Browsing by Author "Chen, C. T."

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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.
Effectiveness of organic molecules for spin filtering in an organic spin valve: Reaction-induced spin polarization for Co atop Alq(3)
(2015-01-21)
Tu-Ngoc Lam
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Lai, Yu-Ling
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Chen, Chih-Han
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Chen, Po-Hung
;
Chan, Yuet-Loy
The spin polarization of organic-ferromagnetic interfaces in an organic spin valve critically affects the efficiency of spin injection or detection. We examined the chemical and electronic properties of ferromagnetic Co deposited on organic Alq(3) and the interfacial spin-polarized capability of the electronic states. Our x-ray photoemission spectra and calculations with density-functional theory indicate a sequential and unequal distribution of charge from Co clusters to N and then to O atoms in Alq(3). The preferential orbital hybridization at specific functional sites produces efficient spin polarization of organic molecules. Element-specific measurements of x-ray magnetic circular dichroism demonstrate the preferential spin polarization in the lowest unoccupied molecular orbital state of N atoms at the complex interface for Co atop Alq(3), which agrees satisfactorily with calculation. Our results indicate that an induced interfacial spin polarization on engineering the dominant reaction of Co with mainly N and O atoms in Alq(3) might pave a way for effective spin filtering in organic spintronics.
Electrically controllable spontaneous magnetism in nanoscale mixed phase multiferroics
(2011-03-01)
He, Q.
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Chu, Y. -H.
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Heron, J. T.
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Yang, S. Y.
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Liang, W. I.
Magnetoelectrics and multiferroics present exciting opportunities for electric-field control of magnetism. However, there are few room-temperature ferromagnetic-ferroelectrics. Among the various types of multiferroics the bismuth ferrite system has received much attention primarily because both the ferroelectric and the antiferromagnetic orders are quite robust at room temperature. Here we demonstrate the emergence of an enhanced spontaneous magnetization in a strain-driven rhombohedral and super-tetragonal mixed phase of BiFeO(3). Using X-ray magnetic circular dichroism-based photoemission electron microscopy coupled with macroscopic magnetic measurements, we find that the spontaneous magnetization of the rhombohedral phase is significantly enhanced above the canted antiferromagnetic moment in the bulk phase, as a consequence of a piezomagnetic coupling to the adjacent tetragonal-like phase and the epitaxial constraint. Reversible electric-field control and manipulation of this magnetic moment at room temperature is also shown.
Electronic structure investigation of a charge density wave coupled to a metal-to-metal transition in Ce3Co4Sn13
(2018-12-18)
Singh, A.
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Huang, H. Y.
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Chin, Y. Y.
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Liao, Y. F.
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Huang, T. C.
We study the electronic structure of the skutterudite Ce3Co4Sn13, which is known to exhibit a charge density wave (CDW) transition, at temperature T-CDW similar to 160 K, coupled to a metal-to-metal transition. We use temperature dependent hard x-ray photoemission spectroscopy (HAXPES) and x-ray absorption spectroscopy (XAS) to investigate the occupied and unoccupied electronic states of Ce3Co4Sn13. The Co 2p and Sn 3p core level spectra show small but finite shifts in binding energy positions across T-CDW while Ce 3d core level spectra do not show any change across the transition. The Ce M-4,M-5-edge XAS spectrum compared with calculations indicate a typical trivalent ionic Ce3+ spectral shape, ruling out Kondo screening in Ce3Co4Sn13. In contrast, the Co L-2,L-3-edge XAS spectrum compared with a calculated spectrum shows evidence for hybridization with neighboring Sn atoms in a trigonal prismatic co-ordination. Temperature dependent XAS across the Co L-2,L-3-edge shows a small shift across T-CDW, consistent with HAXPES results. Detailed XAS measurements as a function of temperature show that the spectral shifts occur with a hysteresis across T-CDW , indicative of a first-order transition. Valence band spectra show a normal Fermi edge above and below T-CDW. The Co 3d states are observed at a binding energy of similar to 2 eV while the Ce 4f states occur as a weak feature within 0.5 eV of the Fermi level. The results suggest an unusual CDW transition coupled to a metal-to-metal transition in Ce3Co4Sn13.
Interfacial symmetry of Co-Alq(3)-Co hybrid structures for effective spin filtering
(2015-11-01)
Lam, Tu-Ngoc
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Lai, Yu-Ling
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Chen, Chih-Han
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Chen, Po-Hung
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Wei, Der-Hsin
Understanding the interfacial behavior at FM-OSC-FM hybrid structures for both the bottom contact (Alq(3) adsorption on Co, Co/Alq(3)) and the top contact (Co atop Alq(3), Alq(3)/Co) is crucial for efficient spin filtering with transport of spin-polarized charge carriers through these interfaces. X-ray photoelectron spectroscopy (XPS) spectra indicate a symmetry of charge transfer from Co to Alq(3) and the corresponding orbital hybridization to a certain extent at both contacts. The alignment of energy levels at both Alq(3)/Co and Co/Alq(3) heterostructures is depicted with ultraviolet photoelectron spectroscopy (UPS). Through magnetic images acquired with a X-ray photoemission electron microscope (XPEEM), the strong hybridization of the top contact presents no micromagnetic domain but still shows magnetic coupling, to some extent, to the bottom contact in the Co-Alq(3)-Co trilayer structure. Measurements of X-ray magnetic circular dichroism (XMCD) demonstrate the induced spin-polarization of non-magnetic Alq(3) at both contacts, proving Alq(3) a unique and promising organic material for spin filtering in OSV. (C) 2015 Elsevier B.V. All rights reserved.
Interfacial symmetry of Co-Alq(3)-Co hybrid structures for effective spin filtering (vol 354, pg 90, 2015)
(2018-05-31)
Tu-Ngoc Lam
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Lai, Yu-Ling
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Chen, Chih-Han
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Chen, Po-Hung
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Wei, Der-Hsin
Involvement of transcriptional regulator Ndt80 in stress responses of Candida albicans
(2012-06-01)
Yang, Y. L.
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Wu, S. Y.
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Hsiung, C. A.
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Chen, C. T.
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Hsiao, C. D.
Magnetic ground state and transition of a quantum multiferroic LiCu(2)O(2)
(2008-08-15)
Huang, S. W.
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Huang, D. J.
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Okamoto, J.
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Mou, C. Y.
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Wu, W. B.
Based on resonant soft x-ray magnetic scattering, we report that LiCu(2)O(2) exhibits a large interchain coupling which suppresses quantum fluctuations along spin chains, and a quasi-2D short-range magnetic order prevails at temperatures above the magnetic transition. These observations unravel the fact that the ground state of LiCu(2)O(2) possesses long-range 2D-like incommensurate magnetic order rather than being a gapped spin liquid as expected from the nature of quantum spin-1/2 chains. In addition, the spin coupling along the c axis is found to be essential for inducing electric polarization.
Magnetic order of LiCu2O2 studied by resonant soft x-ray magnetic scattering
(2008-08-01)
Huang, S. W.
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Huang, D. J.
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Okamoto, J.
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Wu, W. B.
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Chen, C. T.
We present results of resonant soft x-ray magnetic scattering measurements on single crystals of LiCu2O2 to address the magnetic order. LiCu2O2 exhibits a long-range magnetic order incommensurate with the lattice, and the modulation vector is (0.5, zeta, 0) in reciprocal lattice units with zeta similar to 0.174, depending upon the temperature. The spin-spin correlation length along the spin chain deduced from the width of momentum scan is larger than 2100 angstrom. The inter-chain correlation length that lies within the ab plane was discovered to be substantial, similar to 690 angstrom. (c) 2008 Elsevier Ltd. All rights reserved.
Microscopic Origin of the Giant Ferroelectric Polarization in Tetragonal-like BiFeO(3)
(2011-09-29)
Zhang, J. X.
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He, Q.
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Trassin, M.
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Luo, W.
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Yi, D.
We report direct experimental evidence for a room-temperature, similar to 130 mu C/cm(2) ferroelectric polarization from the tetragonal-like BiFeO(3) phase. The physical origin of this remarkable enhancement of ferroelectric polarization has been investigated by a combination of x-ray absorption spectroscopy, scanning transmission electron microscopy, and first principles calculations. A large strain-induced Fe-ion displacement relative to the oxygen octahedra, combined with the contribution of Bi 6s lone pair electrons, is the mechanism driving the large ferroelectric polarization in this tetragonal-like phase.
Microscopic Origin of the Giant Ferroelectric Polarization in Tetragonal-like BiFeO3
(2011-09-29)
Zhang, J. X.
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He, Q.
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Trassin, M.
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Luo, W.
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Yi, D.
We report direct experimental evidence for a room-temperature, similar to 130 mu C/cm(2) ferroelectric polarization from the tetragonal-like BiFeO3 phase. The physical origin of this remarkable enhancement of ferroelectric polarization has been investigated by a combination of x-ray absorption spectroscopy, scanning transmission electron microscopy, and first principles calculations. A large strain-induced Fe-ion displacement relative to the oxygen octahedra, combined with the contribution of Bi 6s lone pair electrons, is the mechanism driving the large ferroelectric polarization in this tetragonal-like phase.
Molecular beam epitaxy preparation and in situ characterization of FeTe thin films
(2020-02-28)
Pereira, V. M.
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Wu, C. N.
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Liu, C. E.
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Liao, S-S
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Chang, C. F.
We have synthesized Fe1+y Te thin films by means of molecular beam epitaxy (MBE) under Te-limited growth conditions. We found that epitaxial layer-by-layer growth is possible for a wide range of excess Fe values, wider than expected from what is known from studies on the bulk material. Using x-ray magnetic circular dichroism spectroscopy at the Fe L-2(,3) and Te M-4(,5) edges, we observed that films with high excess Fe contain ferromagnetic clusters while films with lower excess Fe remain nonmagnetic. Moreover, x-ray absorption spectroscopy showed that it is possible to obtain films with very similar electronic structure as that of a high quality bulk single crystal Fe-1.(14) Te. Our results suggest that MBE with Te-limited growth may provide an opportunity to synthesize FeTe films with smaller amounts of excess Fe as to come closer to a possible superconducting phase.
Multiferroic nanoregions and a memory effect in cupric oxide
(2010-05-01)
Wu, W. B.
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Huang, D. J.
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Okamoto, J.
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Huang, S. W.
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Sekio, Y.
We report the observation of multiferroic nanoregions in cupric oxide, CuO, by means of resonant soft x-ray magnetic scattering. There exists an anomalous memory effect for the direction of the electric polarization in the commensurate-incommensurate magnetic transition that coincides with the ferroelectric transition. Scattering results, incorporated with simulations of diffuse scattering, lead us to propose a scenario that preserved spin handedness in the multiferroic nanoregions is responsible for this memory effect in the magnetically induced ferroelectric properties of CuO.
Non-lethal Candida albicans cph1/cph1 efg1/efg1 transcription factor mutant establishing restricted zone of infection in a mouse model of systemic infection
(2006-07-01)
Chen, C. G.
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Yang, Y. L.
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Cheng, H. H.
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Su, C. L.
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Huang, S. F.
The cph1/cph1 efg1/efg1andida albicans mutant cells were non-lethal in a mouse model of systemic infection. We investigated in vivo proliferation and invasion of C albicans cells in infected mice to elucidate the interaction between the host and the pathogen. Homogenates of kidneys from the mice infected with the wild-type and the mutant C albicans cells yielded a mean of 2.1 x 10(7)CFU/g and 2.2 x 10(6) CFU/g, respectively. The kidneys from the mice infected with the wild-type cells showed extensive renal cortical necrosis associated with neutrophilic infiltration. There were also wild-type hyphal cells present in abundance. Hence, tubular necrosis leading to renal failure in the mice may be the cause of death. Although the cph1/cph1 efg1/efg1 mutant cells were not lethal, they were capable of establishing restricted zones of infection and colonization near the renal pelvis instead of simply being cleared by the immune system in mice.
Orbital ordering of layered manganites from resonant soft X-ray scattering
(2007-03-01)
Chao, K. S.
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Huang, D. J.
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Okamoto, J.
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Lin, H.-J.
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Hsu, C.-H.
We present measurements of Mn L-edge resonant soft X-ray scattering on single-layered manganites with different sizes of cations. Orbital ordering of Pr0.5Ca1.5MnO4 exhibits a stronger three-dimensional character and a dramatically enhanced transition temperature, as compared with those of La0.5Sr1.5MnO4. The c-axis correlation length of orbital ordering in Pr0.5Ca1.5MnO4 is about half of the inplane correlation length. Our results indicate that reduction in one-electron bandwidth and quenched disorder strongly enhances the stabilization of charge-orbital ordering. (c) 2006 Published by Elsevier B. V.
Orbital polarization and Jahn-Teller distortion of strained La0.5Sr0.5MnO3 thin films
(2007-03-01)
Wu, W. B.
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Huang, D. J.
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Huang, C.-M.
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Hsu, C.-H.
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Chang, C. F.
We report spectroscopic evidence for orbital-mediated phases in strained La0.5Sr0.5MnO3 thin films by combining soft X-ray spectroscopy and synthesis of manganite thin films. Measurements of polarization-dependent soft X-ray absorption reveal that electronic states responsible for the lowest-energy excitations in C-type antiferromagnetic La0.5Sr0.5MnO3 films have an orbital symmetry of 3z(2) - r(2), while those in A-type AFM. films have an orbital symmetry of x(2)-y(2). Such orbital polarizations in strained films of manganite result from a combined effect of the Jahn-Teller distortion and the electron correlations of Mn 3d electrons. (c) 2006 Published by Elsevier B.V.
Orthorhombic BiFeO3
(2012-12-14)
Yang, J. C.
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He, Q.
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Suresha, S. J.
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Kuo, C. Y.
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Peng, C. Y.
A new orthorhombic phase of the multiferroic BiFeO3 has been created via strain engineering by growing it on a NdScO3(110)(o) substrate. The tensile-strained orthorhombic BiFeO3 phase is ferroelectric and antiferromagnetic at room temperature. A combination of nonlinear optical second harmonic generation and piezoresponse force microscopy revealed that the ferroelectric polarization in the orthorhombic phase is along the in-plane < 110 >(pc) directions. In addition, the corresponding rotation of the antiferromagnetic axis in this new phase was observed using x-ray linear dichroism. DOI: 10.1103/PhysRevLett.109.247606
Quasi-two-dimensional d-spin and p-hole ordering in the three-dimensional perovskite La(1/3)Sr(2/3)FeO(3)
(2010-10-22)
Okamoto, J.
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Huang, D. J.
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Chao, K. S.
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Huang, S. W.
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Hsu, C. -H.
Measurements of resonant soft x-ray magnetic scattering reveal anomalous ordering of 3d spins and 2p holes in perovskite La(1/3)Sr(2/3)FeO(3), which is known to exhibit three-dimensional (3D) charge and spin ordering along the (111) direction. Upon cooling, a quasi-two-dimensional phase consisting of ferromagnetic and charge-disproportionated [111] trilayers of Fe(3+)-Fe(5+)-Fe(3+) precedes the development of 3D magnetic ordering at the transition temperature, in sharp contrast to an ordinary transition from the paramagnetic and charge-uniform phase to the antiferromagnetic and charge-disproportionated phase expected according to conventional wisdom.
Quasi-two-dimensional d-spin and p-hole ordering in the three-dimensional perovskite La1/3Sr2/3FeO3
(2010-10-22)
Okamoto, J.
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Huang, D. J.
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Chao, K. S.
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Huang, S. W.
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Hsu, C. -H.
Measurements of resonant soft x-ray magnetic scattering reveal anomalous ordering of 3d spins and 2p holes in perovskite La1/3Sr2/3FeO3, which is known to exhibit three-dimensional (3D) charge and spin ordering along the (111) direction. Upon cooling, a quasi-two-dimensional phase consisting of ferromagnetic and charge-disproportionated [111] trilayers of Fe3+-Fe5+-Fe3+ precedes the development of 3D magnetic ordering at the transition temperature, in sharp contrast to an ordinary transition from the paramagnetic and charge-uniform phase to the antiferromagnetic and charge-disproportionated phase expected according to conventional wisdom.