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. 5 In this sense, it is very difficult to find a real Mott insulator.
Doped Mott insulator (transition-metal oxides) 6
Hartree-Fock calculation for a doped Mott insulator lower Hubband upper Hubband 1 1 1 -x 2 x 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)](BF 4)2 ptz=1 -propyltetrazole [Fe(2 -pic)]Cl 3・Et. OH [Fe(Htrz)3 -3 x(4 -NH 2 trz)3 x](Cl. O 4)・n. H 2 O trz=triazole [Fe(Htrz)3]-Nafion Fe 2+ low-spin (S=0) → high-spin (S=2) 11
JEOL JPS 9200 Photon Energy: 1486. 6 e. V Energy Resolution: 500 me. V Space Resolution: 30 mm Electron Analyzer photoelectron Cleaver x-ray laser sample Nd: YAG Laser Monochromator 12
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. Cox. O 2 • photo-induced electronic structural change in La 2 -2 x. Sr 1+2 x. Mn 2 O 7 • photo-induced melting of charge order in Cs 2 Au 2 Br 6 13
Photo-excited (photo-injected) carriers in VO 2/Ti. O 2 and YBCO/STO Muraoka and Hiroi, 2002 14
15
Laser frequency dependence of photovoltage for 1 m. J/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 Ti 1 -x. Cox. O 2 • photo-induced electronic structural change in La 2 -2 x. Sr 1+2 x. Mn 2 O 7 • photo-induced melting of charge order in Cs 2 Au 2 Br 6 18
Co 2 p XPS of Ti 1 -x. Cox. O 2 Co 2ｐ Li. Co. O 2 high spin Co 2+ state just like Co. O 19
Cluster model analysis of Co 2 p XPS high spin Co 2+ D = 4. 0 e. V U = 6. 5 e. V (pds) = -1. 1 e. V EA = D – 7 B+7 C-W/2 ~ EG 20
O 1 s by Co doping in Ti 1 -x. Cox. O 2 O 1 s Energy shift of 0. 6 e. V 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 Ti 1 -x. Cox. O 2 Valence Co 3 d impurity band grows within the band gap of Ti. O 2. 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 Ti 1 -x. Cox. O 2 O 1 s Co １０％ Energy shift of 0. 3 e. V for x=0. 05 and 0. 1: reduction of band bending Exchange splitting: 0. 3 e. V No energy shift for x=0 O 1 s Co ０％ 24
Origin of ferromagnetism in Ti 1 -x. Cox. O 2 t 1 t 1 ~ -0. 5 e. V, t 2 ~ -0. 5 e. V EA ~ EG Exchange splitting: Eex ~ 0. 3 e. V 25
Photoemission study: • Photo-excited (photo-injected) carriers in YBCO/STO • photo-induced potential shift in Ti 1 -x. Cox. O 2 • photo-induced electronic structural change in La 2 -2 x. Sr 1+2 x. Mn 2 O 7 • photo-induced melting of charge order in Cs 2 Au 2 Br 6 26
La 2 -2 x. Sr 1+2 x. Mn 2 O 7 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). 27
La 2 -2 x. Sr 1+2 x. Mn 2 O 7 x=0. 4 Ferromagmetic metal Spectral weight at EF 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 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 Cs 2 Au 2 Cl 6 and Cs 2 Au 2 Br 6 Jahn-Teller type distortion of Au 3+ site is important to stabilize the charge order. Photoemission data of Au 4 f 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. 31
Photo-induced valence transition in Cs 2 Au 2 Br 6 Charge order in Cs 2 Au 2 Cl 6 is very robust under strong illumination larger than 1 m. J/pulse. Au+ Au 3+ Charge order in Cs 2 Au 2 Br 6 can be destroyed by weak illumination. Au+ + Au 3+ → 2 Au 2+ The photo-induced change is enhanced at the surface region. 32
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+ → 2 Au 2+ induced by the light illumination. The spectral weight at the Fermi level is still very small even in the Au 2+ 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 Ti 1 -x. Cox. O 2 , the interaction between the localized high-spin Co 2+ state and the itinerant Ti 3 d 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 -2 x. Sr 1+2 x. 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. 34
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