Browsing by Author "Yang, T. J."

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Anisotropic Magnetism and Magnetoresistance in Iron Nanowire Arrays
(2009-12-01)
Ou, M. N.
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Yang, T. J.
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Chen, Y. Y.
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電子物理學系
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Department of Electrophysics
Highly ordered ferromagnetic iron nanowire (Fe NW) arrays with diameters of about 60 nm and 200 nm were fabricated by electro-chemical deposition into anodic aluminum oxide (AAO) templates. The magnetization measurements show the 60 nm NWs as having a larger anisotropic magnetization. This result was illustrated by the magnetic easy axis and preferred crystal orientation of [110] along with the longitudinal direction of the NWs, revealed by the X-ray diffraction (XRD) patterns. The obtained quadratic magnetic field dependence of the normalized magnetoresistance (MR) at low field for both 60 and 200 nm NWs is attributed to the anisotropic magnetoresistance in low-dimensional systems. Furthermore, the enhanced AIR, observed in the 60 nm NWs is attributed to additional spin-diffusion mechanisms.
Anisotropic pinning in Nb thin films with triangular pinning arrays
(2007-07-01)
Cao, R.
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Wu, T. C.
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Kang, P. C.
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Wu, J. C.
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Yang, T. J.
A superconducting thin film with regular triangular arrays has been fabricated to explore the anisotropic vortex pinning behavior. We found that the critical currents for these films depend strongly on the current directions. Some interesting temperature-dependent phenomena are observed and discussed. We also made molecular dynamic simulations to study the dynamics of the vortex motion. The simulation results confirm the anisotropic features we found in the experiments. (c) 2007 Elsevier Ltd. 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.
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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.
Electrical and thermal transport in single nickel nanowire
(2008-02-11)
Ou, M. N.
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Yang, T. J.
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Harutyunyan, S. R.
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Chen, Y. Y.
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Chen, C. D.
The thermal conductivity and electrical resistivity of a suspended nickel nanowire have been measured for T=15-300 K. The temperature dependence of the thermal conductivity and the Lorenz number strongly differ from the bulk. The comparison of the transports in the Ni nanowire shows, that at temperatures 75 < T < 300 K Wiedemann-Franz (WF) law holds, whereas at temperatures T < 75 K the WF law is violated, indicating that thermal current in this material is suppressed more than electrical current. The results are explained by combined effect of confined dimension, enhanced disorder, and grown contribution of N-processes. (C) 2008 American Institute of Physics.
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.
Matching effect and flux pinning force in a Nb superconducting thin film with a triangular lattice of artificial pinning centers
(2004-05-01)
Horng, Lance
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Wu, J. C.
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Kang, P. C.
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Wu, T. C.
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Yang, T. J.
A strong pinning force in a Nb superconducting thin film is formed by a triangular lattice of defects due to the structural corrugation that acts as pinning centers. The magnetoresistance measurement close to the transition temperature shows a set of minima of magnetoresistance at equal external field intervals. A crossover in current-voltage relations with matching field and half-integer matching field occurs for currents exceeding a certain threshold value. The phenomenon is related to the presence of pinning structures at matching fields. (C) 2003 Elsevier B.V. All rights reserved.
Photonic band gaps of polygonal and circular dielectric rods in square lattices
(2008-12-01)
Chang, K. P.
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Yang, S. L.
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Shen, L. F.
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Yang, T. J.
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電子物理學系
The plane-wave method is employed to determine the band structures and the field patterns of the photonic crystals with polygonal and circular rods in square lattices. Isotropic and anisotropic N-fold polygonal rods are used to assess the effects of the shapes and symmetries of polygonal rods on E-and H-polarization modes. The photonic band structures of the polygonal rods approach those of the circular rods as N increases above a value that is determined by the optical properties of the photonic crystal. Moreover, for comparable geometric structures, anisotropic polygonal-rod photonic crystals can provide larger absolute photonic band gaps than isotropic photonic crystals. (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3039213]
Pressure dependence of the electronic structure and spin state in Fe(1.01)Se superconductors probed by x-ray absorption and x-ray emission spectroscopy
(2011-09-09)
Chen, J. M.
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Haw, S. C.
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Lee, J. M.
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Chou, T. L.
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Chen, S. A.
Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe(1.01)Se superconductors up to similar to 66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of similar to 7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe1.01Se decreases progressively with pressure up to similar to 10 GPa. A new prepeak at energy of similar to 7113.7 eV develops for pressure above similar to 13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe(1.01)Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe(1.01)Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe(1.01)Se. A discontinuous variation of the integrated absolute difference values of the K beta emission line was observed, originating from a phase transition of Fe(1.01)Se for a pressure >12 GPa. Fe(1.01)Se shows a small net magnetic moment of Fe(2 broken vertical bar) at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line K beta' was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe(1.01)Se superconductors. The experimental results provide insight into the spin state of Fe(1.01)Se superconductors under pressure.
Pressure dependence of the electronic structure and spin state in Fe1.01Se superconductors probed by x-ray absorption and x-ray emission spectroscopy
(2011-09-09)
Chen, J. M.
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Haw, S. C.
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Lee, J. M.
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Chou, T. L.
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Chen, S. A.
Pressure dependence of electronic structures and spin states of iron-chalcogenide Fe1.01Se superconductors up to similar to 66 GPa has been investigated with x-ray emission spectra and x-ray absorption spectra with partial-fluorescence yield. The intensity of the pre-edge peak at energy of similar to 7112.7 eV of the Fe K-edge x-ray absorption spectrum of Fe1.01Se decreases progressively with pressure up to similar to 10 GPa. A new prepeak at energy of similar to 7113.7 eV develops for pressure above similar to 13 GPa, indicating formation of a new phase. The experimental and the calculated Fe K-edge absorption spectra of Fe1.01Se using the FDMNES code agree satisfactorily. The larger compression accompanied by significant distortion around the Fe atoms along the c axis in Fe1.01Se upon applying pressure suppresses the Fe 3d-Se 4p and Fe 4p-Se 4d hybridization. The applied pressure suppresses the nearest-neighbor ferromagnetic superexchange interaction and enhances spin fluctuations on the Fe sites in Fe1.01Se. A discontinuous variation of the integrated absolute difference values of the K beta emission line was observed, originating from a phase transition of Fe1.01Se for a pressure >12 GPa. Fe1.01Se shows a small net magnetic moment of Fe-2 broken vertical bar at ambient pressure, probably arising from strong Fe-Fe spin fluctuations. The satellite line K beta' was reduced in intensity upon applying pressure and became absent for pressure >52 GPa, indicating a continuous reduction of the spin moment of Fe in Fe1.01Se superconductors. The experimental results provide insight into the spin state of Fe1.01Se superconductors under pressure.
Pressure-dependent electronic structures in multiferroic DyMnO(3): A combined lifetime-broadening-suppressed x-ray absorption spectroscopy and ab initio electronic structure study
(2010-10-21)
Chen, J. M.
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Lee, J. M.
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Chou, T. L.
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Chen, S. A.
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Huang, S. W.
Variations in the electronic structure and structural distortion in multiferroic DyMnO(3) were probed by synchrotron x-ray diffraction, lifetime-broadening-suppressed x-ray absorption spectroscopy (XAS), and ab initio electronic structure calculations. The refined x-ray diffraction data enabled an observation of a diminished local Jahn-Teller distortion of Mn sites within MnO(6) octahedra in DyMnO(3) on applying the hydrostatic pressure. The intensity of the white line in Mn K-edge x-ray absorption spectra of DyMnO(3) progressively increased with the increasing pressure. With the increasing hydrostatic pressure, the absorption threshold of an Mn K-edge spectra of DyMnO(3) shifted toward a greater energy, whereas the pre-edge line slightly shifted to a smaller energy. We provide the spectral evidence for the pressure-induced bandwidth broadening for manganites. The intensity enhancement of the white line in Mn K-edge spectra is attributed to a diminished Jahn-Teller distortion of MnO(6) octahedra in compressed DyMnO(3). A comparison of the pressure-dependent XAS spectra with the ab initio electronic structure calculations and full calculations of multiple scattering using the code FDMNES shows the satisfactory agreement between experimental and calculated Mn K-edge spectra. (C) 2010 American Institute of Physics. [doi:10.1063/1.3490400]
Pressure-dependent electronic structures in multiferroic DyMnO3: A combined lifetime-broadening-suppressed x-ray absorption spectroscopy and ab initio electronic structure study
(2010-10-21)
Chen, J. M.
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Lee, J. M.
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Chou, T. L.
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Chen, S. A.
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Huang, S. W.
Variations in the electronic structure and structural distortion in multiferroic DyMnO3 were probed by synchrotron x-ray diffraction, lifetime-broadening-suppressed x-ray absorption spectroscopy (XAS), and ab initio electronic structure calculations. The refined x-ray diffraction data enabled an observation of a diminished local Jahn-Teller distortion of Mn sites within MnO6 octahedra in DyMnO3 on applying the hydrostatic pressure. The intensity of the white line in Mn K-edge x-ray absorption spectra of DyMnO3 progressively increased with the increasing pressure. With the increasing hydrostatic pressure, the absorption threshold of an Mn K-edge spectra of DyMnO3 shifted toward a greater energy, whereas the pre-edge line slightly shifted to a smaller energy. We provide the spectral evidence for the pressure-induced bandwidth broadening for manganites. The intensity enhancement of the white line in Mn K-edge spectra is attributed to a diminished Jahn-Teller distortion of MnO6 octahedra in compressed DyMnO3. A comparison of the pressure-dependent XAS spectra with the ab initio electronic structure calculations and full calculations of multiple scattering using the code FDMNES shows the satisfactory agreement between experimental and calculated Mn K-edge spectra. (C) 2010 American Institute of Physics. [doi:10.1063/1.3490400]
Pressure-induced structural distortion of TbMnO(3): A combined x-ray diffraction and x-ray absorption spectroscopy study
(2009-04-01)
Chen, J. M.
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Chou, T. L.
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Lee, J. M.
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Chen, S. A.
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Chan, T. S.
The variation of electronic states and structural distortion in highly compressed multiferroic TbMnO(3) was probed by x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) using synchrotron radiation. Refined XRD data enabled observation of a reduced local Jahn-Teller (JT) distortion of Mn sites within MnO(6) octahedra in TbMnO(3) with increasing hydrostatic pressure. A progressively increasing intensity of the white line in Mn K-edge x-ray absorption spectra of TbMnO(3) was detected with increasing pressure. The absorption threshold of Mn K-edge spectra of TbMnO(3) is shifted toward higher energy, whereas the pre-edge peak is slightly shifted to lower energy with increasing hydrostatic pressure. We provide spectral evidence for pressure-induced bandwidth broadening for mangnites. The enhanced intensity of the white line and the shifted absorption threshold of Mn K-edge spectra are explained in terms of a reduced JT distortion of MnO(6) octahedra in compressed TbMnO(3). Comparison of XAS data with full-multiple-scattering calculations using code FDMNES shows satisfactory agreement between experimental and calculated Mn K-edge spectra.
Pressure-induced structural distortion of TbMnO3: A combined x-ray diffraction and x-ray absorption spectroscopy study
(2009-04-01)
Chen, J. M.
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Chou, T. L.
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Lee, J. M.
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Chen, S. A.
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Chan, T. S.
The variation of electronic states and structural distortion in highly compressed multiferroic TbMnO3 was probed by x-ray diffraction (XRD) and x-ray absorption spectroscopy (XAS) using synchrotron radiation. Refined XRD data enabled observation of a reduced local Jahn-Teller (JT) distortion of Mn sites within MnO6 octahedra in TbMnO3 with increasing hydrostatic pressure. A progressively increasing intensity of the white line in Mn K-edge x-ray absorption spectra of TbMnO3 was detected with increasing pressure. The absorption threshold of Mn K-edge spectra of TbMnO3 is shifted toward higher energy, whereas the pre-edge peak is slightly shifted to lower energy with increasing hydrostatic pressure. We provide spectral evidence for pressure-induced bandwidth broadening for mangnites. The enhanced intensity of the white line and the shifted absorption threshold of Mn K-edge spectra are explained in terms of a reduced JT distortion of MnO6 octahedra in compressed TbMnO3. Comparison of XAS data with full-multiple-scattering calculations using code FDMNES shows satisfactory agreement between experimental and calculated Mn K-edge spectra.
Properties of vortex propagation in a niobium film with spacing-graded density of pinning sites
(2007-05-01)
Horng, Lance
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Wu, T. C.
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Wu, J. C.
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Cao, R.
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Yang, T. J.
The vortex propagation has been investigated in Nb superconductors with one dimension spacing-graded density of submicrometer-scaled holes in this research. A rectification effect can be discerned because the applied ac is transformed into a net dc voltage. These results demonstrate that the rectification is mainly characterized by the asymmetric pinning landscape and the rectified voltage depends considerably on temperature and the amplitude of applied ac. The dc voltage increases steeply up to the maximum value and then decreases to zero dc voltage with the increase of ac. The shape of peaks in the dc voltage signal become steeper and move to higher applied ac regime with the decrease of temperature.
Resonant x-ray emission spectroscopy of multiferroic TbMnO3
(2007-07-30)
Chen, J. M.
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Chen, C. K.
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Chou, T. L.
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Jarrige, I.
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Ishii, H.
The Mn 3d valence states in single-crystalline TbMnO3 were probed using x-ray absorption spectroscopy and resonant x-ray emission spectroscopy (RXES). The polarized Mn K-edge x-ray absorption spectra show a strong polarization dependence, particularly for the white line region, indicating the strong anisotropic Mn-O bonding within the ab plane in TbMnO3. The RXES data obtained at the Mn K edge clearly reveal that unoccupied Mn 3d states exhibit a relatively delocalized character, stemming from hybridization of the Mn 3d states with the neighboring Mn 4p orbitals. The authors demonstrated that resonant x-ray emission spectroscopy is able to characterize the degree of localization of the unoccupied states or hole carriers in manganites.
Superconducting transition parameters in aluminum-lithium alloys (0-10 at.% Li)
(2007-05-01)
Ou, M. N.
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Yang, T. J.
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Chen, B. J.
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Chen, Y. Y.
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Ho, J. C.
The superconducting transition temperatures T-C of face-centered cubic Al1-x-Li-x alloys (x = 0-0.10) exhibit a minimum near x = 0.03 (3 at.% Li). The McMillan strong-coupling T, equation yields a similar trend of the electron-phonon coupling constant.. Meanwhile, the density of states at the Fermi level N(0) decreases monotonically with increasing x. It appears that T-C drops initially due to a reduced N(0), which is then overtaken by alloying-enhanced factors of phonon or electron-phonon interaction. (c) 2007 Elsevier Ltd. All rights reserved.
Switchable tunneling mode for cylindrical photonic quantum well consisting of photonic crystals containing liquid crystal
(2013-06-15)
Hu, C. A.
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Yang, S. L.
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Yang, T. J.
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電子物理學系
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Department of Electrophysics
We propose a cylindrical photonic quantum well made of photonic crystals containing liquid crystals, the properties of which are theoretically calculated and investigated by the transfer matrix method in the cylindrical symmetry system. Liquid crystals are introduced into the photonic quantum well structure as tunable defect layers. When the liquid crystals are pseudo-isotropic state and the azimuthal mode order of incident waves are m=0, there were two pass-bands around certain wavelength. When the liquid crystals are homeotropic state, the reflectance of pass-band at shorter wavelength decreases from 0.75 to 0.05 in the TM mode, but the reflectance does not change in the TE mode. When mode order m=1 and the liquid crystals are pseudo-isotropic state, the reflectance of defect mode stayed the same as m=0. However, the result is reversed while the phase of liquid crystals change from pseudo-isotropic to homeotropic state. The reflectance is the same as in the TM mode, but that in the TE mode decreases substantially from 0.75 to 0.05. The application of our structure to switching device is highly potential. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.