1 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

2 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.

3 Band insulator

4 Diluted magnetic semiconductor (transition-metal doped band insulator) cluster-type model or Anderson impurity model Mizokawa and Fujimori, 1993

5 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.

6 Doped Mott insulator (transition-metal oxides)

x 2x Hartree-Fock calculation for a doped Mott insulator lower Hubband band upper Hubband band

8 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

9 photo-induced ferromagnetism in diluted magnetic semiconductors S. Koshihara et al., PRL 78, 4617 (1997)

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

11 [Fe(ptz) 6 )](BF 4 ) 2 ptz=1-propyltetrazole [Fe(2-pic)]Cl 3 ・ EtOH [Fe(Htrz) 3-3x (4-NH 2 trz) 3x ](ClO 4 ) ・ nH 2 O trz=triazole [Fe(Htrz) 3 ]-Nafion Fe 2+ 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)

12 Electron Analyzer Monochromator Cleaver Nd:YAG Laser JEOL JPS9200 Photon Energy: eV Energy Resolution: 500 meV Space Resolution: 30  m sample x-ray photoelectron laser

13 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 Ti 1-x Co x O 2 photo-induced electronic structural change in La 2-2x Sr 1+2x Mn 2 O 7 photo-induced melting of charge order in Cs 2 Au 2 Br 6

14 Photo-excited (photo-injected) carriers in VO 2 /TiO 2 and YBCO/STO Muraoka and Hiroi, 2002

15

16 Laser frequency dependence of photovoltage for 1 mJ/pulse

17 undoped (Mott insulator) hole-doped Hole injetion:  ~ 30 ms Photo-carrier injection in YBCO/STO

18 Photoemission study: Photo-excited (photo-injected) carriers in YBCO/STO photo-induced potential shift in Ti 1-x Co x O 2 photo-induced electronic structural change in La 2-2x Sr 1+2x Mn 2 O 7 photo-induced melting of charge order in Cs 2 Au 2 Br 6

19 Co2 ｐ CoO Co 2p XPS of Ti 1-x Co x O 2 LiCoO 2 high spin Co 2+ state just like CoO

20 Cluster model analysis of Co 2p XPS high spin Co 2+  = 4.0 eV U = 6.5 eV (pd  ) = -1.1 eV E A =  –7B+7C-W/2 ~ E G

21 O1s O 1s by Co doping in Ti 1-x Co x O 2 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.

22 Valence Valence band XPS of Ti 1-x Co x O 2 Co 3d impurity band grows within the band gap of TiO 2.

23 Band bending near surface can be reduced by photo-excited carriers. Effect of band bending Photoemission spectra are shifted and broadened by band bending.

24 O1s Co １０％ O1s Co ０％ Core-level shift induced by laser illumination in Ti 1-x Co x O 2 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

25 Origin of ferromagnetism in Ti 1-x Co x O 2 t 1 ~ -0.5 eV, t 2 ~ -0.5 eV E A ~ E G Exchange splitting: E ex ~ 0.3 eV t1t1 t2t2

26 Photoemission study: Photo-excited (photo-injected) carriers in YBCO/STO photo-induced potential shift in Ti 1-x Co x O 2 photo-induced electronic structural change in La 2-2x Sr 1+2x Mn 2 O 7 photo-induced melting of charge order in Cs 2 Au 2 Br 6

27 D. S. Dessau et al., Science 287, 767 (2000). La 2-2x Sr 1+2x Mn 2 O 7 x=0.4 M. Kubota et al., JPSJ 69, 1606 (2000) Phase competition at x=0.5

28 La 2-2x Sr 1+2x Mn 2 O 7 x=0.4 Ferromagmetic metal Spectral weight at E F is suppressed. Polaron formation? x=0.5 A-type antiferromagnetic state CE-type antiferromagnetic state Mn 3+ :Mn 4+ =1:1 at x=0.5 Mn 4+ Mn 3+ In going from x=0.4 to x=0.5: chemical potential shift by hole doping

29 photo-induced energy shift is enhanced for x=0.5 large electronic structural change related to the phase competition?

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

31 charge and orbital order in Cs 2 Au 2 Cl 6 and Cs 2 Au 2 Br 6 Photoemission data of Au 4f core level indicate that the charge order becomes stronger in going from Cs 2 Au 2 Br 6 to Cs 2 Au 2 Cl 6. Jahn-Teller type distortion of Au 3+ site is important to stabilize the charge order.

32 Photo-induced valence transition in Cs 2 Au 2 Br 6 Au 3+ Au + Charge order in Cs 2 Au 2 Cl 6 is very robust under strong illumination larger than 1 mJ/pulse. Charge order in Cs 2 Au 2 Br 6 can be destroyed by weak illumination. Au + + Au 3+ → 2Au 2+ The photo-induced change is enhanced at the surface region.

33 Photo-induced valence-band change in Cs 2 Au 2 Br 6 Valence band of Cs 2 Au 2 Br 6 is largely changed by the valence transition Au + + Au 3+ → 2Au 2+ induced by the light illumination. The spectral weight at the Fermi level is still very small even in the Au 2+ state.

34 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 Ti 1-x Co x O 2, the interaction between the localized high-spin Co 2+ 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 2-2x Sr 1+2x Mn 2 O 7 and Cs 2 Au 2 Br 6. While the Jahn-Teller systems are largely affected by the illumination, the dimer system is less sensitive to the illumination..