J-H Guo

Publications (78)95.5 Total impact

• Article: Electronic structure of the kagome staircase compounds Ni3V2O8 and Co3V2O8

  J. Laverock, B. Chen, A. R. H. Preston, K. E. Smith, N. R. Wilson, G. Balakrishnan, P. -A. Glans, J. -H. Guo
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  ABSTRACT: The electronic structure of the kagome staircase compounds, Ni3V2O8 and Co3V2O8, has been investigated using soft x-ray absorption, soft x-ray emission, and resonant inelastic x-ray scattering (RIXS). Comparison between the two compounds, and with first principles band structure calculations and crystal-field multiplet models, provide unique insight into the electronic structure of the two materials. Whereas the location of the narrow (Ni,Co) d bands is predicted to be close to EF, we experimentally find they lie deeper in the occupied O 2p and unoccupied V 3d manifolds, and determine their energy via measured charge-transfer excitations. Additionally, we find evidence for a dd excitation at 1.5 eV in Ni3V2O8, suggesting the V d states may be weakly occupied in this compound, contrary to Co3V2O8. Good agreement is found between the crystal-field dd excitations observed in the experiment and predicted by atomic multiplet theory.
  05/2013;
• Article: The band structure of WO3 and non-rigid-band behaviour in Na0.67WO3 derived from soft x-ray spectroscopy and density functional theory.

  B Chen, J Laverock, L F J Piper, A R H Preston, S W Cho, A Demasi, K E Smith, D O Scanlon, G W Watson, R G Egdell, P-A Glans, J-H Guo
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  ABSTRACT: The electronic structure of single-crystal WO3 and Na0.67WO3 (a sodium-tungsten bronze) has been measured using soft x-ray absorption and resonant soft x-ray emission oxygen K-edge spectroscopies. The spectral features show clear differences in energy and intensity between WO3 and Na0.67WO3. The x-ray emission spectrum of metallic Na0.67WO3 terminates in a distinct Fermi edge. The rigid-band model fails to explain the electronic structure of Na0.67WO3 in terms of a simple addition of electrons to the conduction band of WO3. Instead, Na bonding and Na 3s-O 2p hybridization need to be considered for the sodium-tungsten bronze, along with occupation of the bottom of the conduction band. Furthermore, the anisotropy in the band structure of monoclinic γ-WO3 revealed by the experimental spectra with orbital-resolved geometry is explained via density functional theory calculations. For γ-WO3 itself, good agreement is found between the experimental O K-edge spectra and the theoretical partial density of states of O 2p orbitals. Indirect and direct bandgaps of insulating WO3 are determined from extrapolating separations between spectral leading edges and accounting for the core-hole energy shift in the absorption process. The O 2p non-bonding states show upward band dispersion as a function of incident photon energy for both compounds, which is explained using the calculated band structure and experimental geometry.
  Journal of Physics Condensed Matter 04/2013; 25(16):165501. · 2.55 Impact Factor
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  Available from: ArXiv

  Article: Strain dependence of bonding and hybridization across the metal-insulator transition of VO2

  J. Laverock, L. F. J. Piper, A. R. H. Preston, B. Chen, J. McNulty, K. E. Smith, S. Kittiwatanakul, J. W. Lu, S. A. Wolf, P. -A. Glans, J. -H. Guo
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  ABSTRACT: Soft x-ray spectroscopy is used to investigate the strain dependence of the metal-insulator transition of VO2. Changes in the strength of the V 3d - O 2p hybridization are observed across the transition, and are linked to the structural distortion. Furthermore, although the V-V dimerization is well-described by dynamical mean-field theory, the V-O hybridization is found to have an unexpectedly strong dependence on strain that is not predicted by band theory, emphasizing the relevance of the O ion to the physics of VO2.
  02/2012;
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  Available from: Martin Magnuson

  Article: The electronic structure of poly(pyridine-2,5-diyl) investigated by soft x-ray absorption and emission spectroscopies

  M. Magnuson, L Yang, J-H Guo, C. Såthe, A. Agui, J. Nordgren, Y Luo, H. Ågren, N. Johansson, W. R. Salaneck, L. E. Horsburgh, A. P. Monkman
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  ABSTRACT: The electronic structure of the poly-pyridine conjugated polymer has been investigated by resonant and nonresonant inelastic X-ray scattering and X-ray absorption spectroscopies using synchrotron radiation. The measurements were made for both the carbon and nitrogen contents of the polymer. The analysis of the spectra has been carried out in comparison with molecular orbital calculations taking the repeat-unit cell as a model molecule of the polymer chain. The simulations indicate no significant differences in the absorption and in the non-resonant X-ray scattering spectra for the different isomeric geometries, while some isomeric dependence of the resonant spectra is predicted. The resonant emission spectra show depletion of the {\pi} electron bands in line with symmetry selection and momentum conservation rules. The effect is most vizual for the carbon spectra; the nitrogen spectra are dominated by lone pair n orbital emission of {\sigma} symmetry and are less frequency dependent.
  01/2012;
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  Available from: Martin Magnuson

  Article: Resonant inelastic soft-x-ray scattering spectra at the N1s and C1s edges of poly(pyridine-2,5-diyl)

  M. Magnuson, L Yang, J-H Guo, C. Såthe, A. Agui, J. Nordgren, Y Luo, H. Ågren, N. Johansson, W. R. Salaneck, L. E. Horsburgh, A. P. Monkman
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  ABSTRACT: Resonant inelastic scattering measurements of poly(pyridine-2,5-diyl) have been performed at the N1s and C1s edges using synchrotron radiation. For comparison, molecular orbital calculations of the spectra have been carried out with the repeat unit as a model molecule of the polymer chain. The resonant emission spectra show depletion of the p electron bands which is consistent with symmetry selection and momentum conservation rules. The depletion is most obvious in the resonant inelastic scattering spectra of carbon while the nitrogen spectra are dominated by lone pair n orbital emission of s symmetry and are less excitation energy dependent. By comparing the measurements to calculations an isomeric dependence of the resonant spectra is found giving preference to two of the four possible isomers in the polymer.
  01/2012;
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  Available from: Martin Magnuson

  Article: The electronic structure of polyaniline and doped phases studied by soft X-ray absorption and emission spectroscopies

  M. Magnuson, J-H Guo, S. M. Butorin, A. Agui, C. Såthe, J. Nordgren, A. P. Monkman
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  ABSTRACT: The electronic structure of the conjugated polymer, polyaniline, has been studied by resonant and nonresonant X-ray emission spectroscopy using synchrotron radiation for the excitation. The measurements were made on polyaniline and a few doped (protonated) phases for both the carbon and nitrogen contents. The resonant X-ray emission spectra show depletion of the {\pi} electron bands due to the selective excitation which enhances the effect of symmetry selection rules. The valence band structures in the X-ray emission spectra attributed to the {\pi} bands show unambiguous changes of the electronic structure upon protonation. By comparing to X-ray absorption measurements, the chemical bonding and electronic configuration is characterized.
  01/2012;
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  Available from: Martin Magnuson

  Article: Electronic structure investigation of CeB6 by means of soft X-ray scattering

  M. Magnuson, S. M. Butorin, J-H Guo, A. Agui, J. Nordgren, H Ogasawara, A. Kotani, T Takahashi, S. Kunii
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  ABSTRACT: The electronic structure of the heavy fermion compound CeB6 is probed by resonant inelastic soft X-ray scattering using photon energies across the Ce 3d and 4d absorption edges. The hybridization between the localized 4f orbitals and the delocalized valence-band states is studied by identifying the different spectral contributions from inelastic Raman scattering and normal fluorescence. Pronounced energy-loss structures are observed below the elastic peak at both the 3d and 4d thresholds. The origin and character of the inelastic scattering structures are discussed in terms of charge-transfer excitations in connection to the dipole allowed transitions with 4f character. Calculations within the single impurity Anderson model with full multiplet effects are found to yield consistent spectral functions to the experimental data.
  01/2012;
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  Available from: Martin Magnuson

  Article: Electronic structure investigation of CoO by means of soft X-ray scattering

  M. Magnuson, S. M. Butorin, J-H Guo, J. Nordgren
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  ABSTRACT: The electronic structure of CoO is studied by resonant inelastic soft X-ray scattering spectroscopy using photon energies across the Co 2p absorption edges. The different spectral contributions from the energy-loss structures are identified as Raman scattering due to d-d and charge-transfer excitations. For excitation energies close to the L3 resonance, the spectral features are dominated by quartet-quartet and quartet-doublet transitions of the 3d7 configuration. At excitation energies corresponding to the satellites in the Co 2p X-ray absorption spectrum of CoO, the emission features are instead dominated by charge-transfer transitions to the 3d8L-1 final state. The spectra are interpreted and discussed with the support of simulations within the single impurity Anderson model with full multiplet effects which are found to yield consistent spectral functions to the experimental data.
  01/2012;
• Article: Role of 3d electrons in the rapid suppression of superconductivity in the dilute V doped spinel superconductor LiTi2O4

  C L Chen, C L Dong, K Asokan, J L Chen, Y S Liu, J-H Guo, W L Yang, Y Y Chen, F C Hsu, C L Chang, M K Wu
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  ABSTRACT: The microscopic effects of V doping in LiTi2O4 have been poorly understood. The present study employs x-ray absorption near-edge structure (XANES) and resonant inelastic soft-x-ray scattering (RIXS) spectroscopy to understand the change in the electronic structure due to dilute V doping in spinel LiTi2O4 and the possible origin for the rapid suppression of superconductivity in these compounds. Results from the XANES spectra at Ti L and K edges and Ti L-RIXS show that Ti exists in a mixed-valence state and, with V doping, the unoccupied states of Ti in the t2g band increase. The rapid suppression of superconductivity is associated with the change in Ti 3d electrons and Ti–O hybridization.
  Superconductor Science and Technology 09/2011; 24(11):115007. · 2.66 Impact Factor
• Article: X-Ray spectra and electronic correlations of FeSe(1-x)Te(x).

  C L Chen, C L Dong, J L Chen, J-H Guo, W L Yang, C C Hsu, K W Yeh, T W Huang, B H Mok, T S Chan, J F Lee, C L Chang, S M Rao, M K Wu
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  ABSTRACT: Critical issues concerning emerging Fe-based superconductors include the degree of electron correlation and the origin of the superconductivity. X-Ray absorption spectra (XAS) and resonant inelastic X-ray scattering spectra (RIXS) of FeSe(1-x)Te(x) (x = 0-1) single crystals were obtained to study their electronic properties that relate to electron correlation and superconductivity. The linewidth of Fe L(2,3)-edges XAS of FeSe(1-x)Te(x) is narrower than that of Fe-pnictides, revealing the difference between their hybridization effects and localization character and those of other Fe-pnictides. While no significant differences exist between the Fe L-edge XAS and RIXS of FeSe(1-x)Te(x) and those of Fe-pnictides, Se K-edge and Te K-edge XAS exhibit substantial edge shift, suggesting that the superconductivity in an Fe-Se superconductor is strongly associated with the ligand states. A comparison of the Se K-edge and Te K-edge spectra reveals that the charge transfer may occur between Se and Te. Given the Coulomb interaction and the bandwidth, the spectral results indicate that FeSe(1-x)Te(x) is unlikely to be a weakly correlated system unlike the Fe-pnictides of the "1111" and "122" families. The spectral results further demonstrate that superconductivity in this class of Fe-based compounds is strongly associated with the ligand 4p hole state.
  Physical Chemistry Chemical Physics 09/2011; 13(34):15666-72. · 3.57 Impact Factor
• Article: X-Ray spectra and electronic correlations of FeSe1–xTex

  C. L. Chen, C. L. Dong, J. L. Chen, J.-H. Guo, W. L. Yang, C. C. Hsu, K. W. Yeh, T. W. Huang, B. H. Mok, T. S. Chan, J. F. Lee, C. L. Chang, S. M. Rao, M. K. Wu
  Physical Chemistry Chemical Physics 07/2011; · 3.57 Impact Factor
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  Available from: Martin Magnuson

  Article: Uranium oxides investigated by X-ray absorption and emission spectroscopies

  M. Magnuson, S. M. Butorin, L Werme, J. Nordgren, K. Ivanov, J-H Guo, D. K. Shuh
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  ABSTRACT: X-ray absorption and resonant X-ray emission measurements at the O 1s edge of the uranium oxides UO2, U3O8 and UO3 are presented. The spectral shapes of the O K{\alpha} X-ray emission spectra of UO3 exhibit significant excitation energy dependence, from an asymmetric to a symmetric form, which differs from those of UO2 and U3O8. This energy dependence is attributed to a significant difference in the oxygen-uranium hybridization between two different sites in the crystal structure of UO3. The spectral shapes of UO2 and U3O8 are also found to be different but without significant energy dependence. The experimental spectra of the valence and conduction bands of the uranium oxides are compared to the results of electronic structure calculations available in the literature.
  02/2011;
• Article: Soft X-Ray Spectroscopic Study of Dense Strontium-Doped Lanthanum Manganite Cathodes for Solid Oxide Fuel Cell Applications

  L. F.J. Piper, A. R.H. Preston, S.-W. Cho, A. DeMasi, B. Chen, J. Laverock, K. E. Smith, L. J. Miara, J. N. Davis, S. N. Basu, U. Pal, S. Gopalan, Laxmikant Saraf, Tiffany Kaspar, A. Y. Matsuura, P.-A. Glans, J.-H. Guo
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  ABSTRACT: The evolution of the Mn charge state, chemical composition, and electronic structure of (LSMO) cathodes during the catalytic activation of solid oxide fuel cell (SOFC) has been studies using X-ray spectroscopy of as-processed, exposed, and activated dense thin LSMO films. Comparison of O -edge and Mn -edge X-ray absorption spectra from the different stages of LSMO cathodes revealed that the largest change after the activation occurred in the Mn charge state with little change in the oxygen environment. Core-level X-ray photoemission spectroscopy and Mn resonant photoemission spectroscopy studies of exposed and as-processed LSMO determined that the SOFC environment ( ambient pressure of ) alone results in La deficiency (severest near the surface with Sr doping ) and a stronger contribution, leading to the increased insulating character of the cathode prior to activation. Meanwhile, O -edge X-ray absorption measurements support enrichment nearer the surface, along with the formation of mixed and/or passive and SrO species.
  Journal of The Electrochemical Society. 01/2011; 158(2):B99-B105.
• Source

  Available from: S. M. Rao

  Article: X-ray absorption spectroscopy (XAS) investigation of the electronic structure of superconducting FeSex single crystals

  C L Chen, S. M. Rao, C L Dong, J. L. Chen, T. W. Huang, B. H. Mok, M. C. Ling, W. C. Wang, C L Chang, T S Chan, J F Lee, J-H Guo, M. K. Wu
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  ABSTRACT: X-ray absorption spectroscopy (XAS) Fe K-edge spectra of the FeSex (x=1-0.8) single crystals cleaved in situ in vacuum reveal characteristic Fe 4sp states, a lattice distortion and the Se K-edge spectra point to a strong Fe 3d-Se 4p hybridization giving rise to itinerant charge carriers. A formal charge of ~1.8+ for Fe and ~2.2- for Se were evaluated from these spectra in the FeSex (x=0.88). The charge balance between Fe and Se is assigned itinerant electrons located in the Fe-Se hybridization bond. As x decreases the 4p hole count increases and a crystal structure distortion is observed that in turn causes the Fe separation in the ab plane change from 4p orbital to varying (modulating) coordination. Powder x-ray diffraction (XRD) measurements also show a slight increase in lattice parameters as x decreases (increasing Se deficiency).
  05/2010;
• Article: Electronic excitations in the correlated metal BaV(0.98)Ti(0.02)S(3) studied using resonant inelastic soft x-ray scattering.

  T Learmonth, P-A Glans, J-H Guo, M Greenblatt, K E Smith
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  ABSTRACT: Electronic excitations in the correlated metal BaTi(0.02)V(0.98)S(3) have been studied using resonant inelastic soft x-ray scattering at the V L edge. The intensities of the intra-atomic d-d(*) excitations and the elastic x-ray scattering feature are found to be temperature dependent, with the intensity increasing with decreasing temperature until saturation is reached near 100 K. The behavior of the spectral features is interpreted as evidence of a shift in the 3d electrons from more band-like states at higher temperature to more localized states at low temperature.
  Journal of Physics Condensed Matter 01/2010; 22(2):025504. · 2.55 Impact Factor
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  Available from: escholarship.org

  Article: Electronic structure of the 1D conductor K0.3MoO3 studied using resonant inelastic x-ray scattering and soft x-ray emission spectroscopy

  T Learmonth, P-A Glans, C McGuinness, L Plucinski, Y Zhang, J-H Guo, M Greenblatt, K E Smith
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  ABSTRACT: The electronic structure of the quasi-one-dimensional conductor K0.3MoO3 has been measured using high resolution resonant inelastic x-ray scattering and x-ray absorption spectroscopy. The results are compared to those for the related two-dimensional insulator α-MoO3. Features of the scattering from both oxides are observed and are explained in terms of the band momentum selectivity of the scattering process, allowing a comparison of the scattering data with recent band structure calculations.
  Journal of Physics Condensed Matter 09/2008; 20(41):415219. · 2.55 Impact Factor
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  Available from: arxiv.org

  Article: Observation of Spin Fluctuations in a High-Tc Parent Compound Using Resonant Inelastic X-ray Scattering

  B. Freelon, P. G. Medaglia, A. Tebano, G. Balestrino, K Okada, A. Kotani, F. Vernay, T. P. Devereaux, P-A Glans, T. Learmonth, K. E. Smith, A. L. D. Kilcoyne, B. Rude, I. Furtado, J-H Guo
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  ABSTRACT: We report the first observation of soft-x-ray scattering from spin fluctuations in a high-Tc parent compound. An antiferromagnetic charge transfer insulator, CaCuO2, was irradiated by Cu M-edge soft x-rays. Ultra-high resolution measurements of scattered intensity revealed magnon-magnon excitations, due to spin exchange scattering, as low-energy loss features. The process is analogous to optical Raman scattering. The spectra provide the first measurement of the two-magnon excitation energy and the antiferromagnetic exchange parameter in infinite-layer CaCuO2. The results reveal resonant inelastic soft x-ray scattering as a novel probe of the spin dynamics in cuprates.
  07/2008;
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  Available from: sinica.edu.tw

  Article: Effect of surface treatments on the electronic properties of ultra-nanocrystalline diamond films

  C L Dong, S S Chen, J W Chiou, Y Y Chen, J.-H Guo, H F Cheng, I N Lin, C L Chang
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  ABSTRACT: We present the soft x-ray spectroscopic study of the ultra-nanocrystalline diamond (UNCD) films with different surface treatments. The samples were prepared by means of microwave plasma enhanced chemical vapor deposition (MPECVD) and the different surface treatments are applied to alter their field emission properties. The electronic properties were subsequently examined by the soft x-ray absorption and x-ray emission spectroscopy at carbon 1s threshold. From the experimental results, there is no significant variation in electronic structure of oxygen-and hydrogen-plasma treated UNCD films. On the other hand, the biased treated UNCD film shows more remarkable change on the sp 2 and sp 3 states. The formation of sp 2 bonding and the reduction of sp 3 bonding are the consequence of the improved electron field emission properties.
  01/2008;
• Article: System for in situ studies of atmospheric corrosion of metal films using soft x-ray spectroscopy and quartz crystal microbalance.

  J Forsberg, L-C Duda, A Olsson, T Schmitt, J Andersson, J Nordgren, J Hedberg, C Leygraf, T Aastrup, D Wallinder, J-H Guo
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  ABSTRACT: We present a versatile chamber ("atmospheric corrosion cell") for soft x-ray absorption/emission spectroscopy of metal surfaces in a corrosive atmosphere allowing novel in situ electronic structure studies. Synchrotron x rays passing through a thin window separating the corrosion cell interior from a beamline vacuum chamber probe a metal film deposited on a quartz crystal microbalance (QCM) or on the inside of the window. We present some initial results on chloride induced corrosion of iron surfaces in humidified synthetic air. By simultaneous recording of QCM signal and soft x-ray emission from the corroding sample, correlation between mass changes and variations in spectral features is facilitated.
  Review of Scientific Instruments 09/2007; 78(8):083110. · 1.37 Impact Factor
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  Available from: Kousik Samanta

  Article: Structural and magnetic properties of (In1-xFex)2O3 (0.0 <x< 0.25) system: Prepared by gel combustion method

  O.D. Jayakumar, I.K. Gopalakrishnan, S.K. Kulshreshtha, A. Gupta, K.V. Rao, D.V. Louzguine-Luzgin, A. Inoue, P.-A. Glans, J.-H. Guo, K. Samanta, M.K. Singh, R.S. Katiyar
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  ABSTRACT: �In1−xFex�2O3 polycrystalline samples with x=�0.0, 0.05, 0.10, 0.15, 0.20, and 0.25� have been synthesized by a gel combustion method. Reitveld refinement analysis of x-ray diffraction data indicated the formation of single phase cubic bixbyite structure without any parasitic phases. This observation is further confirmed by high resolution transmission electron microscopy imaging, indexing of the selected-area electron diffraction patterns, x-ray absorption spectroscopy, and Raman Spectroscopy. dc magnetization studies as a function of temperature and field indicate that they are ferromagnetic with Curie temperature �TC� well above room temperature.
  Applied Physics Letters 07/2007; 91:052504. · 3.84 Impact Factor