Publications (16)24.81 Total impact
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Article: Pressure-dependent electronic structures in multiferroic DyMnO(3): A combined lifetime-broadening-suppressed x-ray absorption spectroscopy and ab initio electronic structure study.
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ABSTRACT: 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.The Journal of chemical physics 10/2010; 133(15):154510. · 3.09 Impact Factor -
Article: Spin-phonon coupling effects in antiferromagnetic Cr2O3 nanoparticles.
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ABSTRACT: In the present work we use Raman microscopy to investigate the size effect of spin-phonon coupling in antiferromagnetic Cr2O3 nanoparticles. The peculiarities of the dependency of the phonon wave number on temperature (with a relatively weak peak at 293 cm(-1)) can be attributed to the spin-phonon coupling. The variation with temperature of the spin-phonon mode develops when the antiferromagnetic state is near the ordering temperature of the Cr spins. The observations can be reasonably well interpreted by describing the order parameter. A shift in frequency is caused by a strong spin-phonon interaction in Cr2O3. The obtained s-ph coefficients are found to be consistent with the strain, where raising the strain results in weakening of the s-ph coupling.Journal of Nanoscience and Nanotechnology 07/2010; 10(7):4596-601. · 1.56 Impact Factor -
Article: Sodium ion ordering of Na_ {0.77} CoO_ {2} under competing multivacancy cluster, superlattice, and domain formation
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ABSTRACT: Hexagonal superlattice formed by sodium multivacancy-cluster ordering in Na0.77CoO2 has been proposed based on synchrotron x-ray Laue-diffraction study on electrochemically fine-tuned single crystals. The title compound sits closely to the proposed lower end of the miscibility gap of x∼0.77–0.82 phase-separated range. The average sodium vacancy-cluster size is estimated to be 4.5 Na vacancies per layer within a large superlattice size of √19a×√19a×3c. The exceptionally large Na vacancy-cluster size favors large twinned simple hexagonal superlattice of √19a, in competition with the smaller divacancy, trivacancy, and quadrivacancy clusters formed superlattices of √12a and √13a. Competing electronic correlations are revealed by the observed spin-glasslike magnetic hysteresis below ∼3 K and the twin domain, triple domain, and monodomain transformations during thermal cycling between 273 and 373 K.Phys. Rev. B. 10/2009; 80(14). -
Article: Sodium ion ordering of Na0.77CoO2 under competing multi-vacancy cluster, superlattice and domain formation
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ABSTRACT: Hexagonal superlattice formed by sodium multi-vacancy cluster ordering in Na$_{0.77}$CoO$_2$ has been proposed based on synchrotron X-ray Laue diffraction study on electrochemically fine-tuned single crystals. The title compound sits closely to the proposed lower end of the miscibility gap of x ~ 0.77-0.82 phase separated range. The average sodium vacancy cluster size is estimated to be 4.5 Na vacancies per layer within a large superlattice size of sqrt{19}a*sqrt{19}a*3c. The exceptionally large Na vacancy cluster size favors large twinned simple hexagonal superlattice of sqrt{19}a, in competition with the smaller di-, tri- and quadri-vacancy clusters formed superlattices of sqrt{12}a and sqrt{13}a. Competing electronic correlations are revealed by the observed spin glass-like magnetic hysteresis below ~ 3K and the twin, triple and mono domain transformations during thermal cycling between 273-373K. Comment: 7 pages, 6 figures09/2009; -
Article: Crystal structure and electronic and thermal properties of TbFeAsO0.85
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ABSTRACT: The crystal structure and the electronic and thermal properties of a high-quality polycrystalline TbFeAsO0.85 sample made by a high-pressure technique are investigated. The crystal structure, as determined by synchrotron X-ray powder diffraction, possesses a tetragonal unit cell (space group: P4/nmm) with lattice parameters of a = b = 3.8851 Å and c = 8.3630 Å. In order to elucidate the electronic structure and oxidation states of corresponding elements, X-ray absorption near-edge structure (XANES) spectra are presented. The XANES spectra confirm that the oxidation states of Fe, As, and Tb in the TbFeAsO0.85 sample are ∼ Fe2+, ∼ As3−, and ∼ Tb3+, respectively, which are consistent with the previously reported band structure calculations. The n-type character of the charge carriers as revealed from XANES spectra is corroborated by the negative sign of the Seebeck coefficient (S) in the present study. The heat capacity (CP) measurement shows an anomaly in the vicinity of the superconducting transition temperature (Tc = 42.5 K), which confirms the bulk nature of the superconductivity in this material.Applied Physics Letters 05/2009; 94(19):192507-192507-3. · 3.84 Impact Factor -
Article: X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering in γ-Na_ {x} CoO_ {2}(x= 0.71 and 0.84)
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ABSTRACT: We have recently demonstrated that x=0.71 and 0.84 are the two most stable single-phase compounds above x=0.5 in γ-NaxCoO2 [ G. J. Shu et al. Phys. Rev. B 76 184115 (2007); F. C. Chou et al. Phys. Rev. Lett. 101 127404 (2008)], and this structural investigation was performed on the single crystals and pulverized samples elaborately synthesized therein. Using the complementary techniques of x-ray and electron diffractions, we unambiguously established the existence of superlattices in x=0.71 and 0.84, √12ap×√12ap×3cp, and √13ap×√13ap×3cp (ap and cp, the hexagonal primitive cell parameters), respectively. The exceptionally large superlattice for x=0.71 arises from the long-range three-dimensional sodium ordering, consisting of the spiral-like sodium-trimer chain screwing along c axis and being decorated with alternating truncated-triangle and honeycomb sodium sublattices in ab plane. The trimers in the neighboring sodium planes show corner-shared-like characteristics along the chain direction. A larger interplane separation of the trimers that was expected to minimize the trimer interlayer Coulomb repulsion was, however, not observed, and all Co ions in the superlattice show similar charge characteristics. In x=0.84, the large superlattice is rather a result of the long-range ordering of sodium monomers both in ab plane and along c axis. The differences in the superlattice structure and the corresponding long-range sodium ordering between x=0.71 and 0.84 may provide the critical information in understanding their distinctly different physical properties from structural aspects.Phys. Rev. B. 01/2009; 79(1). -
Article: Sodium ion ordering and vacancy cluster formation in NaxCoO2 (x=0.71 and 0.84) single crystals by synchrotron X-Ray diffraction.
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ABSTRACT: In the rich phase diagram of NaxCoO2, x=0.71 enjoys special stability and is called the Curie-Weiss metal due to its anomalous properties. Similarly, x=0.84 prepared from high temperature melt is a special end point beyond which the system phase separates. Using synchrotron x-ray diffraction on single crystals, we discovered sqrt[12]a and sqrt[13]a superlattice structures which we interpret as the ordering of Na (vacancy) clusters. These results lead to a picture of coexisting local moments and itinerant carriers and form the first step towards understanding the many anomalous properties of cobaltates.Physical Review Letters 10/2008; 101(12):127404. · 7.37 Impact Factor -
Article: Sodium vacancy ordering and the co-existence of localized spins and itinerant charges in NaxCoO2
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ABSTRACT: The sodium cobaltate family (NaxCoO2) is unique among transition metal oxides because the Co sits on a triangular lattice and its valence can be tuned over a wide range by varying the Na concentration x. Up to now detailed modeling of the rich phenomenology (which ranges from unconventional superconductivity to enhanced thermopower) has been hampered by the difficulty of controlling pure phases. We discovered that certain Na concentrations are specially stable and are associated with superlattice ordering of the Na clusters. This leads naturally to a picture of co-existence of localized spins and itinerant charge carriers. For x = 0.84 we found a remarkably small Fermi energy of 87 K. Our picture brings coherence to a variety of measurements ranging from NMR to optical to thermal transport. Our results also allow us to take the first step towards modeling the mysterious ``Curie-Weiss'' metal state at x = 0.71. We suggest the local moments may form a quantum spin liquid state and we propose experimental test of our hypothesis.10/2007; -
Article: Searching for Stable Na-ordered Phases in Single Crystal Samples of gamma-NaxCoO2
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ABSTRACT: We report on the preparation and characterization of single crystal gamma phase NaxCoO2 with 0.25 < x < 0.84 using a non-aqueous electrochemical chronoamperemetry technique. By carefully mapping the overpotential versus x (for x < 0.84), we find six distinct stable phases with Na levels corresponding to x ~ 0.75, 0.71, 0.50, 0.43, 0.33 and 0.25. The composition with x ~0.55 appears to have a critical Na concentration which separates samples with different magnetic behavior as well as different Na ion diffusion mechanisms. Chemical analysis of an aged crystal reveals different Na ion diffusion mechanisms above and below x_c ~ 0.53, where the diffusion process above x_c has a diffusion coefficient about five times larger than that below x_c. The series of crystals were studied with X-ray diffraction, susceptibility, and transport measurements. The crystal with x = 0.5 shows a weak ferromagnetic transition below T=27 K in addition to the usual transitions at T = 51 K and 88 K. The resistivity of the Curie-Weiss metallic Na0.71CoO2 composition has a very low residual resistivity, which attests to the high homogeneity of the crystals prepared by this improved electrochemical method. Our results on the various stable crystal compositions point to the importance of Na ion ordering across the phase diagram. Comment: 9 pages, 9 figures08/2007; -
Article: Influence of oxygen defects on the crystal structure and magnetic properties of the (Tb1-xNax)MnO3-y (0<or=x<or=0.3) system.
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ABSTRACT: The crystallographic and magnetic behaviors of (Tb1-xNax)MnO3-y (0<or=x<or=0.3) have been studied by neutron powder diffraction (NPD), synchrotron X-ray powder diffraction, and Raman spectroscopy techniques. Although Na+ ions have larger ionic radii than Tb3+ ions, analysis of NPD data reveals a decrease in cell volume upon Na-doping, which can be explained solely by the occurrence of oxygen deficiencies and not by the size effect. The Raman spectrum represents the variation in bond length and bond angle, which originates from the balance of ions, asymmetric structure, and defects in the system. Na-doping causes an oxygen deficiency, and consequently, a peak shift is seen in the Raman spectrum because of the structural adjustment resulting from the doping. The observed effective moments decrease with increasing x because of the replacement of Tb3+ ions by Na+ ions. The well-defined peak at approximately 45 K (labeled TMn) of the x=0.3 sample is associated with Mn spin ordering, while the magnetic responses associated with TMn are not clearly present in the x=0.15 and x=0 samples.Inorganic Chemistry 06/2007; 46(11):4575-82. · 4.60 Impact Factor -
Article: Effect of Ca doping on the optical properties of La1.2Sr1.8Mn2O7
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ABSTRACT: We present x-ray-diffraction, infrared reflectivity, and Raman-scattering measurements of La <sub>1.2</sub>( Sr <sub>1.8-x</sub> Ca <sub>x</sub>) Mn <sub>2</sub> O <sub>7</sub> as a function of temperature and doping ( x=0.0 , 0.4, 0.6, and 0.8). At room temperature, x-ray-diffraction data show that the replacement of Sr ions with smaller Ca ions causes a shrinkage of the unit-cell volume. The far-infrared conductivity spectra of all doped samples are typical of an insulator, showing only phonons. When doped with Ca on Sr and a corresponding systematic reduction in Curie temperature (T<sub>c</sub>) , there is a suppression of the low-frequency optical spectral weight. For the x=0.0 compound, the Raman-active phonon associated with the internal stretching of the oxygen atoms in Mn O <sub>6</sub> octahedra shows a noticeable hardening below T<sub>c</sub> . A polaron transport can be attributed to the observed frequency shifts. Interestingly, this hardening is reduced as the Ca content increases. These observations suggest that Ca doping disrupts the effects of the double-exchange interaction on the lattice degree of freedom, thereby accounting for the characteristic changes in the phase diagram.Journal of Applied Physics 02/2006; · 2.17 Impact Factor -
Article: Structural, electrical and magnetic properties of two-dimensional La1.2(Sr1.8-xCax)Mn2O7 manganites
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ABSTRACT: The effects of structural, electrical and magnetic properties with the isovalent chemical substitution of Ca <sup> 2+ </sup> into the Sr <sup> 2+ </sup> sites in La <sub> 1.2 </sub> (Sr <sub>1.8-x</sub> Ca <sub>x</sub>) Mn <sub> 2 </sub> O <sub> 7 </sub> (x=0–0.8) are investigated. The highest magnetoresistance ratio [ρ(0)/ρ(H)] of 208% (H=1.5 T) at a temperature of 102 K was observed for the x=0.4 sample. The Curie temperatures decreased from 135 to 102 K for x=0–0.4, respectively. Moreover, the Weiss constants θ were varied from the positive to negative value with increasing Ca concentration. The antiferromagnetic behavior with Néel temperature around 30 K was found in the x=0.8 sample. The magnetization measurements show that the hysteresis phenomenon appeared at the temperatures below the Curie or Néel temperatures. © 1999 American Institute of Physics.Journal of Applied Physics 09/1999; · 2.17 Impact Factor -
Article: X-ray and electron diffraction studies of superlattices and long-range three-dimensional Na ordering in gamma-Na[subscript x]CoO[subscript 2] (x=0.71 and 0.84)
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ABSTRACT: We have recently demonstrated that x=0.71 and 0.84 are the two most stable single-phase compounds above x=0.5 in gamma-Na[subscript x]CoO[subscript 2] [G. J. Shu et al., Phys. Rev. B 76, 184115 (2007); F. C. Chou et al., Phys. Rev. Lett. 101, 127404 (2008)], and this structural investigation was performed on the single crystals and pulverized samples elaborately synthesized therein. Using the complementary techniques of x-ray and electron diffractions, we unambiguously established the existence of superlattices in x=0.71 and 0.84, sqrt12a[subscript p]×sqrt12a[subscript p]×3c[subscript p], and sqrt13a[subscript p]×sqrt13a[subscript p]×3c[subscript p] (a[subscript p] and c[subscript p] the hexagonal primitive cell parameters), respectively. The exceptionally large superlattice for x=0.71 arises from the long-range three-dimensional sodium ordering, consisting of the spiral-like sodium-trimer chain screwing along c axis and being decorated with alternating truncated-triangle and honeycomb sodium sublattices in ab plane. The trimers in the neighboring sodium planes show corner-shared-like characteristics along the chain direction. A larger interplane separation of the trimers that was expected to minimize the trimer interlayer Coulomb repulsion was, however, not observed, and all Co ions in the superlattice show similar charge characteristics. In x=0.84, the large superlattice is rather a result of the long-range ordering of sodium monomers both in ab plane and along c axis. The differences in the superlattice structure and the corresponding long-range sodium ordering between x=0.71 and 0.84 may provide the critical information in understanding their distinctly different physical properties from structural aspects. National Science Council of the Republic of China National Taiwan University Excellence ProjectAPS. -
Article: Searching for stable Na-ordered phases in single-crystal samples of γ-Na_ {x} CoO_ {2}
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ABSTRACT: We report on the preparation and characterization of single-crystal γ phase NaxCoO2 with 0.25≤x≤0.84 using a nonaqueous electrochemical chronoamperemetry technique. By carefully mapping the overpotential versus x (for x<0.84), we find six distinct stable phases with Na levels corresponding to x∼ 0.75, 0.71, 0.50, 0.43, 0.33, and 0.25. The composition with x≃0.55 appears to have a critical Na concentration which separates samples with different magnetic behavior as well as different Na ion diffusion mechanisms. Chemical analysis of an aged crystal reveals different Na ion diffusion mechanisms above and below xc∼0.53, where the diffusion process above xc has a diffusion coefficient about five times larger than that below xc. The series of crystals were studied with x-ray diffraction, susceptibility, and transport measurements. The crystal with x=0.5 shows a weak ferromagnetic transition below T=27 K in addition to the usual transitions at T=51 and 88 K. The resistivity of the Curie-Weiss metallic Na0.71CoO2 composition has a very low residual resistivity, which attests to the high homogeneity of the crystals prepared by this improved electrochemical method. Our results on the various stable crystal compositions point to the importance of Na ion ordering across the phase diagram.Phys. Rev. B. 76(18). -
Article: Effect of Jahn-Teller distortion on magnetic ordering in Dy (Fe, Mn) O_ {3} perovskites
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ABSTRACT: The structural, magnetic properties of the rare-earth perovskites, DyFe1-xMnxO3 (space group, orthorhombic Pbnm), were studied, and the magnetic phase diagram of the solid solution was investigated, which unveiled an unexplored hidden field-induced weak ferromagnetism in the material system with x = 0.5–0.9 (antiferromagnetically ordered along the b axis and canted along the c axis). In the solid solution, the Jahn-Teller distortion contributed by Mn3+ gives rise to the predominance of the bc plane sublattice with increasing Mn and also the effective onset of orbital ordering in the ab plane for x = 0.5 and above. These distinct features in the respective lattice and orbital degrees of freedom induce the Dy-(Fe,Mn) electronic interaction anisotropy primarily in the bc plane and the growing in-(ab)plane electronic anisotropy, effectively competing with each other above x = 0.5. The close entanglement of these anisotropies introduced by the various lattice, orbital, and correlated electronic characters results in the favoring of the Dzyaloshinskii-Moriya interaction dictating along the a axis, in contrast to the conventionally observed b axis and accounting for the hidden field-induced canted magnetic ordering in the bc plane. The complex interplays of these multiple factors are discussed in this work and satisfactorily explain the rich magnetic phase diagram of the solid solution.Phys. Rev. B. 83(24). -
Article: Pressure-induced structural distortion of TbMnO_ {3}: A combined x-ray diffraction and x-ray absorption spectroscopy study
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ABSTRACT: 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.Phys. Rev. B. 79(16).
Top co-authors
Top Journals
Institutions
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2010
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National Dong Hwa University
- Department of Physics
Hualian, Taiwan, Taiwan
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1999–2009
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National Synchrotron Radiation Research Center (NSRRC)
Hsinchu, Taiwan, Taiwan
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1999–2008
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National Taiwan University
- • Center for Condensed Matter Sciences
- • Department of Chemistry
Taipei, Taipei, Taiwan
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2006
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National Taiwan Normal University
- Department of Physics
Taipei, Taipei, Taiwan
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