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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

17 June 2005, 基盤 A 研究会

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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.

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Band insulator

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Diluted magnetic semiconductor (transition-metal doped band insulator)

cluster-type model or Anderson impurity model Mizokawa and Fujimori, 1993

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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.

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Doped Mott insulator(transition-metal oxides)

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1 1

1-x 1-x2x

Hartree-Fock calculation for a doped Mott insulator

lower Hubband band

upper Hubband band

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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

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photo-induced ferromagnetism in diluted magnetic semiconductors

S. Koshihara et al., PRL 78, 4617 (1997)

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photo-induced melting of charge disproportionation X. J. Liu et al., PRB 61, 20 (2000)

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[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)

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)

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Electron Analyzer

Monochromator

Cleaver

Nd:YAG Laser

JEOL JPS9200Photon Energy: 1486.6 eVEnergy Resolution: 500 meVSpace Resolution: 30 m

sample

x-ray

photoelectron

laser

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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

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Photo-excited (photo-injected) carriers in VO2/TiO2 and YBCO/STO

Muraoka and Hiroi, 2002

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Laser frequency dependence of photovoltage for 1 mJ/pulse

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undoped(Mott insulator)

hole-dopedHole injetion: ~ 30 ms

Photo-carrier injection in YBCO/STO

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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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Inte

nsi

ty

800 795 790 785 780

Binding Energy [eV]

Co 5% Co 10%

Co2ｐ

CoO

Co 2p XPS of Ti1-xCoxO2

LiCoO2

high spin Co2+ state just like CoO

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Cluster model analysis of Co 2p XPS

high spin Co2+

= 4.0 eVU = 6.5 eV(pd) = -1.1 eV

EA = –7B+7C-W/2 ~ EG

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Inte

nsi

ty

536 534 532 530 528 526 524 522

Binding Energy [eV]

Co 0% Co 5% Co 10%

O1s

O 1s by Co doping in Ti1-xCoxO2

Energy shift of 0.6 eV between x=0 and x=0.10 is probably due toband bending and/or exchange splitting of the conduction band.

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Inte

nsit

y

10 8 6 4 2 0 -2

Binding Energy [eV]

Co 0% Co 5% Co 10%

Valence

Valence band XPS of Ti1-xCoxO2

Co 3d impurity band grows within the band gap of TiO2.

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Band bending near surface can be reduced by photo-excited carriers.

Effect of band bending

Photoemission spectra are shifted and broadened by band bending.

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Inte

nsit

y

536 534 532 530 528 526 524 522Binding Energy [eV]

no UV UV UV-cut-1 UV-cut-2

O1s

Co １０％

Inte

nsi

ty

536 534 532 530 528 526 524 522

Binding Energy [eV]

no UV UV UV-cutO1s

Co ０％

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

Exchange splitting: 0.3 eV

No energy shift for x=0

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Origin of ferromagnetism in Ti1-xCoxO2

t1 ~ -0.5 eV, t2 ~ -0.5 eVEA ~ EG

Exchange splitting:Eex ~ 0.3 eV

GA

GA

GAGA

ex

EExt

EExt

xtxt

H

EE

t

EE

txE

pdpdt

ddt

02

02

220

44

))((

)(

2

1

21

2

2

2

1

2

1

t1

t2

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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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D. S. Dessau et al., Scienc

e 287, 767 (2000).

La2-2xSr1+2xMn2O7 x=0.4

M. Kubota et al., JPSJ 69,

1606 (2000).

0.3 0.5

Phase competition at x=0.5

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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

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photo-induced energy shift is enhanced for x=0.5

large electronic structural changerelated to the phase competition?

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Photo-induced melting of charge disproportionation X. J. Liu et al., PRB 61, 20 (2000)

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charge and orbital order in Cs2Au2Cl6 and Cs2Au2Br6

Photoemission data of Au 4f core level indicate that the charge order becomes stronger in going from Cs2Au2Br6 to Cs2Au2Cl6.

Jahn-Teller type distortion of Au3+ site i

s important to stabilize the charge order.

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Photo-induced valence transition in Cs2Au2Br6

Au3+

Au+

Charge order in Cs2Au2Cl6 is very robust under strong illumination larger than 1 mJ/pulse.

Charge order in Cs2Au2Br6 can be destroyed by weak illumination.

Au+ + Au3+ → 2Au2+

The photo-induced change is enhanced at the surface region.

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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 levelis still very small even in the Au2+ state.

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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 La

2-2xSr1+2xMn2O7 and Cs2Au2Br6. While the Jahn-Teller systems are largely affected by the illum

ination, the dimer system is less sensitive to the illumination.

• .