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T. Mizokawa , J.-Y. Son, J. Quilty, D. Asakura, T.-T. Tran, K. Takubo

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

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T. Mizokawa , J.-Y. Son, J. Quilty, D. Asakura, T.-T. Tran, K. Takubo

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  1. 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

  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)

  7. Hartree-Fock calculation for a doped Mott insulator lower Hubband band upper Hubband band 1 1 1-x 1-x 2x

  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. 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)

  12. 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

  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 Ti1-xCoxO2 • photo-induced electronic structural change in La2-2xSr1+2xMn2O7 • photo-induced melting of charge order in Cs2Au2Br6

  14. Photo-excited (photo-injected) carriers in VO2/TiO2 and YBCO/STO Muraoka and Hiroi, 2002

  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

  18. Co2ｐ Co 2p XPS of Ti1-xCoxO2 LiCoO2 high spin Co2+ state just like CoO CoO

  19. 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. 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 to band bending and/or exchange splitting of the conduction band.

  21. Valence Valence band XPS of Ti1-xCoxO2 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. O1s O1s Co １０％ 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

  24. Origin of ferromagnetism in Ti1-xCoxO2 t2 t1 t1~ -0.5 eV, t2 ~ -0.5 eV EA ~ EG Exchange splitting: Eex ~ 0.3 eV

  25. 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

  26. La2-2xSr1+2xMn2O7 x=0.4 0.3 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).

  27. 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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