Browsing by Author "Lee, J. F."

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Determination of the microstructure of Eu-treated ZnO nanowires by x-ray absorption
(2010-02-08)
Huang, W. L.
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Labis, J.
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Ray, S. C.
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Liang, Y. R.
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Pao, C. W.
X-ray absorption near-edge structure (XANES), extended x-ray absorption fine structures (EXAFS), and photoluminescence measurements were used to elucidate the microstructural and photoluminescence properties of ZnO nanowires (ZnO-NWs) that had been treated with Eu by thermal diffusion. The O K- and Eu L(3)-edge XANES and EXAFS spectra at the Zn K- and Eu L(3)-edge verified the formation of Eu(2)O(3)-like layer on the surface of ZnO-NWs. X-ray diffraction, XANES and EXAFS measurements consistently suggest the lack of substitutional doping of Eu ions at the Zn ion sites in the interior of ZnO-NWs. The clear sharp and intense emission bands in the range 610-630 nm of Eu-treated ZnO-NWs originated from the intra-4f transition of Eu ions in the Eu(2)O(3)-like surface layer.
Effect of geometry on the magnetic properties of CoFe2O4-PbTiO3 multiferroic composites
(2013)
Wang, B. Y.
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Wang, H. T.
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Singh, Shashi B.
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Shao, Y. C.
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Wang, Y. F.
In this study, X-ray absorption near-edge structure (XANES), extended X-ray absorption fine structure (EXAFS), X-ray magnetic circular dichroism (XMCD) and element-and site-specific magnetic hysteresis (ESMH) are used to elucidate the effect of geometry (0-3- and 2-2-type) on the magnetic properties of CoFe2O4-PbTiO3 (CFO-PTO) multiferroic composites by comparison with those of the reference CFO and PTO powders. Magnetic Co ions in CFO have been confirmed to be located at both the tetrahedral (A)- and octahedral (B)-sites. CFO retains its mixed-spinel structure as verified by the EXAFS, XMCD and ESMH measurements. ESMH measurements further demonstrate that the magnetic moments of Co2+ and Fe3+/Fe2+ cations at both the A-and B-sites in the composites are smaller than those of the CFO powder. The reduction of the magnetic moments in the 2-2-type composite was larger than that in the 0-3-type composite. The reduction of the magnetic moments in the composites was attributable to the formation of anti-phase boundaries owing to the compressive strain in CFO, which is the largest strain in the 2-2-type composite. Based on the Ti L-3,L-2-edge XMCD measurements of the CFO-PTO composites, no induced magnetic moment was observed at the Ti sites in the PTO matrix, excluding the possibility that the Ti ions in the PTO matrix affect the magnetic properties of these CFO-PTO composites.
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.
Effects of water intercalation in Na(x)CoO(2)center dot yH(2)O
(2009)
Lin, J. Y.
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Chen, Y. L.
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Hsu, Ben
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Liu, C. J.
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Tai, Y. L.
Water intercalation is necessary for the occurrence of superconductivity in NaxCoO(2) center dot yH(2)O. It is known that water insertion leads to the elongation of the c-axis. Beyond that, it remains controversial about the effects of water intercalation on the electronic structure of Na(x)CoO(2) center dot yH(2)O. In this paper, we report the Co-K edge spectroscopy on y = 0.7 and 1.4 samples, respectively. The spectra are compared with those of the reference samples. It is found that the variation of the Co valence can not be explained by the simple electron counting. These results are consistent with those of the previous chemical titration.
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.
Loycal atomic and electronic structures and ferroelectric properties of PbZr(0.52)Ti(0.48)O(3): An x-ray absorption study
(2011-07-25)
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.
This work investigates local atomic and electronic structures of PbZr(0.52)Ti(0.48)O(3) (PZT) thin films with < 001 >, < 101 >, and < 111 > orientations using extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectroscopy with theta = 0 degrees and 70 degrees incident angles. The EXAFS result indicates that the < 001 >-oriented PZT film has a polarization dominantly along the c-axis, while both < 101 >- and < 111 >-oriented PZT films have a dominant in-ab-plane polarization. The hysteresis-loop measurements show that the < 001 >-oriented PZT film has a much larger coercive field than those of other two PZT films, which indicates that the double-well potential along the c-axis is much deeper than that in the ab-plane. (C) 2011 American Institute of Physics. [doi:10.1063/1.3607475]
Loycal atomic and electronic structures and ferroelectric properties of PbZr0.52Ti0.48O3: An x-ray absorption study
(2011-07-25)
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.
This work investigates local atomic and electronic structures of PbZr0.52Ti0.48O3 (PZT) thin films with < 001 >, < 101 >, and < 111 > orientations using extended x-ray absorption fine structure (EXAFS) and x-ray absorption near-edge structure (XANES) spectroscopy with theta = 0 degrees and 70 degrees incident angles. The EXAFS result indicates that the < 001 >-oriented PZT film has a polarization dominantly along the c-axis, while both < 101 >- and < 111 >-oriented PZT films have a dominant in-ab-plane polarization. The hysteresis-loop measurements show that the < 001 >-oriented PZT film has a much larger coercive field than those of other two PZT films, which indicates that the double-well potential along the c-axis is much deeper than that in the ab-plane. (C) 2011 American Institute of Physics. [doi:10.1063/1.3607475]
Polaronic effect in the x-ray absorption spectra of La1-xCaxMnO3 manganites
(2019-05-15)
Huang, S. W.
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Liu, Y. T.
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Lee, J. M.
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Chen, J. M.
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Lee, J. F.
X-ray absorption spectroscopy (XAS) is performed to study changes in the electronic structures of colossal magnetoresistance (CMR) and charged ordered (CO) La1-xCaxMnO3 manganites with respect to temperature. The pre-edge features in O and Mn K-edge XAS spectra, which are highly sensitive to the local distortion of MnO6 octahedral, exhibit contrasting temperature dependence between CMR and CO samples. 'the seemingly counter-intuitive XAS temperature dependence can be reconciled in the context of polarons. These results help identify the most relevant orbital states associated with polarons and highlight the crucial role played by polarons in understanding the electronic structures of manganites.
Size dependence of the electronic structures and electron-phonon coupling in ZnO quantum dots
(2007-12-24)
Ray, S. C.
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Low, Y.
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Tsai, H. M.
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Pao, C. W.
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Chiou, J. W.
The electronic structures and optical properties of various sizes of ZnO quantum dots (QDs) were studied using x-ray absorption, photoluminescence, and Raman spectroscopy. The increase in the intensity ratio of the second-order Raman spectra of longitudinal optical mode and its fundamental mode, which is related to the strength of the electron-phonon coupling (EPC), is found to increase with the size of QD. The trend of EPC also correlates with the increase of the intensity ratio of the O 2p(pi) (I(pi)) and 2p(sigma) (I(sigma)) orbital features in the O K-edge x-ray absorption near-edge structure (XANES) as the size of QD increases. The EPC and XANES results suggest that the crystal orientations of ZnO QDs are approximately aligned with the c axis parallel with the polarization of x-ray photons. (c) 2007 American Institute of Physics.
The unconventional doping in YBa2Cu3O7-x/La0.7Ca0.3MnO3 heterostructures by termination control
(2017-01-16)
Tra, V. T.
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Huang, R.
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Gao, X.
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Chen, Y. -J.
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Liu, Y. T.
In strongly correlated oxides, heterostructures provide a powerful route to manipulate the charge, spin, orbital, and lattice degrees of freedom to create distinctive functionalities. In this work, we have achieved atomically precise interface control in YBa2Cu3O7-x/La0.7Ca0.3MnO3 (YBCO/LCMO) heterostructures and find a hidden effective doping. This mechanism is responsible for higher Tc in the sample with the MnO2-terminated interface than in that with the La0.7Ca0.3O-terminated interface. The MnO2-terminated sample also shows a larger magnetic moment of Mn together with a lower valence state. For more than a decade, the control of T-c in these heterostructures prior to this work has been solely via the variation of YBCO or LCMO thickness. This work hints at an alternative way of exploiting and exploring the interactions between superconductivity and magnetism in this system. Published by AIP Publishing.
Wide-Range Tuning of the Mo Oxidation State in La1-xSrxFe2/3Mo1/3O3 Perovskites
(2016-06)
Istomin, Sergey Ya.
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Chernova, Varvara V.
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Antipov, Evgeny V.
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Lobanov, Maxim V.
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Bobrikov, Ivan A.
La1-xSrxFe2/3Mo1/3O3 (0x1) perovskites allow exceptionally wide tuning of the Mo oxidation state from +3 (x = 0) to +6 (x = 1) with the Fe-III oxidation state virtually unchanged. The end members of this series show antiferromagnetic ordering in LaFe2/3Mo1/3O3 at T-N = 520 K and ferrimagnetic ordering in SrFe2/3Mo1/3O3 at T-C = 420 K. The crucial role of electron correlations in the Mo 4d shell suggests that LaFe2/3Mo1/3O3 with a half-filled band is a Mott insulator, whereas SrFe2/3Mo1/3O3 is a band insulator. In both cases, the magnetic structure is dictated by antiferromagnetic superexchange between localized magnetic moments. At intermediate compositions, an interplay of antiferromagnetic-superexchange and double-exchange interactions results in nonmonotonous variations of both the magnetic-ordering temperature and saturation magnetization.