17 June 2005, 基盤A研究会
光電子分光でプローブする
遷移金属酸化物薄膜の光照射効果
Photo-induced phenomena in transition-metal thin films
probed by photoemission spectroscopy
T. Mizokawa, J.-Y. Son, J. Quilty, D. Asakura, T.-T. Tran, K. Takubo
PRESTO-JST & Graduate School of Frontier Sciences, University of Tokyo
H. Toyosaki, T. Fukumura, and M. Kawasaki
IMR, Tohoku University
Y. Muraoka and Z. Hiroi
ISSP, University of Tokyo
1
strongly-correlated electron systems
• Electron-electron and electron-lattice interactions provide correlated
ground states that cannot be described by Hartree-Fock method:
doped Mott insulators, diluted magnetic semiconductors, …
• Some correlated electron systems show competition between various
ordered states:
ferromagnetism, superconductivity, charge order, orbital order, …
• Dramatic response to external field is expected:
magnetic field, electric field, pressure, photo-excitation, …
In particular, surfaces of strongly correlated materials are unexplored.
2
Band insulator
3
Diluted magnetic semiconductor
(transition-metal doped band insulator)
cluster-type model or Anderson impurity model
Mizokawa and Fujimori, 1993
4
Mott insulator
(transition-metal oxides)
Insulating state with spin and orbital order can be described by Hartee-Fock method.
In this sense, it is very difficult to find a real Mott insulator.
5
Doped Mott insulator
(transition-metal oxides)
6
Hartree-Fock calculation for a doped Mott insulator
lower Hubband
band
upper Hubband
band
1
1
1-x
2x
1-x
7
Effect of photo-excitation in strongly correlated systems
• Photo-excited (photo-injected) carriers may induce phase transition:
photo-induced ferromagnetism in diluted magnetic semiconductors
• Photo-excitation may change local lattice distortion and destroy charge
and orbital order:
photo-induced metal-insulator transition of Mott insulators and
charge-ordered insulators
• Photo-excitation may change local electronic configuration:
photo-induced low-spin to high-spin transition in Mott insulators
8
photo-induced ferromagnetism in diluted magnetic semiconductors
S. Koshihara et al., PRL 78, 4617 (1997)
9
photo-induced melting of charge disproportionation
X. J. Liu et al., PRB 61, 20 (2000)
10
photo-induced spin state transition
S. Decurtins et al., CPL, 105, 1 (1984)
Y. Ogawa et al., PRL 84, 3181 (2000)
Y. Moritomo et al., JPSJ 71, 1015 (2002)
[Fe(ptz)6)](BF4)2
ptz=1-propyltetrazole
[Fe(2-pic)]Cl3・EtOH
[Fe(Htrz)3-3x(4-NH2trz)3x](ClO4)・nH2O trz=triazole
[Fe(Htrz)3]-Nafion
Fe2+
low-spin (S=0) → high-spin (S=2)
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JEOL JPS9200
Photon Energy: 1486.6 eV
Energy Resolution: 500 meV
Space Resolution: 30 mm
Electron Analyzer
photoelectron
x-ray
Cleaver
laser
sample
Nd:YAG Laser
Monochromator
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Photoemission study under light illumination:
Nd:YAG laser 532 nm and 355 nm
• Photo-excited (photo-injected) carriers in YBCO/STO
• photo-induced potential shift in Ti1-xCoxO2
• photo-induced electronic structural change in La2-2xSr1+2xMn2O7
• photo-induced melting of charge order in Cs2Au2Br6
13
Photo-excited (photo-injected) carriers in VO2/TiO2 and YBCO/STO
Muraoka and Hiroi, 2002
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15
Laser frequency dependence of photovoltage for 1 mJ/pulse
16
Photo-carrier injection in YBCO/STO
hole-doped
Hole injetion: t ~ 30 ms
undoped
(Mott insulator)
17
Photoemission study:
• Photo-excited (photo-injected) carriers in YBCO/STO
• photo-induced potential shift in Ti1-xCoxO2
• photo-induced electronic structural change in La2-2xSr1+2xMn2O7
• photo-induced melting of charge order in Cs2Au2Br6
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Co 2p XPS of Ti1-xCoxO2
Co2ｐ
Intensity
Co 5%
Co 10%
800
795
790
785
780
LiCoO2
Binding Energy [eV]
high spin Co2+ state just like CoO
CoO
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Cluster model analysis of Co 2p XPS
high spin Co2+
D = 4.0 eV
U = 6.5 eV
(pds) = -1.1 eV
EA = D –7B+7C-W/2 ~ EG
20
O 1s by Co doping in Ti1-xCoxO2
O1s
Intensity
Co 0%
Co 5%
Co 10%
536
534
532
530
528
526
524
522
Binding Energy [eV]
Energy shift of 0.6 eV between x=0 and x=0.10 is probably due to
band bending and/or exchange splitting of the conduction band.
21
Valence band XPS of Ti1-xCoxO2
Intensity
Valence
Co 0%
Co 5%
Co 10%
10
8
6
4
2
0
-2
Binding Energy [eV]
Co 3d impurity band grows within the band gap of TiO2.
22
Effect of band bending
Photoemission spectra are shifted and broadened by band bending.
Band bending near surface can be reduced by photo-excited carriers.
23
Core-level shift induced by laser illumination in Ti1-xCoxO2
Energy shift of 0.3 eV for
x=0.05 and 0.1:
reduction of band bending
no UV
UV
UV-cut-1
UV-cut-2
Intensity
O1s
Co １０％
Exchange splitting: 0.3 eV
536
534
532
530
528
Binding Energy [eV]
524
522
no UV
UV
UV-cut
O1s
Intensity
526
No energy shift for x=0
Co ０％
536
534
532
530
528
Binding Energy [eV]
526
524
522
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Origin of ferromagnetism in Ti1-xCoxO2
t1  (dds )
( pds )( pd )
D
2
2
 4t1
4t 2 
Eex  x 


 E A  EG E A  EG 
t2 
t2
t1
 0

H   2t1 x
 2t x
 2
2t1 x
E A  EG
0
2t 2 x 

0 
E A  EG 
t1 ~ -0.5 eV, t2 ~ -0.5 eV
EA ~ EG
Exchange splitting:
Eex ~ 0.3 eV
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Photoemission study:
• Photo-excited (photo-injected) carriers in YBCO/STO
• photo-induced potential shift in Ti1-xCoxO2
• photo-induced electronic structural change in La2-2xSr1+2xMn2O7
• photo-induced melting of charge order in Cs2Au2Br6
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La2-2xSr1+2xMn2O7
0.3
x=0.4
0.5
Phase competition at x=0.5
M. Kubota et al., JPSJ 69,
1606 (2000).
D. S. Dessau et al., Science
287, 767 (2000).
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La2-2xSr1+2xMn2O7
x=0.4 Ferromagmetic metal
Spectral weight at EF is suppressed.
Polaron formation?
x=0.5 A-type antiferromagnetic state
CE-type antiferromagnetic state
Mn3+:Mn4+=1:1 at x=0.5
Mn4+
Mn3+
In going from x=0.4 to x=0.5:
chemical potential shift by hole doping
28
photo-induced energy shift is enhanced for x=0.5
large electronic structural change
related to the phase competition?
29
Photo-induced melting of charge disproportionation
X. J. Liu et al., PRB 61, 20 (2000)
30
charge and orbital order in Cs2Au2Cl6 and Cs2Au2Br6
Jahn-Teller type distortion of Au3+ site is
important to stabilize the charge order.
Photoemission data of Au 4f core
level indicate that the charge order
becomes stronger in going from
Cs2Au2Br6 to Cs2Au2Cl6.
31
Photo-induced valence transition in Cs2Au2Br6
Charge order in Cs2Au2Cl6 is very
robust under strong illumination
larger than 1 mJ/pulse.
Au+
Au3+
Charge order in Cs2Au2Br6 can be
destroyed by weak illumination.
Au+ + Au3+ → 2Au2+
The photo-induced change is
enhanced at the surface region.
32
Photo-induced valence-band change in Cs2Au2Br6
Valence band of Cs2Au2Br6 is largely
changed by the valence transition
Au+ + Au3+ → 2Au2+ induced by the
light illumination.
The spectral weight at the Fermi level
is still very small even in the Au2+ state.
33
Summary
• Nature of photo-excited (photo-injected) carriers in
YBCO/STO has been studied by the core level shift.
The life time of the injected holes is about 30 ms.
• In Ti1-xCoxO2 , the interaction between the localized
high-spin Co2+ state and the itinerant Ti 3d xy state
gives the ferromagnetism. Photo-excited carriers at
surface are trapped by the Co impurity in the depletion
layer and reduce the band bending.
• Photo-induced electronic structural change in correlated
systems with strong electron-lattice coupling has been
studied in La2-2xSr1+2xMn2O7 and Cs2Au2Br6.
While the Jahn-Teller systems are largely affected by
the illumination, the dimer system is less sensitive to
the illumination.
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