Koji Fujita

Kyoto University, Kioto, Kyōto, Japan

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Publications (135)333.31 Total impact

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    ABSTRACT: The hydride reduction of a tetragonal layered perovskite LaSrCoO4 is known to yield orthorhombic LaSrCoO3H0.7 with a complete hydride/oxide order within the ab plane. In this study, epitaxial thin films of LaSrCoO4 with a-axis and c-axis orientations have been deposited on (100) and (001) LaSrAlO4 (LSAO) substrates, respectively, and allowed to react with hydride to convert into oxyhydrides. X-ray diffraction, secondary ion mass spectroscopy and thermal desorption spectroscopy experiments indicate that both films are topochemically reduced and can integrate hydride ions with a chemical composition close to that obtained for the powder. A significant reduction in the a-axis was observed for the a-axis oriented LaSrCoO3H0.7 film, indicating hydride/oxide order, as previously reported. In contrast, the c-axis oriented LaSrCoO3H0.7 film remains tetragonal, suggesting hydride/oxide disorder. These results demonstrate that strain engineering can lead to new materials with designed anion arrangement in mixed anion materials.
    CrystEngComm 09/2014; · 3.88 Impact Factor
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    ABSTRACT: The effective magneto-optical properties of novel nonanuclear Tb(III) complexes with Tb-O lattice (specifically, [Tb9(sal-R)16(μ-OH)10](+)NO3(-), where sal-R = alkyl salicylate (R = -CH3 (Me), -C2H5 (Et), -C3H7 (Pr), or -C4H9 (Bu)) are reported. The geometrical structures of these nonanuclear Tb(III) complexes were characterized using X-ray single-crystal analysis and shape-measure calculation. Optical Faraday rotation was observed in nonanuclear Tb(III) complexes in the visible region. The Verdet constant per Tb(III) ion of the Tb9(sal-Me) complex is 150 times larger than that of general Tb(III) oxide glass. To understand their large Faraday rotation, electron paramagnetic resonance measurements of Gd(III) complexes were carried out. In this Report, the magneto-optical relation to the coordination geometry of Tb ions is discussed.
    Inorganic chemistry. 06/2014;
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    ABSTRACT: Rotations of oxygen octahedra are ubiquitous, but they cannot break inversion symmetry in simple perovskites. However, in a layered oxide structure, this is possible, as we demonstrate here in A-site ordered Ruddlesden-Popper NaRTiO_{4} (R denotes rare-earth metal), previously believed to be centric. By revisiting this series via synchrotron x-ray diffraction, optical second-harmonic generation, piezoresponse force microscopy, and first-principles phonon calculations, we find that the low-temperature phase belongs to the acentric space group P4[over ¯]2_{1}m, which is piezoelectric and nonpolar. The mechanism underlying this large new family of acentric layered oxides is prevalent, and could lead to many more families of acentric oxides.
    Physical Review Letters 05/2014; 112(18):187602. · 7.73 Impact Factor
  • Phys. Rev. Lett. 05/2014; 112(18):187602.
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    ABSTRACT: We have prepared amorphous xFeO(100 ? x)SiO2 (x = 38.2, 54.8, and 67.5 in mol %) thin films by a pulsed laser deposition method and examined their magnetic properties. The temperature dependence of zero-field-cooled magnetization manifests a cusp-like peak, i.e., spin-freezing in a low temperature region. The spin-freezing temperature depends on the frequency of ac magnetic field, and the dependence is explainable in terms of the scaling law. The aging-memory effect characteristic of spin glass is also observed. Furthermore, superparamagnetic behavior is found in the variation of magnetization with magnetic field at high temperatures. These observations indicate that there exist magnetic clusters of Fe2+ ions in the amorphous FeO?SiO2 thin films and that the intercluster interactions bring about the superspin glass transition at low temperatures.
    Japanese Journal of Applied Physics 04/2014; 53(5S1):05FB11. · 1.07 Impact Factor
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    ABSTRACT: We report on novel light–emitting properties from monodispersed mesoporous silica particles embedded with β–Ga2O3 nanocrystals that were fabricated through a chemical approach followed by thermal annealing in specific atmosphere. The emission spectrum of such nanocomposites consists of several sharp peaks where the dominant one regularly shifts with variation of the excitation wavelength, leading to observation of multiple–color light emissions ranging from blue, green, to white light wavelength regions. We suggest that the donor levels created by oxygen vacancy while multiple acceptor levels induced by gallium vacancy or gallium oxide vacancy account for the emission features of multiple bands.
    Optical Materials Express 03/2014; 4(3). · 2.92 Impact Factor
  • Physical Review Letters 01/2014; 112(18):187602. · 7.73 Impact Factor
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    ABSTRACT: Large-scale battery systems are essential for efficiently utilizing renewable energy power sources from solar and wind, which can generate electricity only intermittently. The use of lithium-ion batteries to store the generated energy is one solution. A long cycle life is critical for lithium-ion battery when used in these applications; this is different from portable devices which require 1,000 cycles at most. Here we demonstrate a novel co-substituted lithium iron phosphate cathode with estimated 70%-capacity retention of 25,000 cycles. This is found by exploring a wide chemical compositional space using density functional theory calculations. Relative volume change of a compound between fully lithiated and delithiated conditions is used as the descriptor for the cycle life. On the basis of the results of the screening, synthesis of selected materials is targeted. Single-phase samples with the required chemical composition are successfully made by an epoxide-mediated sol-gel method. The optimized materials show excellent cycle-life performance as lithium-ion battery cathodes.
    Nature Communications 01/2014; 5:4553. · 10.02 Impact Factor
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    ABSTRACT: A novel silver-containing perovskite, AgCu3V4O12, was synthesized under high-pressure and high-temperature conditions. It crystallizes in an A-site-ordered perovskite structure (space group Im3̅), in which silver ions occupy the 12-coordinated A sites forming regular icosahedra, and exhibits metallic behavior. Bond-valence-sum calculations and X-ray photoemission spectroscopy reveal that Ag ions are present in the mixed-valence state, most likely attributable to the coexistence of Ag(+) and Ag(3+), unlike the case of well-known perovskite-type AgNbO3 and AgTaO3 containing only Ag(+) ions. We discuss metallic conduction in relation to electronic structure calculations.
    Inorganic Chemistry 12/2013; 52(24):13824-13826. · 4.59 Impact Factor
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    ABSTRACT: Remarkable magneto-optical properties of a new isolator material, that is, europium sulfide nanocrystals with gold (EuS-Au nanosystem), has been demonstrated for a future photo-information technology. Attachment of gold particles that exhibit surface plasmon resonance leads to amplification of the magneto-optical properties of the EuS nanocrystals. To construct the EuS-Au nanosystems, cubic EuS and spherical Au nanocrystals have been joined by a variety of organic linkers, that is, 1,2-ethanedithiol (EDT), 1,6-hexanedithiol (HDT), 1,10-decanedithiol (DDT), 1,4-bisethanethionaphthalene (NpEDT), or 1,4-bisdecanethionaphthalene (NpDDT) . Formation of these systems was observed by XRD, TEM, and absorption spectra measurements. The magneto-optical properties of the EuS-Au nanosystem have been characterized by using Faraday rotation spectroscopy. The Faraday rotation angle of the EuS-Au nanosystem is dependent on the Au particle size and interparticle distance between EuS and Au nanocrystals. Enhancement of the Faraday rotation of EuS-Au nanosystems was observed. The spin configuration in the excited state of the EuS-Au nanosystem was also investigated using photo-assisted electron paramagnetic resonance.
    Chemistry 09/2013; · 5.93 Impact Factor
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    ABSTRACT: A novel cubic perovskite MnCu3V4O12 has been synthesized at a high pressure and high temperature of 12 GPa and 1373 K. This compound crystallizes in the A-site-ordered perovskite structure (space group Im3̅) with lattice constant a = 7.26684(10) Å at room temperature. The most notable feature of this compound lies in the fact that the Mn(2+) ion is surrounded by 12 equidistant oxide ions to form a regular icosahedron; the situation of Mn(2+) is unprecedented for the crystal chemistry of an oxide. An anomalously large atomic displacement parameter Uiso= 0.0222(8) Å(2) is found for Mn(2+) at room temperature, indicating that the thermal oscillation of the small Mn(2+) ion in a large icosahedron is fairly active. Magnetic susceptibility and electric resistivity measurements reveal that 3d electrons of Mn(2+) ions are mainly localized, while 3d electrons in Cu(2+) and V(4+) ions are delocalized and contribute to the metallic conduction.
    Inorganic Chemistry 09/2013; · 4.59 Impact Factor
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    ABSTRACT: Magnetic, optical, and magneto-optical properties have been examined for aluminoborosilicate glasses with high concentration of divalent europium ions. An analysis on temperature dependence of magnetic susceptibility yields positive Weiss temperatures, indicating that magnetic moments of 4f electrons of Eu2+ interact ferromagnetically with each other in the glasses. The glasses show large Faraday rotation angles in the visible range; glass with a composition of 58.0EuO·12.0Al2O3·20.0B2O3·10.0SiO2 (mol%) has a Verdet constant of -1.03 min/Oe cm (-300 rad/T m) at 633 nm. Wavelength dependence of the Verdet constant is interpreted in terms of Eu2+ 4f7 to 4f65d1 electronic transitions based on the Van Vleck-Hebb theory.
    Optical Materials 09/2013; 35(11):1997-2000. · 1.92 Impact Factor
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    ABSTRACT: A SPASER, short for surface plasmon amplification by stimulated emission of radiation, is key to accessing coherent optical fields at the nanoscale. Nevertheless, the realization of a SPASER in the visible range still remains a great challenge because of strong dissipative losses. Here, we demonstrate that room-temperature SPASER emission can be achieved by amplifying longitudinal surface plasmon modes supported in gold nanorods as plasmon nanocavities and utilizing laser dyes to supply optical gain for compensation of plasmon losses. By choosing a particular organic dye and adjusting the doping level, the resonant wavelength of the SPASER emission can be tuned from 562 to 627 nm with a spectral linewidth narrowed down to 5 ~ 11 nm. This work provides a versatile route towards SPASERs at extended wavelength regimes.
    Nano Letters 08/2013; · 13.03 Impact Factor
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    ABSTRACT: Random lasing properties of dye solutions suspended with gold–silica core–shell nanoparticles are investigated. The core–shell architecture allows adjustment of plasmon coupling strength between metal and optical gain media by varying the thickness of dielectric shell. Consequently, multiple aspects of random lasers can be controlled, such as mode interactions, lasing spikiness, and pump threshold. The results show that bare gold nanoparticles give rise to the most profound lasing spikes that are evenly separated in resonant wavelength. The lowest threshold is observed when the shell thickness is ~14.6 nm. The experimental observations are interpreted in terms of resonant coupling between metal nanoparticles and fluorophores, localization of pump light and lasing modes at the surface of metal, as well as plasmonic modifications of absorption and pump rates of fluorophores. The results suggest that metal–dielectric core–shell nanostructures can serve as promising candidates towards development of well-controlled random lasers.
    Advanced Optical Materials. 08/2013; 1(8).
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    ABSTRACT: Amorphous Eu2TiO4 and EuTiO3 have been studied by a combination of the Faraday effect in the visible region and polarization-dependent x-ray absorption spectroscopy at the Eu M4,5 and L2,3 edges to examine the role of Eu 4f-5d exchange interactions on the ferromagnetic behavior. The bulk-sensitive x-ray absorption spectra (XAS) for Eu L2,3 edges show that most of the europium ions are present as the divalent state in the amorphous Eu2TiO4 and EuTiO3. The Eu M4,5 edge x-ray magnetic circular dichroism (XMCD) signals, measured for the amorphous Eu2TiO4, dramatically increase upon cooling through the Curie temperature (16 K) determined by a superconducting quantum interference device (SQUID) magnetometer. Sum-rule analysis of the XMCD at Eu M4,5 edges measured at 10 K yields a 4f spin magnetic moment of 6.6μB per Eu2+ ion. These results confirm that the ferromagnetic properties exclusively arise from 4f spins of Eu2+. In addition, for both the amorphous Eu2TiO4 and EuTiO3, the temperature and magnetic-field dependence of Eu L2,3 edge XMCD signals can be scaled with the corresponding magnetization measured by SQUID, indicating that the 5d magnetic polarization of Eu2+ is involved in the process to cause the ferromagnetic interaction between Eu2+ ions. We further discuss the origin of ferromagnetism in the amorphous system on the basis of the energy diagram of Eu 4f and 5d levels deduced from the Faraday effect in the visible region. From the wavelength dependence of Faraday rotation angles of the amorphous EuO-TiO2 system in comparison with those of the divalent Eu chalcogenides as reported previously, it is found that the magnitude of crystal-field splitting of Eu 5d levels in the former is on the same order as that in the latter, which explains an enhanced ferromagnetic exchange interaction between Eu 4f and 5d states.
    Physical Review B 07/2013; 88(2). · 3.66 Impact Factor
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    ABSTRACT: Lattice volume expansion or amorphization renders EuTiO3 ferromagnetic, although the stable phase of crystalline EuTiO3 is an antiferromagnet. The lattice volume expansion is induced into the crystalline EuTiO3 thin film by utilizing the lattice mismatch between the thin film and a substrate. The magnetization at low temperatures monotonically increases with an increase in lattice volume for the crystalline EuTiO3 thin film, coincident with the results of calculations based on the hybrid Hartree–Fock density functional approach. The ferromagnetic interaction between Eu2+ ions is enhanced by the amorphization as well; the amorphous EuTiO3 thin film becomes a ferromagnet, and the Curie temperature is higher for amorphous Eu2TiO4 than for its crystalline counterpart. The phenomenon, that is, the volume expansion- and amophization-induced ferromagnetism, is explained in terms of the competition between ferromagnetic and antiferromagnetic interactions among Eu2+ ions.
    Journal of Materials Research 04/2013; 28(08):1031-1041. · 1.82 Impact Factor
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    ABSTRACT: Novel EuS nanocrystals containing paramagnetic Mn(II), Co(II) or Fe(II) ions have been reported as advanced semiconductor materials with effective optical rotation under magnetic field, Faraday rotation. The EuS nanocrystals with transition metal ions, EuS:M nanocrystals, were prepared by the reduction of the Eu(III) dithiocarbamate complex (tetraphenylphosphonum tetrakis (diethyldithiocarbamate) europium (III)) with transition metal complexes at 300°C. The EuS:M nanocrystals thus prepared were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), inductively coupled plasma atomic emission spectroanalysis (ICP-AES) and a superconducting quantum interference device magnetometer (SQUID). Enhanced Faraday rotations of the EuS:M nanocrystals were observed around 550 nm, and their enhanced spin polarization was estimated using electron paramagnetic resonance (EPR) measurements. In this report, the magneto-optical relationship between the Faraday rotation efficiency and spin polarization is discussed.
    Journal of the American Chemical Society 01/2013; · 10.68 Impact Factor
  • Advanced Functional Materials 01/2013; 23(15):1864-1872. · 10.44 Impact Factor
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    ABSTRACT: The spaser, a quantum amplifier of surface plasmons by stimulated emission of radiation, is recognized as a coherent light source capable of confining optical fields at subwavelength scale. The control over the directionality of spasing has not been addressed so far, especially for a single-particle spasing nanocavity where optical feedback is solely provided by a plasmon resonance. In this work we numerically examine an asymmetric spaser - a resonant system comprising a dielectric core capped by a metal semishell. The proposed spaser emits unidirectionally along the axis of the semishell; this directionality depends neither on the incident polarization nor on the incident angle of the pump. The spasing efficiency of the semishell-capped resonator is one order of magnitude higher than that in the closed core-shell counterpart. Our calculations indicate that symmetry breaking can serve as a route to create unidirectional, highly intense, single-particle, coherent light sources at subwavelength scale.
    Scientific Reports 01/2013; 3:1241. · 5.08 Impact Factor
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    ABSTRACT: First-principles calculations reveal that in divalent europium perovskites EuMO3 (M = Ti, Zr, and Hf), antiferromagnetic superexchange interactions via nd states of the B-site M cations (n = 3, 4, and 5, respectively) are enhanced by rotations of the MO6 octahedra. The octahedral rotations involved in a structural change from cubic to orthorhombic Pbnm structures not only reduce energy gaps between the Eu 4f and M nd bands but also point the M nd orbitals at the Eu sites, leading to a significant overlap between the M nd and Eu 4f orbitals. These results reveal that the octahedral rotations are indispensable for antiferromagnetic ordering observed for EuZrO3 and EuHfO3, and put these perovskites into a class of materials exhibiting a novel type of strong coupling between their magnetism and octahedral rotations.
    Advanced Functional Materials 01/2013; 23(15):1864-1872. · 10.44 Impact Factor

Publication Stats

467 Citations
333.31 Total Impact Points

Institutions

  • 1997–2014
    • Kyoto University
      • • Department of Material Chemistry
      • • Department of Materials Science and Engineering
      • • Fukui Institute for Fundamental Chemistry
      Kioto, Kyōto, Japan
  • 2009–2010
    • Japan Science and Technology Agency (JST)
      Edo, Tōkyō, Japan
  • 2002
    • Kyoto Institute of Technology
      • Department of Chemistry and Materials Technology
      Kioto, Kyōto, Japan