Browsing by Author "Wu, WB"

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Charge-orbital ordering and verwey transition in magnetite measured by resonant soft X-ray scattering
(2006-03-10)
Huang, DJ
;
Lin, HJ
;
Okamoto, J
;
Chao, KS
;
Jeng, HT
We report experimental evidence for the charge-orbital ordering in magnetite below the Verwey transition temperature T-V. Measurements of O K-edge resonant x-ray scattering on magnetite reveal that the O 2p states in the vicinity of the Fermi level exhibit a charge-orbital ordering along the c axis with a spatial periodicity of the doubled lattice parameter of the undistorted cubic phase. Such a charge-orbital ordering vanishes abruptly above T-V and exhibits a thermal hysteresis, correlating closely with the Verwey transition in magnetite.
Charge-orbital ordering of magnetite studied by soft resonant X-ray scattering
(2006-05-01)
Chao, KS
;
Huang, DJ
;
Lin, HJ
;
Okamoto, J
;
Hsu, CH
We report direct experimental evidence for the existence of charge and orbital ordering in magnetite. In particular, measurements of oxygen K-edge resonant X-ray scattering on magnetite reveal that the oxygen 2p states in the vicinity of the Fermi level exhibit a charge-orbital ordering with a spatial periodicity of the doubled lattice parameter of the undistorted cubic phase. (c) 2006 Published by Elsevier B.V.
Evolution of the yeast protein interaction network
(2003-10-28)
Qin, H
;
Lu, HHS
;
Wu, WB
;
Li, WH
;
統計學研究所
To study the evolution of the yeast protein interaction network, we first classified yeast proteins by their evolutionary histories into isotemporal categories, then analyzed the interaction tendencies within and between the categories, and finally reconstructed the main growth path. We found that two proteins tend to interact with each other if they are in the same or similar categories, but tended to avoid each other otherwise, and that network evolution mirrors the universal tree of life. These observations suggest synergistic selection during network evolution and provide insights into the hierarchical modularity of cellular networks.
Magnetic circular dichroism in Fe 2p resonant photoemission of magnetite
(2004-02-01)
Chen, J
;
Huang, DJ
;
Tanaka, A
;
Chang, CF
;
Chung, SC
Measurements of magnetic circular dichroism in Fe 2p resonant photoemission and calculations based on a full-multiplet cluster model reveal the parameters of the electronic structure of Fe3O4. We obtained on-site Coulomb energy U-dd of 3d electrons, charge-transfer energy Delta which is defined as the energy required to transfer an electron from O 2p to Fe 3d, and the hybridization strength between Fe 3d and O 2p at different Fe ion sites of Fe3O4 by analyzing the data of magnetic circular dichroism. The charge-transfer energy of Fe3+ ions was found to be significantly smaller than that of Fe2+ ions. We determined the energy positions of the low-lying multiplets of each Fe ion site. These energy positions for Fe3+ ion sites are 1.5-2.5 eV higher than those obtained from band-structure calculations. Our calculations also show that the photoemission spectral weight in binding energy bigger than 10 eV is substantial, in contrast to the results of band-structure calculations. Our findings conclude that Fe3O4 is a system with strong electron-electron interactions.
Orbital moments of CrO2 and Fe3O4 studied by MCD in soft X-ray absorption
(2004-07-01)
Huang, DJ
;
Chang, CF
;
Chen, J
;
Lin, HJ
;
Chung, SC
We present studies of orbital magnetic moments of CrO2 and Fe3O4 by measuring magnetic circular dichroism (MCD) in soft X-ray absorption. The results show that Fe3O4 exhibits large orbital moments, as a consequence of the strong Coulomb interactions of Fe 3d electrons. In contrast, orbital moments of Cr in CrO2 are nearly quenched, revealing that Cr 3d electrons are strongly hybridized with O 2p electrons and more delocalized as compared with those of Fe in Fe3O4. Comparing the MCD data with the band structure calculations based on local spin density approximation with on-site Coulomb energy U taken into account, we conclude that to include the on-site Coulomb interactions of 3d electrons is essential for adequately describing the electronic structure of CrO2 and Fe3O4. (C) 2004 Elsevier B.V. All rights reserved.
Orbital ordering in La0.5Sr1.5MnO4 studied by soft x-ray linear dichroism
(2004-02-27)
Huang, DJ
;
Wu, WB
;
Guo, GY
;
Lin, HJ
;
Hou, TY
We found that the conventional model of orbital-ordering of 3x(2)-r(2)/3y(2)-r(2) type in the e(g) states of La0.5Sr1.5MnO4 is incompatible with measurements of linear dichroism in the Mn 2p-edge x-ray absorption, whereas these e(g) states exhibit predominantly cross-type orbital ordering of x(2)-z(2)/y(2)-z(2). LDA+U band-structure calculations reveal that such a cross-type orbital-ordering results from a combined effect of antiferromagnetic structure, Jahn-Teller distortion, and on-site Coulomb interactions.
Orbital polarization of LaSrMnO4 studied by soft X-ray linear dichroism
(2004-07-01)
Wu, WB
;
Huang, DJ
;
Guo, GY
;
Lin, H
;
Hou, TY
With polarization-dependent soft X-ray absorption, we determined the orbital character of occupied and the lowest unoccupied e, orbitals of prototype single-layered perovskite LaSrMnO4, Our measurements indicate that LaSrMnO4 exhibits strong orbital polarization in the e, states of Mn3+ sites, leading us to conclude that LaSrMnO4 has a "ferro-orbital" ordering of 3z(2)-r(2). We found that linear dichroism in soft X-ray absorption is an effective experimental method to identify the nature of orbital ordering in transition-metal oxides such as manganites. (C) 2004 Elsevier B.V. All rights reserved.
Orbital symmetry and electron correlation in NaxCoO2
(2005-04-15)
Wu, WB
;
Huang, DJ
;
Okamoto, J
;
Tanaka, A
;
Lin, HJ
Measurements of polarization-dependent soft x-ray absorption reveal that the electronic states determining the low-energy excitations of NaxCoO2 have predominantly a(1g) symmetry with significant O 2p character. In contrast to the prediction of band theory, doping-dependent O 1s x-ray absorption shows a large transfer of spectral weight, providing spectral evidence for strong electron correlations of the layered cobaltates. We also found that NaxCoO2 exhibits a charge-transfer electronic character rather than a Mott-Hubbard character.
Orbital symmetry and electron correlation in NaxCoO2
(2006-05-01)
Wu, WB
;
Huang, DJ
;
Okamoto, J
;
Lin, HJ
;
Chou, FC
We present measurements of polarization-dependent soft X-ray absorption on NaxCoO2. The results reveal that the electronic states determining the low-energy excitations of NaxCoO2 have predominantly a(1g) symmetry with significant O 2p character. A large transfer of spectral weight observed in O 1s X-ray absorption provides spectral evidence for strong electron correlations in NaxCoO2 with the Na doping higher than half. For the Na concentration less than half, our measurements of O 1s absorption suggest that Na2CoO2 exhibits band-like electronic character. (c) 2006 Published by Elsevier B.V.
Spin and orbital magnetic moments of Fe3O4
(2004-08-13)
Huang, DJ
;
Chang, CF
;
Jeng, HT
;
Guo, GY
;
Lin, HJ
We present measurements of the spin and orbital magnetic moments of Fe3O4 by using SQUID and magnetic circular dichroism in soft x-ray absorption. The measurements show that Fe3O4 has a noninteger spin moment, in contrast to its predicted half-metallic feature. Fe3O4 also exhibits a large unquenched orbital moment. Calculations using the local density approximation including the Hubbard U method and the configuration interaction cluster-model suggest that strong correlations and spin-orbit interaction of the 3d electrons result in the noninteger spin and large orbital moments of Fe3O4.