Browsing by Author "Chung, SC"

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Anomalous spin polarization and dualistic electronic nature of CrO(2)
(2003-06-01)
Huang, DJ
;
Tjeng, LH
;
Chen, J
;
Chang, CF
;
Wu, WP
Spin-resolved O 1s x-ray absorption measurements on CrO(2) reveal that the spin polarization of the states at the Fermi level approaches 100%, providing direct evidence of the half-metallic nature of CrO(2). The measurements also show the existence of an atomiclike Cr 3d state not far away from the Fermi level with a spin polarization of only 50%, establishing its Mott-Hubbard character. We conclude that CrO(2) has a dualistic electronic nature, in which the states at the Fermi level are bandlike, while those at higher energies are strongly localized of completely different origin and symmetry. The finding of an exceptionally large number of O 2p holes suggests their important role in preventing the strongly correlated CrO(2) from being an insulator.
Anomalous spin polarization and dualistic electronic nature of CrO2
(2003-06-01)
Huang, DJ
;
Tjeng, LH
;
Chen, J
;
Chang, CF
;
Wu, WP
Spin-resolved O 1s x-ray absorption measurements on CrO2 reveal that the spin polarization of the states at the Fermi level approaches 100%, providing direct evidence of the half-metallic nature of CrO2. The measurements also show the existence of an atomiclike Cr 3d state not far away from the Fermi level with a spin polarization of only 50%, establishing its Mott-Hubbard character. We conclude that CrO2 has a dualistic electronic nature, in which the states at the Fermi level are bandlike, while those at higher energies are strongly localized of completely different origin and symmetry. The finding of an exceptionally large number of O 2p holes suggests their important role in preventing the strongly correlated CrO2 from being an insulator.
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 magnetic moments of oxygen and chromium in CrO2
(2002-11-01)
Huang, DJ
;
Jeng, HT
;
Chang, CF
;
Guo, GY
;
Chen, J
With measurements of the magnetic circular dichroism and band-structure calculations in the local spin density approximation (LSDA+U) approach, we revealed that the orbital moment of O in CrO2 is ferromagnetically coupled to that of Cr, whereas the spin moment of O is antiferromagnetically coupled to that of Cr. Spin and orbital magnetic moments of Cr and O are enhanced as Coulomb interaction energy U increases. Comparing the magnetic circular diachromism data with the LSDA+U calculations, we conclude that it is essential to include the on-site Coulomb energy for adequately describing the orbital magnetic moments of CrO2.
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.
Performance of a Mott detector for undulator-based spin-resolved spectroscopy
(2002-11-01)
Huang, DJ
;
Wu, WP
;
Chen, J
;
Chang, CF
;
Chung, SC
To measure spin-polarized core-level electron spectra, a spectrometer equipped with a highly efficient retarding-potential Mott spin polarimeter using undulator-based soft-x-ray beamlines has been set up. With a thin film of Au as a target this polarimeter has an efficiency estimated to be similar to2x10(-4). The performance of this system for spin-polarized spectroscopy has been tested using core-level spin-polarized photoemission of magnetic and nonmagnetic thin films excited with linearly and circularly polarized light, respectively. Measurements using a new spin-resolved absorption technique are also discussed. (C) 2002 American Institute of Physics.
Photoemission and photoabsorption study of C-60 adsorption on Cu(111) surfaces
(1997-12-15)
Tsuei, KD
;
Yuh, JY
;
Tzeng, CT
;
Chu, RY
;
Chung, SC
We have carried out an extensive study of C-60 adsorption on Cu(111) surfaces using low-energy electron diffraction, photoemission, and x-ray-absorption spectroscopy. It is found that in valence-band photoemission a state forms right below the Fermi energy for an annealed, well-ordered monolayer, similar to the case of K-doped C-60. This peak disperses across the Fermi energy at off normal emission geometry. The spectra of carbon core-level photoemission show that the line shape is highly asymmetric with a metalliclike tail. The carbon near-edge absorption spectra show that the lowest unoccupied molecular orbital (LUMO) is attenuated, and a clear Fermi edge jump appears at the absorption onset. This evidence indicates that charge transfers from the substrate to the C-60 molecular orbitals and the overlayer becomes metallic. The amount of charge transfer can be determined to be 1.5-2 electrons per molecule from both the area of the occupied LUMO in photoemission and the peak shift in near-edge absorption spectra. It has been reported that many metal surfaces with originally different work functions covered by a monolayer of C-60 have a similar work function of about 5 eV. We suggest that the measured work functions are due to the metallic C-60 overlayers and are similar regardless of the metal substrates. This is in line with the reported alignment of monolayer energy levels to substrate Fermi energy. Since the work functions are similar, the energy levels with respect to the vacuum level are also similar. Finally we compare near-edge x-ray-absorption with inverse photoemission spectroscopy to address the screening effects.