×
LaMnO3
SrMnO3
Under-dopedNearly 3+
Over-dopedNearly 4+
Mn Valence?
? Even in the short periodicity, electronic state is not uniform.
? Charge transfer alone cannot explain the properties.
Additional mechanism?Additional mechanism?
ShortShort--Period LMOPeriod LMO--SMO SLSMO SL
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Good L2S2
HH--Induced InsulatorInduced Insulator--toto--Metal Transition Metal Transition
FM
AFI
Metallic for H >7T : strong coupling with spin/orbital degrees of freedom
Alloy LSMO/LSAT
No large MR effect:
H//film Sr
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10-3
10-2
10-1
100
101
102
ρ (Ω
cm)
-10 -5 0 5 10Magnetic Field (T)
-2
-1
0
1
2
M (μ
B/M
n)
T=10 K
Hann1T
3T7T9T
Hann1T3T7T
zfc
Cr-doped Nd0.5Ca0.5MnO3
T. Kimura et al., PRL 83 (1999)
Magnetorelaxor
History of H-applicationaffects Msat and MR effect.
Phase Separation H-annealing effect
L2S2: MagneticL2S2: Magnetic--Field DependenceField Dependence
FM domain Short-range Orbital-Order (AFI)
Hann
Phase competition at LMOPhase competition at LMO--SMO interfaceSMO interface
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oneone--electron bandwidthelectron bandwidth
==
Phase diagram of perovskite MnPhase diagram of perovskite Mn--oxidesoxides
bibi--critical point: phase competitioncritical point: phase competition
Lattice distortion induces an electronic phase variationLattice distortion induces an electronic phase variation
Tomioka and Tokura, PRB 70, 014432 (2004)
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LMO 2uc < SMO 3uc
LMO 3uc > SMO 2ucFerromagnetic metal(TC=240K, M
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GroundGround--State Phase Diagram State Phase Diagram
Long-Period
2u.c
.
4u
.c. Sr
MnO
3
LaMnO3 layer 2u.c. 6u.c.
FI
Short-Period complicated ground-states
L2S2
L2S3
L3S2 FM
AFIL4S4
L5S5
bulk-like properties
LMO
M-I boundary Insulator (L2S2)Nearly-metal MetalH
CMR
Insulator
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ElectronElectron--doped CaMnOdoped CaMnO33
G-type AF InsulatorInsulator
Caspi et al., PRB 69, 104402 (2004)Tomioka et al., unpublished
MetalMetal--insulator transition with lightly electron dopinginsulator transition with lightly electron doping
Mn4+ (t2g3eg0)
t2g
eg
Ce4+
CaCa11--xxCeCexxMnOMnO33
cG-type AF MetalMetal
C-type AF InsulatorInsulator
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0 50 100 150 200 250 30010-3
10-2
10-1
100
101
102
CMO
0 T 5 T
Temperature (K)
Res
istiv
ity (Ω
cm)
0 50 100 150 200 250 30010-3
10-2
10-1
100
101
102
CCMO8
0 T 5 T
Temperature (K)
Res
istiv
ity (Ω
cm)
0 50 100 150 200 250 30010-3
10-2
10-1
0 T 3 T 5 T 9 T
Res
istiv
ity (Ω
cm)
Temperature (K)
CCMO5x=0 x=0.05 x=0.08
Substrate: (100)-oriented
Ca1-xCexMnO3 (CCMO): x = 0 – 0.08
NAO: NdAlO3 (aa = 0.3751 nm= 0.3751 nm, +0.3%)
aa (CCMO x=0.05) ~ 0.374 nm(CCMO x=0.05) ~ 0.374 nm 1 μm x 1 μm
Xiang et al., APL 94, 062109 (2009)
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KrF Laser
e-
RHEED
NAO
CaMnO3 Ca1-xCexMnO3(x=0.08)
0 50 100 150 200 250 30010-3
10-2
10-1
100
101
102
CMO
CCMO8 CMO CMO@5T CCMO8 CCMO8@5T
Temperature (K)
Res
istiv
ity (Ω
cm)
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[CMO([CMO(xx=0)=0)××mm/CMO(/CMO(xx=0) )=0) )××nn]]CC--AFIAFIGG--AFIAFI
C-AFI
G-AFI
C-AFI
G-AFI
Competition of different spin structuresCompetition of different spin structuresat interfacesat interfaces
Charge transfer +Charge transfer +
Phase separation?Phase separation?
+
interface
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まとめまとめ
強相関酸化物ヘテロ界面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・新機能界面電子相・新機能
今後今後
・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