Browsing by Author "Lin, HJ"

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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.
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.
Comparison of the electronic structures of AIN nanotips grown on p- and n-type Si substrates
(2005-12-07)
Chiou, JW
;
Tsai, HM
;
Pao, CW
;
Dong, CL
;
Chang, CL
At and N K-edge x-ray absorption near-edge structure (XANES), scanning photoelectron microscopy (SPEM) and x-ray emission measurements were performed on AIN nanotips grown on p- and n-type Si substrates (p-AIN and n-AIN). Features and intensities in the Al and N K-edge XANES spectra of these AIN nanotips overall are similar. In contrast, the intensities of the valence-band SPEM spectra of p-AIN are apparently larger than those of n-AIN, which indicates that the valence-band density of states of p-AIN exceeds that of n-AIN. This result may be related to the observed enhancement of field-emission intensity of AIN nanotips grown on the p-type Si substrate.
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.
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 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.
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.
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.