Browsing by Author "Chen, J. M."

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Anisotropic electronic structure in single crystalline orthorhombic TbMnO(3) thin films
(2010-02-01)
Wu, K. H.
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Gou, I. C.
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Luo, C. W.
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Uen, T. M.
;
Lin, J. -Y.
We have deposited the c-axis-oriented orthorhombic TbMnO(3) (o-TMO) films with well-aligned in-plane orientations on NdGaO(3) (001) substrates by using pulsed laser deposition. The distinctive orientation alignments between the film and substrate allow the study of the X-ray absorption spectroscopy (XAS) with the electric field along three major crystallographic directions, respectively. Polarization-dependent XAS spectra show significant anisotropy in the electronic Structure of o-TMO. The correlation between the electronic structure, the bonding anisotropy, and the magnetoelectric effect in the multiferroic materials is revealed. (C) 2009 Elsevier B.V. All rights reserved.
Anisotropic electronic structure in single crystalline orthorhombic TbMnO3 thin films
(2010-02-01)
Wu, K. H.
;
Gou, I. C.
;
Luo, C. W.
;
Uen, T. M.
;
Lin, J. -Y.
We have deposited the c-axis-oriented orthorhombic TbMnO3 (o-TMO) films with well-aligned in-plane orientations on NdGaO3 (001) substrates by using pulsed laser deposition. The distinctive orientation alignments between the film and substrate allow the study of the X-ray absorption spectroscopy (XAS) with the electric field along three major crystallographic directions, respectively. Polarization-dependent XAS spectra show significant anisotropy in the electronic Structure of o-TMO. The correlation between the electronic structure, the bonding anisotropy, and the magnetoelectric effect in the multiferroic materials is revealed. (C) 2009 Elsevier B.V. All rights reserved.
Bonding anisotropy in multiferroic TbMnO(3) probed by polarization dependent x-ray absorption spectroscopy
(2009-01-26)
Chen, J. M.
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Lee, J. M.
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Chen, C. K.
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Chou, T. L.
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Lu, K. T.
O K- and Mn L(2,3)-edges x-ray absorption spectra of orthorhombic TbMnO(3) single crystals show strong polarization dependence, in contrast with results of previous experiments on orthorhombic LaMnO(3) and DyMnO(3) thin films that show nearly isotropic spectral structure. First-principles calculations reveal that TbMnO(3) exhibits a zigzag e(g) orbital ordering ground state. The highly distorted MnO(6) octahedron and orbital ordering in TbMnO(3) produce highly anisotropic Mn-O bonding within the ab plane, a frustration of the magnetic ordering, and the formation of complicated incommensurate magnetic structures.
Bonding anisotropy in multiferroic TbMnO3 probed by polarization dependent x-ray absorption spectroscopy
(2009-01-26)
Chen, J. M.
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Lee, J. M.
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Chen, C. K.
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Chou, T. L.
;
Lu, K. T.
O K- and Mn L-2,L-3-edges x-ray absorption spectra of orthorhombic TbMnO3 single crystals show strong polarization dependence, in contrast with results of previous experiments on orthorhombic LaMnO3 and DyMnO3 thin films that show nearly isotropic spectral structure. First-principles calculations reveal that TbMnO3 exhibits a zigzag e(g) orbital ordering ground state. The highly distorted MnO6 octahedron and orbital ordering in TbMnO3 produce highly anisotropic Mn-O bonding within the ab plane, a frustration of the magnetic ordering, and the formation of complicated incommensurate magnetic structures.
Coherent acoustic phonons in hexagonal HoMnO(3) probed by femtosecond spectroscopy
(2010)
Luo, C. W.
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Shih, H. C.
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Wu, K. H.
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Lin, J-Y
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Juang, J. Y.
We have observed the coherent acoustic phonons in the hexagonal manganite HoMnO(3) single crystals using wavelength-dependent femtosecond pump-probe spectroscopy. The oscillatory component of the photoinduced transient reflectivity change (Delta R/R) at various pump-probe wavelengths is consistent with the model of strain pulse propagation. Furthermore, the oscillation period of the oscillation is strong temperature- and wavelength-dependent. In particular, the period of all oscillations in Delta R/R for various wavelengths are very similar in the vicinity of the Neel temperature (T(N)), which associates with the appearance of the antiferromagnetic (AFM) long-range ordering.
Core-level anionic photofragmentation of gaseous CCl(4) and solid-state analogs
(2009-09-01)
Lu, K. T.
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Chen, J. M.
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Lee, J. M.
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Haw, S. C.
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Chou, T. L.
The dissociation dynamics of anionic and excited neutral fragments of gaseous CCl(4) and CCl(4) adsorbed on Si(100) similar to 90 K following Cl 2p core-level excitations were investigated on combining measurements of photon-induced anionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The transitions of core electrons to high Rydberg states or doubly excited states near Cl 2p ionization thresholds of gaseous CCl(4) remarkably enhance the production of excited neutral fragments (C* and CCl*); this enhancement is attributed to the contribution from the shake-modified resonant Auger decay and/or postcollision interaction (PCI). The Cl-anion is significantly reinforced in the vicinity of the Cl 2p(1/2,3/2) ionization threshold of gaseous CCl(4), originating from PCI-mediated photoelectron recapture. The Cl 2p -> 7a(1)* excitation for CCl(4)/Si(100) at similar to 90 K enhances the Cl(-) desorption yield at a submonolayer level. This resonant enhancement of Cl-yield at the 7a(1)* resonance in the Cl 2p edge at a submonolayer level occurs through the formation of high-lying molecular-ion states of CCl(4) adsorbed on a Si surface.
Core-level anionic photofragmentation of gaseous CCl4 and solid-state analogs
(2009-09-01)
Lu, K. T.
;
Chen, J. M.
;
Lee, J. M.
;
Haw, S. C.
;
Chou, T. L.
The dissociation dynamics of anionic and excited neutral fragments of gaseous CCl4 and CCl4 adsorbed on Si(100) similar to 90 K following Cl 2p core-level excitations were investigated on combining measurements of photon-induced anionic dissociation, x-ray absorption, and uv-visible dispersed fluorescence. The transitions of core electrons to high Rydberg states or doubly excited states near Cl 2p ionization thresholds of gaseous CCl4 remarkably enhance the production of excited neutral fragments (C* and CCl*); this enhancement is attributed to the contribution from the shake-modified resonant Auger decay and/or postcollision interaction (PCI). The Cl-anion is significantly reinforced in the vicinity of the Cl 2p(1/2,3/2) ionization threshold of gaseous CCl4, originating from PCI-mediated photoelectron recapture. The Cl 2p -> 7a(1)* excitation for CCl4/Si(100) at similar to 90 K enhances the Cl- desorption yield at a submonolayer level. This resonant enhancement of Cl-yield at the 7a(1)* resonance in the Cl 2p edge at a submonolayer level occurs through the formation of high-lying molecular-ion states of CCl4 adsorbed on a Si surface.
Dissociation dynamics of anionic and excited neutral fragments of gaseous SiCl(4) following Cl 2p and Si 2p core-level excitations
(2008-10-08)
Chen, J. M.
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Lu, K. T.
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Lee, J. M.
;
Chou, T. L.
;
Chen, H. C.
The state-selective dissociation dynamics for anionic and excited neutral fragments of gaseous SiCl(4) following Cl 2p and Si 2p core-level excitations were characterized by combining measurements of the photon-induced anionic dissociation, x-ray absorption and UV/visible dispersed fluorescence. The transitions of core electrons to high Rydberg states) doubly excited states in the vicinity of both Si 2p and Cl 2p ionization thresholds of gaseous SiCl4 lead to a remarkably enhanced production of anionic, Si-and Cl(-), fragments and excited neutral atomic, Si*, fragments. This enhancement via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of the contributions from the Auger decay of doubly excited states, shake-modified resonant Auger decay, or/and post-collision interaction. These complementary results provide insight into the state-selective anionic and excited neutral fragmentation of gaseous molecules via core-level excitation.
Dissociation dynamics of anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations
(2008-10-08)
Chen, J. M.
;
Lu, K. T.
;
Lee, J. M.
;
Chou, T. L.
;
Chen, H. C.
The state-selective dissociation dynamics for anionic and excited neutral fragments of gaseous SiCl4 following Cl 2p and Si 2p core-level excitations were characterized by combining measurements of the photon-induced anionic dissociation, x-ray absorption and UV/visible dispersed fluorescence. The transitions of core electrons to high Rydberg states) doubly excited states in the vicinity of both Si 2p and Cl 2p ionization thresholds of gaseous SiCl4 lead to a remarkably enhanced production of anionic, Si-and Cl-, fragments and excited neutral atomic, Si*, fragments. This enhancement via core-level excitation near the ionization threshold of gaseous SiCl4 is explained in terms of the contributions from the Auger decay of doubly excited states, shake-modified resonant Auger decay, or/and post-collision interaction. These complementary results provide insight into the state-selective anionic and excited neutral fragmentation of gaseous molecules via core-level excitation.
Dissociation dynamics of positive-ion and negative-ion fragments of gaseous and condensed Si(CH3)(2)Cl-2 via Si 2p, Cl 2p, and Cl 1s core-level excitations
(2006-12-07)
Chen, J. M.
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Lu, K. T.
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Lee, J. M.
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Chen, C. K.
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Haw, S. C.
The state-selective positive-ion and negative-ion dissociation pathways of gaseous and condensed Si(CH3)(2)Cl-2 following Cl 2p, Cl 1s, and Si 2p core-level excitations have been characterized. The excitations to a specific antibonding state (15a(1)(*) state) of gaseous Si(CH3)(2)Cl-2 at the Cl 2p, Cl 1s, and Si 2p edges produce significant enhancement of fragment ions. This ion enhancement at specific core-excited states correlates closely with the ion kinetic energy distribution. The results deduced from ion kinetic energy distribution are consistent with results of quantum-chemical calculations on Si(CH3)(2)Cl-2 using the ADF package. The Cl- desorption yields for Si(CH3)(2)Cl-2/Si(100) at similar to 90 K are notably enhanced at the 15a(1)(*) resonance at both Cl 2p and Si 2p edges. The resonant enhancement of Cl- yield occurs through the formation of highly excited states of the adsorbed molecules. These results provide insight into the state-selective ionic fragmentation of molecules via core-level excitation. (c) 2006 American Institute of Physics.
Doping evolution of Zhang-Rice singlet spectral weight: A comprehensive examination by x-ray absorption spectroscopy
(2013-10-31)
Chen, Y. -J.
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Jiang, M. G.
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Luo, C. W.
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Lin, J. -Y.
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Wu, K. H.
The total spectral weight S of the emergent low-energy quasiparticles in high-temperature superconductors (HTSCs) is explored by x-ray absorption spectroscopy. In order to examine the applicability of the Hubbard model, regimes that cover from zero doping to overdoping are investigated. In contrast to results of mean-field theory, we found that S deviates from linear dependence on the doping level p. The slope of S versus p changes continuously throughout the whole doping range with no sign of saturation up to p = 0.23. Therefore, the picture of the Zhang-Rice singlet remains intact within the most prominent doping regimes of HTSCs.
Effect of Mn impurities on the superconductivity in NaxCoO2 center dot yH(2)O
(2007-09-01)
Chen, Y.-J.
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Liu, C.-J.
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Wang, J.-S.
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Lin, J.-Y.
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Sun, C. P.
To shed light on the symmetry of the superconducting order parameter in NaxCoO2 center dot yH(2)O, the Mn doping effects are studied. X-ray absorption spectroscopy verifies that the doped Mn impurities occupy the Co sites and are with a valance close to +4. Impurity scattering by Mn is in the unitary limit that, however, does not lead to strong T-c suppression. This absence of the strong impurity effects on T-c is inconsistent with the simple picture of a sign-changing order parameter. Coexistence of the s-wave superconducting order parameter and the nodal one is proposed to reconcile all the existing experiments and has been implied by the specific heat experiments.
Electronic structure of pyrochlore Cd(2)Re(2)O(7)
(2009-05-13)
Huang, Shih-Wen
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Jeng, Horng-Tay
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Lin, J-Y
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Chang, W. J.
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Chen, J. M.
Detailed band structure calculations have been performed for Cd(2)Re(2)O(7) in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorporated self-consistently in both the generalized gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U) approaches. It is found that the on-site U has negligible effects on the Re 5d band structures; therefore both the GGA and GGA + U Re 5d band energies agree well with the observed O K-edge x-ray absorption spectroscopy (XAS) spectrum, whereas the Cd 4d band energy observed from photoemission spectroscopy can only be correctly reproduced by GGA + U calculations, indicating the relatively itinerant Re 5d and localized Cd 4d electrons. On the other hand, the spin-orbit coupling gives rise to nontrivial spin and orbital magnetic moments for the middle-T phase. Most unexpectedly, we found that the low-T phase exhibits quasi-two-dimensional Fermi surfaces. The calculated carrier numbers for the three phases are, at least qualitatively, consistent with the measured Hall coefficient.
Electronic structure of pyrochlore Cd2Re2O7
(2009-05-13)
Huang, Shih-Wen
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Jeng, Horng-Tay
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Lin, J-Y
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Chang, W. J.
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Chen, J. M.
Detailed band structure calculations have been performed for Cd2Re2O7 in high-, middle- and low-temperature (T) phases. The calculations are based on the observed lattice structures from x-ray diffraction measurements. The spin-orbit interaction is incorporated self-consistently in both the generalized gradient approximation (GGA) and the GGA plus Hubbard U (GGA + U) approaches. It is found that the on-site U has negligible effects on the Re 5d band structures; therefore both the GGA and GGA + U Re 5d band energies agree well with the observed O K-edge x-ray absorption spectroscopy (XAS) spectrum, whereas the Cd 4d band energy observed from photoemission spectroscopy can only be correctly reproduced by GGA + U calculations, indicating the relatively itinerant Re 5d and localized Cd 4d electrons. On the other hand, the spin-orbit coupling gives rise to nontrivial spin and orbital magnetic moments for the middle-T phase. Most unexpectedly, we found that the low-T phase exhibits quasi-two-dimensional Fermi surfaces. The calculated carrier numbers for the three phases are, at least qualitatively, consistent with the measured Hall coefficient.
Fabrication and X-ray absorption NaxCoO2 spectroscopy in layered cobaltate thin films
(2007-03-01)
Chang, W. J.
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Lin, J.-Y.
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Chung, T. Y.
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Chen, J. M.
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Hsu, C.-H.
NaxCoO2 (x similar to 0.68) thin films were fabricated on sapphire (0 0 0 1) substrates via the lateral diffusion of sodium into Co3O4 (1 1 1) films, which were grown by pulsed-laser deposition. From the results of X-ray diffraction and in-plane resistivity rho(ab), the single phase and the metallic behaviors of these NaxCoO2 films can be identified. For the same sodium content x, rho(ab) is consistent with that of single crystals. In addition, the O 1s X-ray absorption near edge spectra of thin films are measured and compared with those of single crystals. (c) 2006 Elsevier B.V. All rights reserved.
High-resolution XANES study of Eu(Ba1-xRx)(2)Cu3O7+delta (R = Eu, Pr)
(2006-09-27)
Chen, J. M.
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Liu, R. S.
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Lee, J. M.
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T Lu, K.
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Yang, T. J.
We have probed the distribution of hole carriers in Eu(Ba1-xEux)(2) Cu3O7+delta (x = 0 - 0.2) and Eu(Ba1-xPrx)(2)Cu3O7+delta (x = 0 - 0.25) by O K-edge and Cu L-edge x-ray absorption spectra. Upon Eu and Pr substitution at the Ba site in Eu(Ba1-xRx)(2)Cu3O7+delta, the concentration of holes in the CuO2 planes becomes greatly diminished. The depletion rate of hole carriers within the CuO2 planes in Eu(Ba1-xPrx)(2)Cu3O7+delta is greater than that in Eu(Ba1-xEux)(2)Cu3O7+delta. The rate of T-c suppression with Pr doping in Eu(Ba1-xPrx)(2)Cu3O7+delta is accordingly greater than that in Eu(Ba1-xEux)(2)Cu3O7+delta.
Intra- and intersite electronic excitations in multiferroic TbMnO(3) probed by resonant inelastic x-ray scattering
(2010-09-27)
Chen, J. M.
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Lee, J. M.
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Huang, S. W.
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Lu, K. T.
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Jeng, H. T.
Mn 3d valence states and elementary electronic excitations in single-crystalline TbMnO(3) were probed with resonant inelastic x-ray scattering (RIXS) (or resonant x-ray emission spectroscopy, RXES), polarized x-ray absorption spectra and ab initio electron-structure calculations. Polarized Mn K-edge x-ray absorption spectra of TbMnO(3) crystals exhibit a significant anisotropy along three crystallographic directions, particularly for the white-line region. The 1s3p-RXES spectra obtained at the Mn K edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of 1s -> 3d transitions, indicating a relatively delocalized character of unoccupied Mn 3d states. The additional K beta emission profile at energy loss similar to 62 eV is attributed to the off-site dipole Mn 1s-Mn' 3d and/or Mn 1s-Tb 5d transitions, originating from hybridization between Mn 4p states and neighboring Mn' 3d/Tb 5d states. The off-site dipole transition makes a considerable contribution to the pre-edge region of Mn K-edge spectrum. Three prominent RIXS features at similar to 2.9, 7, and 11 eV were observed. Based on GGA+U calculations, the 7 eV region corresponds to transitions from O 2p states to unoccupied minority Mn 3d states, whereas the 11 eV band is ascribed to transitions from the O 2p band to the empty Tb 5d band. The broad 2.9 eV band is attributed to the coexistence of on-site Mn 3d-Mn 3d and off-site Mn 3d-Mn' 3d transitions.
Intra- and intersite electronic excitations in multiferroic TbMnO3 probed by resonant inelastic x-ray scattering
(2010-09-27)
Chen, J. M.
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Lee, J. M.
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Huang, S. W.
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Lu, K. T.
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Jeng, H. T.
Mn 3d valence states and elementary electronic excitations in single-crystalline TbMnO3 were probed with resonant inelastic x-ray scattering (RIXS) (or resonant x-ray emission spectroscopy, RXES), polarized x-ray absorption spectra and ab initio electron-structure calculations. Polarized Mn K-edge x-ray absorption spectra of TbMnO3 crystals exhibit a significant anisotropy along three crystallographic directions, particularly for the white-line region. The 1s3p-RXES spectra obtained at the Mn K edge reveal that the quadrupolar Raman regime is restricted predominantly to below the pre-edge peak whereas the fluorescence regime starts from the first pre-edge peak of 1s -> 3d transitions, indicating a relatively delocalized character of unoccupied Mn 3d states. The additional K beta emission profile at energy loss similar to 62 eV is attributed to the off-site dipole Mn 1s-Mn' 3d and/or Mn 1s-Tb 5d transitions, originating from hybridization between Mn 4p states and neighboring Mn' 3d/Tb 5d states. The off-site dipole transition makes a considerable contribution to the pre-edge region of Mn K-edge spectrum. Three prominent RIXS features at similar to 2.9, 7, and 11 eV were observed. Based on GGA+U calculations, the 7 eV region corresponds to transitions from O 2p states to unoccupied minority Mn 3d states, whereas the 11 eV band is ascribed to transitions from the O 2p band to the empty Tb 5d band. The broad 2.9 eV band is attributed to the coexistence of on-site Mn 3d-Mn 3d and off-site Mn 3d-Mn' 3d transitions.
Local atomic and electronic structures and ferroelectric properties of PbZr(0.52)Ti(0.48)O(3): An x-ray absorption study (vol 99, 042909, 2011)
(2011-11-28)
Ray, S. C.
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Hsueh, H. C.
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Wu, C. H.
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Pao, C. W.
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Asokan, K.
Local atomic and electronic structures and ferroelectric properties of PbZr0.52Ti0.48O3: An x-ray absorption study (vol 99, 042909, 2011)
(2011-11-28)
Ray, S. C.
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Hsueh, H. C.
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Wu, C. H.
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Pao, C. W.
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Asokan, K.