2022/06/28 更新

写真a

カツフジ タクロウ
勝藤 拓郎
所属
理工学術院 先進理工学部
職名
教授

兼担

  • 理工学術院   大学院先進理工学研究科

学内研究所等

  • 2020年
    -
    2022年

    理工学術院総合研究所   兼任研究員

  • 2006年
    -
     

    理工学術院総合研究所(各務記念材料技術研究所)   流動研究員

学歴

  • 1991年
    -
    1995年

    東京大学   大学院理学系研究科   物理学専攻  

  •  
    -
    1991年

    東京大学   理学部   物理学科  

学位

  • 1997年   東京大学   博士(理学)

経歴

  • 2008年
    -
    継続中

    早稲田大学   理工学術院先進理工学部物理学科   教授

  • 2002年
    -
    2007年

    早稲田大学   理工学部物理学科   助教授

  • 1999年
    -
    2002年

    科学技術振興事業団   研究員

  • 1997年
    -
    1999年

    ベル研究所   博士研究員

  • 1995年
    -
    1997年

    東京大学   大学院工学系研究科   助手

所属学協会

  •  
     
     

    American Physical Society

  •  
     
     

    日本物理学会

 

研究分野

  • 磁性、超伝導、強相関系

研究キーワード

  • 強相関電子系、物質開発、光物性

論文

  • Large Negative Magnetoresistance in Ba3−xEuxNb5O15

    Kenta Iwamoto, Wataru Sekino, Satomi Ito, Yumiko Katayama, Kazunori Ueno, Takuro Katsufuji

    Journal of the Physical Society of Japan   91 ( 3 ) 033702  2022年03月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We found a large negative magnetoresistance in Ba3-xEuxNb5O15 with a tetragonal tungsten bronze structure, which changes from a metallic state (x = 0) to an insulating state (x = 3) under variation of x. The magnitude of the magnetoresistance ρc(0)=ρc(H), where ρc(H) is the resistivity along the c axis with the applied magnetic field of H, increases with x and reaches ∼5000 near the metal–insulator phase boundary. Such a large negative magnetoresistance is likely to be due to the coupling between the itinerant 4d electrons in Nb and the localized 4 f spins in Eu, and the disorder arising from Eu ions in this series of compounds.

    DOI

  • Do electron distributions with orbital degree of freedom exhibit anisotropy?

    Taishun Manjo, Shunsuke Kitou, Naoyuki Katayama, Shin Nakamura, Takuro Katsufuji, Yoichi Nii, Taka Hisa Arima, Joji Nasu, Takumi Hasegawa, Kunihisa Sugimoto, Daisuke Ishikawa, Alfred Q.R. Baron, Hiroshi Sawa

    Materials Advances   3 ( 7 ) 3192 - 3198  2022年02月

     概要を見る

    Orbital degrees of freedom appear in partially filled degenerate orbitals and are responsible for a variety of physical properties. Although many methods for directly observing orbital states in ordered states have been proposed, it is difficult to determine degenerate orbital states directly. We found that high-precision diffraction data obtained by synchrotron radiation X-rays can be used to observe only the valence electron density distribution with minimal analytical bias. The results obtained using this method clearly showed the anisotropy of the partially filled degenerate 3d orbital states in spinel vanadium oxide FeV2O4. The 3d orbital quantum state was successfully determined from this anisotropy.

    DOI

  • Selection rule for the photoinduced phase transition dominated by anisotropy of strain in Ti3O5

    Takuo Saiki, Taishi Yoshida, Kaoru Akimoto, Daigo Indo, Mitsutoshi Arizono, Tetsuji Okuda, Takuro Katsufuji

    Physical Review B   105 ( 7 ) 075134  2022年02月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    In photoinduced phase transitions, the relationship between the crystal orientation and the direction of the polarization or wave vector of light seems important, but surprisingly, such directional properties in photoinduced phase transitions have not been well studied experimentally. Ti3O5 exhibits a structural phase transition involving the anisotropic migration of Ti and O ions, and its nanocrystals exhibit a persistent photoinduced phase transition. We found that a photoinduced phase transition occurs even in a single crystal of this compound, but it significantly depends on the orientation of the plane on which the light is irradiated. Such a selection rule for photoinduced phase transitions is dominated by the anisotropy of the strain associated with the phase transition.

    DOI

  • Metal-insulator transition in Ba3-xSrxNb5O15

    Yuhki Kondoh, Ryosuke Takei, Tetsuji Okuda, Kazunori Ueno, Yumiko Katayama, Takuo Saiki, Wataru Sekino, Tomomasa Kajita, Takuro Katsufuji

    Physical Review B   104   125128  2021年09月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We investigated single crystals of Ba3-xSrxNb5O15 with a tetragonal tungsten bronze (TTB) structure, which exhibits a metal-insulator transition with x. We found that there is a one-dimensional anisotropy with a higher dc conductivity along the c axis in the metallic phase, but it disappears in the insulating phase. The optical conductivity spectra of the metallic samples show similar anisotropy to the dc conductivity, but the anisotropy persists even in the insulating phase. These experimental results, together with the result of magnetic susceptibility, indicate that the disorder caused by the Sr ions loosely bound to the surrounding oxygen ions plays an important role in the metal-insulator transition.

    DOI

  • Superstructures arising from V trimers with orbital ordering in BaV10O15

    Tomomasa Kajita, Hideki Kuwahara, Shigeo Mori, Takuro Katsufuji

    PHYSICAL REVIEW RESEARCH   3   033046  2021年07月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    In BaV10O15 with quasitriangular V lattices, we found unique superstructures characterized by (qq0)-type peaks in the momentum space, where q varies continuously with temperature between q = 0 and 1/3 and locks at 1/3. Such a threefold superstructure is likely caused by the dense arrangement of the V trimers associated with the orbital ordering of V. The results indicate that the superstructure of the V trimers, each of which is also a superstructure, gives rise to the ordered phases with peculiar characteristics.

    DOI

  • Examination of Charge Order in Mixed Valence Oxide LuFe2O4 by Mossbauer Quadrupole Effect

    Shin Nakamura, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   90   064702  2021年06月  [査読有り]

     概要を見る

    The Fe2+/Fe3+ charge order (CO) in LuFe2O4 has been examined by Mossbauer quadrupole effect. The stoichiometric single crystal of LuFe2O4 was used to measure the paramagnetic spectrum at 282 K. The spectrum was analyzed by assuming the CO monoclinic structure (space group C2/m) with four inequivalent Fe sites, among which two are occupied by Fe3+ and other two are occupied by Fe2+. The obtained quadrupole parameters, quadrupole splitting QS and the intensity ratio of the two absorption lines I(+/- 3/2)/I(+/- 1/2), coincide well with those calculated from the crystal structure. On the other hand, the parameters obtained by assuming other two crystal structures previously proposed, the hexagonal structure with a single Fe site (R (3) over barm) and the monoclinic structure with a single Fe site (C2/m), are found to be inconsistent with the calculated ones. These results indicate that the CO monoclinic structure is most probable among the structural models proposed so far from the viewpoint of the Mossbauer quadrupole effect.

    DOI

  • Orbital and magnetic ordering and domain-wall conduction in ferrimagnet La5Mo4O16

    Takuro Katsufuji, Masayuki Miyake, Makoto Naka, Masahito Mochizuki, Sota Kogo, Tomomasa Kajita, Yasuhiro Shimizu, Masayuki Itoh, Takatoshi Hasegawa, Shunsuke Shimose, Shunta Noguchi, Takuo Saiki, Takuro Sato, Fumitaka Kagawa

    PHYSICAL REVIEW RESEARCH   3   013105  2021年02月  [査読有り]

    担当区分:筆頭著者, 責任著者

     概要を見る

    We studied the ferrimagnet La5Mo4O16 with Mo ions on quasisquare lattices both experimentally and theoretically. We found that in addition to the antiferromagnetic ordering at T-N = 200 K, a magnetic phase transition into a ferrimagnetic phase occurs at T-s1 = 60 K, which is caused by the orbital ordering of the Mo 4d states. We also found that the relatively small size of ferrimagnetic domains despite the large Ising anisotropy of the Mo spins are formed below T-s1, which are responsible for the relaxation behavior and the memory effect in the magnetization. Nonvolatile positive magnetoresistance observed in this compound below T-s1 can be attributed to the fact that such magnetic domains and the electrical conduction along the domain walls disappear with the applied magnetic field.

    DOI

  • Pump-probe Spectroscopy with Uniaxial Strain for BaV10O15

    Takuo Saiki, Kazuma Funahashi, Teruo Yamazaki, Tomomasa Kajita, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   90   024704  2021年02月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We performed a pump-probe optical reflectivity measurement with an applied uniaxial strain for bulk single crystals of BaV10O15 at room temperature. We found that with a compressive strain of Delta L/L= -3.0 x 10(-3) along the b-axis, the oscillation in the time dependence of the photoinduced reflectivity change, which arises from the propagation of shockwave, becomes longer in period, indicating that the sound velocity decreases on the order of 10(-2). This result implies that the elastic modulus of BaV10O15 is coupled with the correlation of the orbital degree of freedom of V t(2g) electrons.

    DOI

  • Interplay between electronic correlation and atomic disorder in a low carrier density 4d transition-metal oxide

    T. Yasuda, Y. Kondo, T. Kajita, K. Murota, D. Ootsuki, Y. Takagi, A. Yasui, N. L. Saini, T. Katsufuji, T. Mizokawa

    PHYSICAL REVIEW B   102   205133  2020年11月  [査読有り]

     概要を見る

    We have investigated electronic structure evolution by cation substitution in Ba3-xSrxNb5O15 by means of hard x-ray photoemission spectroscopy. Localization of Nb 4d electrons manifests as spectral weight transfer from a coherent component at the Fermi level to an incoherent one at 1-2 eV below it. This behavior is similar to that of electron-doped SrTiO3. On the other hand, Nb 3d and 4p core level spectra exhibit a screening effect with an energy scale of 1-2 eV by the coherent Nb 4d electrons similar to 4d electron systems near Mott transitions. The energy scale indicates that electron correlation is involved in the metal to insulator transition in the present system, although the Nb 4d band is about 1/10 filled. The present results suggest a mechanism of electron localization due to atomic disorder and electron correlation.

    DOI

  • Mössbauer Study of Rare-earth Ferroborate NdFe3(BO3)4

    Shin Nakamura, Takatsugu Masuda, Kenya Ohgushi, Takuro Katsufuji

    Journal of the Physical Society of Japan   89 ( 8 ) 084703 - 084703  2020年08月  [査読有り]

    DOI

  • Structural, magnetic, transport, and thermoelectric properties of the pseudobrookite AlTi2O5−Ti3O5 system

    Ryusei Takahama, Toi Ishii, Daigo Indo, Mitsutoshi Arizono, Chieko Terakura, Yoshinori Tokura, Nao Takeshita, Masaaki Noda, Hideki Kuwahara, Takuo Saiki, Takuro Katsufuji, Ryoichi Kajimoto, Tetsuji Okuda

    Physical Review Materials   4   074401  2020年07月  [査読有り]

    DOI

  • Nucleation and growth of orbital ordering

    Takuro Katsufuji, Tomomasa Kajita, Suguru Yano, Yumiko Katayama, Kazunori Ueno

    Nature Communications   11 ( 1 ) 2324  2020年05月  [査読有り]

    担当区分:筆頭著者, 責任著者

     概要を見る

    The dynamics of the first-order phase transitions involving a large displacement of atoms, for example, a liquid-solid transition, is generally dominated by the nucleation of the ordered phase and the growth of the nuclei, where the interfacial energy between the two phases plays an important role. On the other hand, electronic phase transitions seldom exhibit such a nucleation-growth behavior, probably because two-phase coexistence is not dominated by only the interfacial energy in such phase transitions. In the present paper, we report that the dynamics of a phase transition associated with an ordering of d orbitals in a vanadate exhibits a clear nucleation-growth behavior and that the interfacial energy between the orbital-ordered and -disordered phases dominated by the orbital-spin coupling can be experimentally obtained. The dynamics of many discontinuous phase transitions is determined by the nucleation and growth of regions of the new phase. Katsufuji et al. demonstrate that the orbital ordering transition of doped BaV10O15 presents an electronic example of nucleation-growth behaviour.

    DOI

  • Change in the optical spectrum of BaV10O15 with applied uniaxial strain

    T. Saiki, S. Ohkubo, K. Funahashi, T. Yamazaki, T. Kajita, T. Katsufuji

    Phys. Rev. B   101 ( 12 ) 121111(R)  2020年03月  [査読有り]

    担当区分:最終著者, 責任著者

    DOI

  • Nonlinear Behavior in the Electrical Resistance of Strongly Correlated Insulators

    T. Katsufuji, H. Ikeda, M. Ashizawa, M. Oike, T. Kajita

    J. Phys. Soc. Jpn.   89 ( 4 ) 044702  2020年03月  [査読有り]

    担当区分:筆頭著者, 責任著者

    DOI

  • Competitive Local Structure in Mixed Vanadium Spinel Fe1-xMnxV2O4

    Shin Nakamura, Kaito Tasaki, Takuro Katsufuji

    JPS Conf. Proc.     011142  2020年  [査読有り]

  • Temperature-dependent evolution of Ti 3d spectral features at surface of BaxTi8O16+δ

    S. Dash, H. Enomoto, T. Kajita, K. Ono, K. Horiba, M. Kobayashi, H. Kumigashira, V. Kandyba, A. Giampietri, A. Barinov, F. Stramaglia, N. L. Saini, T. Katsufuji, T. Mizokawa

    Phys. Rev. B   100 ( 12 ) 125153  2019年09月  [査読有り]

    DOI

  • Local Structure and Magnetic Structure of Spinel Oxide MnV2O4 Observed by M&oumlssbauer Spectroscopy

    S. Nakamura, T. Katsufuji

    J. Phys. Soc. Jpn.   88 ( 6 ) 064703  2019年05月  [査読有り]

    DOI

  • Temperature-dependent valence state within the metallic phase of BaV10O15 probed by hard x-ray photoelectron spectroscopy

    S. Dash, M. Okawa, T. Kajita, T. Yoshino, R. Shimoyama, K. Takahashi, Y. Takahashi, R. Takayanagi, T. Saitoh, A. Yasui, E. Ikenaga, N. L. Saini, T. Katsufuji, T. Mizokawa

    Phys. Rev. B   99 ( 3 ) 035122  2019年01月  [査読有り]

    DOI

  • Magnetotransport Properties of the Orbital-Ordered State of Ba1−xSrxV13O18

    K. Funahashi, Y. Kakesu, T. Kajita, Y. Obata, Y. Katayama, K. Ueno, T. Suzuki, J. G. Checkelsky, T. Katsufuji

    J. Phys. Soc. Jpn.   88 ( 2 ) 024708  2019年01月  [査読有り]

    担当区分:最終著者, 責任著者

    DOI

  • Doping effect on orthorhombic Sr2VO4 with s=1/2 dimers

    K. Funahashi, T. Higashide, T. Ueno, K. Tasaki, Y. Tahara, M. Adachi, T. Saiki, T. Kajita, T. Katsufuji

    Phys. Rev. B   98 ( 18 ) 184422  2018年11月  [査読有り]

    担当区分:最終著者, 責任著者

    DOI

  • Strongly correlated oxides for energy harvesting

    Jobu Matsuno, Jun Fujioka, Tetsuji Okuda, Kazunori Ueno, Takashi Mizokawa, Takuro Katsufuji

    SCIENCE AND TECHNOLOGY OF ADVANCED MATERIALS   19 ( 1 ) 899 - 908  2018年10月  [査読有り]

     概要を見る

    We review recent advances in strongly correlated oxides as thermoelectric materials in pursuit of energy harvesting. We discuss two topics: one is the enhancement of the ordinary thermoelectric properties by controlling orbital degrees of freedom and orbital fluctuation not only in bulk but also at the interface of correlated oxides. The other topic is the use of new phenomena driven by spin-orbit coupling (SOC) of materials. In 5d electron oxides, we show some SOC- related transport phenomena, which potentially contribute to energy harvesting. We outline the current status and a future perspective of oxides as thermoelectric materials.[GRAPHICS].

    DOI

  • Transport, magnetic, thermoelectric, and structural properties of hollandite titanates BaxTi8O16+δ

    Y. Yamashita, T. Okuda, S. Mori, Y. Horibe, R. Murata, T. Katsufuji

    Phys. Rev. Materials   2 ( 7 ) 74409 - 74409  2018年07月  [査読有り]

    担当区分:最終著者, 責任著者

    DOI

  • Electronic states and possible origin of the orbital-glass state in a nearly metallic spinel cobalt vanadate: An x-ray magnetic circular dichroism study

    Yosuke Nonaka, Goro Shibata, Rui Koborinai, Keisuke Ishigami, Shoya Sakamoto, Keisuke Ikeda, Zhendong Chi, Tsuneharu Koide, Arata Tanaka, Takuro Katsufuji, Atsushi Fujimori

    Physical Review B   97 ( 20 ) 205126  2018年05月  [査読有り]

     概要を見る

    We have investigated the orbital states of the orbital-glassy (short-range orbital ordered) spinel vanadate Co1.21V1.79O4 using x-ray absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD), and subsequent configuration-interaction cluster-model calculation. From the sign of the XMCD spectra, it was found that the spin magnetic moment of the Co ion is aligned parallel to the applied magnetic field and that of the V ion antiparallel to it, consistent with neutron scattering studies. It was revealed that the excess Co ions at the octahedral site take the trivalent low-spin state and induce a random potential to the V sublattice. The orbital magnetic moment of the V ion is small, suggesting that the ordered orbitals mainly consist of real-number orbitals.

    DOI

  • Anomalous double-stripe charge ordering in β−NaFe2O3 with double triangular layers consisting of almost perfect regular Fe4 tetrahedra

    S. Kobayashi, H. Ueda, C. Michioka, K. Yoshimura, S. Nakamura, T. Katsufuji, H. Sawa

    Phys. Rev. Materials   2 ( 5 ) 054402  2018年05月  [査読有り]

    DOI

  • Thermal conductivity of SrVO3−SrTiO3 thin films: Evidence of intrinsic thermal resistance at the interface between oxide layers

    T. Katsufuji, T. Saiki, S. Okubo, Y. Katayama, K. Ueno

    Phys. Rev. Materials   2 ( 5 ) 051002(R)  2018年05月  [査読有り]

    担当区分:筆頭著者, 責任著者

    DOI

  • Anomalous metallic state with strong charge fluctuations in BaxTi8O16+δ revealed by hard x-ray photoemission spectroscopy

    S. Dash, T. Kajita, M. Okawa, T. Saitoh, E. Ikenaga, N. L. Saini, T. Katsufuji, T. Mizokawa

    Phys. Rev. B   97 ( 16 ) 165116  2018年04月  [査読有り]

    DOI

  • V 2p core-level spectroscopy of V2+/V3+mixed valence AV<inf>10</inf>O<inf>15</inf>(A = Ba, Sr) and Ba<inf>0.9</inf>Sr<inf>0.1</inf>V<inf>13</inf>O<inf>18</inf>

    Dash, S, Kajita, T, Yoshino, T, Saini, N. L, Katsufuji, T, Mizokawa, T

    Journal of Electron Spectroscopy and Related Phenomena   223   11 - 20  2018年02月  [査読有り]

    DOI

  • Trimerization and orbital ordering in Ba1−xSrxV13O18

    T. Kajita, Y. Obata, Y. Kakesu, Y. Imai, Y. Shmizu, M. Itoh, H. Kuwahara, T. Katsufuji

    Physical Review B   96 ( 24 ) 245126-1 - 245126-7  2017年12月  [査読有り]

    担当区分:最終著者, 責任著者

    DOI

  • Inhomogeneous electronic states associated with charge-orbital order/disorder in BaV10O15 probed by photoemission spectromicroscopy

    T. Yoshino, K. Wakita, E. Paris, A. Barinov, T. Kajita, T. Katsufuji, V. Kandyba, T. Sugimoto, T. Yokoya, N. L. Saini, T. Mizokawa

    PHYSICAL REVIEW B   96 ( 11 ) 115161 - 115161  2017年09月  [査読有り]

     概要を見る

    We have performed scanning photoemission spectromicroscopy of BaV10O15 across the metal-insulator transition at 123 K, which is accompanied by V 3d charge/orbital order and V trimerization. Nucleation of metallic domains is observed at the cleaved surface of BaV10O15 single crystals, similar to Cr-doped V2O3 in which electronic configurations of Cr3+ and V3+ are the same as those of V2+ and V3+ in BaV10O15. Typical domain size is similar to 5- 10 mu m at 150 K, just above the transition temperature. The metallic domains continuously grow up to 240 K, well above the transition temperature. The temperature evolution of the metallic phase in BaV10O15 is different from that of Cr-doped V2O3, probably due to the charge degrees of freedom in BaV10O15.

    DOI

  • Variety of elastic anomalies in an orbital-active nearly itinerant cobalt vanadate spinel

    Tadataka Watanabe, Shogo Yamada, Rui Koborinai, Takuro Katsufuji

    PHYSICAL REVIEW B   96 ( 1 ) 014422  2017年07月  [査読有り]

     概要を見る

    We perform ultrasound velocity measurements on a single crystal of nearly metallic spinel Co1.21V1.79O4 which exhibits a ferrimagnetic phase transition at T-C similar to 165 K. The experiments reveal a variety of elastic anomalies in not only the paramagnetic phase above T-C but also the ferrimagnetic phase below TC, which should be driven by the nearly itinerant character of the orbitally degenerate V 3d electrons. In the paramagnetic phase above TC, the elastic moduli exhibit elastic-mode-dependent unusual temperature variations, suggesting the existence of a dynamic spin-cluster state. Furthermore, above T-C, the sensitive magnetic-field response of the elastic moduli suggests that, with the negative magnetoresistance, the magnetic-field-enhanced nearly itinerant character of the V3d electrons emerges from the spin-cluster state. This should be triggered by the inter-V-site interactions acting on the orbitally degenerate 3d electrons. In the ferrimagnetic phase below T-C, the elastic moduli exhibit distinct anomalies at T-1 similar to 95K and T-2 similar to 50 K, with a sign change of the magnetoresistance at T-1 (positive below T-1) and an enhancement of the positive magnetoresistance below T-2, respectively. These observations below TC suggest the successive occurrence of an orbital glassy order at T-1 and a structural phase transition at T-2, where the rather localized character of the V 3d electrons evolves below T-1 and is further enhanced below T-2.

    DOI

  • Photoinduced phase transitions over three phases in Ba0.9Sr0.1V13O18

    T. Saiki, N. Yamaguchi, Y. Obata, Y. Kakesu, T. Kajita, T. Katsufuji

    PHYSICAL REVIEW B   96 ( 3 ) 035133  2017年07月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    Ba0.9Sr0.1V13O18 exhibits two ordered phases caused by the charge/orbital ordering of the d electrons on the V ions. We have succeeded in observing photoinduced phase transitions over three phases in this compound: a transition from the low-temperature phase to the high-temperature phase within 0.3 ps, followed by a transition to the intermediate-temperature phase within 2 ps. The relationship between the symmetry at each of the three phases plays a key role in the dynamics of the photoinduced phase transitions in such a compound.

    DOI

  • Time-of-Flight Elastic and Inelastic Neutron Scattering Studies on the Localized 4d Electron Layered Perovskite La5Mo4O16

    Kazuki Iida, Ryoichi Kajimoto, Yusuke Mizuno, Kazuya Kamazawa, Yasuhiro Inamura, Akinori Hoshikawa, Yukihiko Yoshida, Takeshi Matsukawa, Toru Ishigaki, Yukihiko Kawamura, Soshi Ibuka, Tetsuya Yokoo, Shinichi Itoh, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   86 ( 6 ) 064803  2017年06月  [査読有り]

     概要を見る

    The magnetic structure and spin-wave excitations in the quasi-square-lattice layered perovskite compound La5Mo4O16 were studied by a combination of neutron diffraction and inelastic neutron scattering techniques using polycrystalline sample. Neutron powder diffraction refinement revealed that the magnetic structure is ferrimagnetic in the ab plane with antiferromagnetic stacking along the c-axis where the magnetic propagation vector is k = (0, 0, 1/2). The ordered magnetic moments are estimated to be 0.54(2)mu(B) for Mo5+ (4d(1)) ions and 1.07(3)mu(B) for Mo4+ (4d(2)) ions at 4K, which are about half of the expected values. The inelastic neutron scattering results display strong easy-axis magnetic anisotropy along the c-axis due to the spin-orbit interaction in Mo ions evidenced by the spin gap at the magnetic zone center. The model Hamiltonian consisting of in-plane anisotropic exchange interactions, the interlayer exchange interaction, and easy-axis single-ion anisotropy can explain our inelastic neutron scattering data well. Strong Ising-like anisotropy and weak interlayer coupling compared with the intralayer exchange interaction can explain both the high-temperature magnetoresistance and long-time magnetization decay recently observed in La5Mo4O16.

    DOI

  • Anomalous relaxation behavior in the resistivity and magnetization of La5Mo4O16

    Y. Mizuno, T. Hasegawa, T. Katsufuji

    PHYSICAL REVIEW B   95 ( 22 ) 224436  2017年06月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We found anomalous relaxation behaviors in a two-dimensional magnet La5Mo4O16, in a pure crystalline form in both the resistivity and magnetization, whose characteristics are similar to those of disordered materials. We also found that such anomalous relaxation behaviors disappear in the impurity-doped sample. We propose a possible scenario for the relaxation behavior that the charge rearrangement of Mo4+ and Mo5+ causes a change in the interlayer magnetic interaction in this compound.

    DOI

  • Electronic properties of Ba1−xSrxV13O18 (x=0,0.2,1) studied using hard x-ray photoelectron spectroscopy

    S. Dash, M. Okawa, T. Kajita, T. Yoshino, R. Shimoyama, K. Takahashi, Y. Takahashi, R. Takayanagi, T. Saitoh, D. Ootsuki, T. Yoshida, E. Ikenaga, N. L. Saini, T. Katsufuji, T. Mizokawa

    Phys. Rev. B   95 ( 19 ) 195116  2017年05月  [査読有り]

    DOI

  • Orbital reorientation in MnV2O4 observed by V NMR

    Euna Jo, Sejun Park, Jooseop Lee, Soonchil Lee, Jeong Hyun Shim, Takehito Suzuki, Takuro Katsufuji

    SCIENTIFIC REPORTS   7   2178  2017年05月  [査読有り]

     概要を見る

    The simultaneous occurrence of the structural and magnetic phase transitions observed in MnV2O4 is one clear example of strong interplay among the spin, orbital and lattice degrees of freedom. The structure of MnV2O4 is switched by the magnetic field and the linear magnetostriction is very high. The orbital order mediates the interaction between the spin and the lattice generating these phenomena. In this work, we present experimental evidence of an orbital order in MnV2O4 and its reorientation under a rotating magnetic field as obtained by nuclear magnetic resonance(NMR). The shift in the resonance frequency of the V NMR spectrum is symmetrical with respect to 45 degrees as an external magnetic field of 7 T rotates from the c-axis to the b-axis, indicating that the initial easy axis flips to the orthogonal direction most parallel to the field direction. The spectrum of V3+ ions splits into four peaks with a maximum shift of 40 MHz. Our analysis revealed that this is the combined effect of the anisotropic hyperfine field due to an ordered orbital and the dipolar hyperfine field. Reorientation of the orbital order in response to an external magnetic field accompanies the macroscopically observed magnetostriction in MnV2O4 .

    DOI

  • Unusual valence state and metal-insulator transition in BaV10 O-15 probed by hard x-ray photoemission spectroscopy

    T. Yoshino, M. Okawa, T. Kajita, S. Dash, R. Shimoyama, K. Takahashi, Y. Takahashi, R. Takayanagi, T. Saitoh, D. Ootsuki, T. Yoshida, E. Ikenaga, N. L. Saini, T. Katsufuji, T. Mizokawa

    PHYSICAL REVIEW B   95 ( 7 )  2017年02月  [査読有り]

     概要を見る

    We have studied the electronic structure of BaV10O15 across the metal-insulator transition with V trimerization by means of hard-x-ray photoemission spectroscopy (HAXPES) and mean-field calculations. The V 2p HAXPES indicates V2.5+-V3+ charge fluctuation in the metallic phase, and V2+-V3+ charge order in the insulating phase. The V2.5+-V3+ charge fluctuation is consistent with the mean-field solution where a V 3d a(1g) electron is shared by two V sites with face-sharing VO6 octahedra. The valence- band HAXPES of the metallic phase exhibits pseudogap opening at the Fermi level associated with the charge fluctuation, and a band gap similar to 200 meV is established in the insulating phase due to the switching of charge correlation.

    DOI

  • Reversible Single-Crystal-to-Single-Crystal Phase Transition of Chiral Salicylidenephenylethylamine

    Akifumi Takanabe, Takuro Katsufuji, Kohei Johmoto, Hidehiro Uekusa, Motoo Shiro, Hideko Koshima, Toru Asahi

    CRYSTALS   7 ( 1 )  2017年01月  [査読有り]

     概要を見る

    The chiral crystal of enantiomeric (S)-N-3,5-di-tert-butylsalicylidene-1-phenylethylamine in the enol form [enol-(S)-1] undergoes a reversible single-crystal-to-single-crystal (SCSC) phase transition at T-c approximate to 3 degrees C from the room temperature beta-form in orthorhombic space group P2(1)2(1)2(1) (Z' = 1) to the low temperature -form in the monoclinic space group P2(1) (Z' = 2) with a thermal hysteresis of approximately 1.7 degrees C. A detailed comparison of the crystal structures of the alpha- and beta-forms revealed that the 5-tert-butyl group of one molecule in the asymmetric unit of the beta-form rotated by ca. 60 degrees, and the dihedral angle between the phenyl and salicyl planes increased slightly in the beta-form crystal. However, the changes in the molecular conformation and packing arrangement are small, which leads to the reversible SCSC phase transition with no destruction of the crystal lattice. The dielectric constant along the b-axis was small, probably due to the weak intermolecular interactions in the crystals.

    DOI

  • Photoinduced dynamics in doped Mott insulators with polaronic conduction: Ba2Ti13O22 and BaxTi8O16

    N. Yamaguchi, A. Furuhashi, H. Nishihara, R. Murata, K. Takayama, T. Katsufuji

    PHYSICAL REVIEW B   94 ( 4 ) 045119  2016年07月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    To clarify the dynamics of polarons in doped Mott insulators, we performed pump-probe reflectivity measurements for two barium titanates, Ba2Ti13O22 and BaxTi8O16, with a noninteger average number of d electrons per Ti. We found that the photoinduced dynamics in Ba2Ti13O22 with a three-dimensional electronic structure is characterized by a broadening of the polaron-excitation peak immediately after photoirradiation, whereas that in BaxTi8O16 with a one-dimensional electronic structure is characterized by a change in the intensities of the polaron-excitation peak and charge-gap-excitation peak with time. This indicates that a difference in dimensionality results in different photoinduced dynamics of polarons in doped Mott insulators.

    DOI

  • Magnetic field and in situ stress dependence of elastic behavior in EuTiO3 from resonant ultrasound spectroscopy

    Jason Schiemer, Leszek J. Spalek, Siddharth S. Saxena, Christos Panagopoulos, Takuro Katsufuji, Annette Bussmann-Holder, Juergen Koehler, Michael A. Carpenter

    PHYSICAL REVIEW B   93 ( 5 ) 054108  2016年02月  [査読有り]

     概要を見る

    Magnetoelectric coupling phenomena in EuTiO3 are of considerable fundamental interest and are also understood to be key to reported multiferroic behavior in strained films, which exhibit distinctly different properties to the bulk. Here, the magnetoelastic coupling of EuTiO3 is investigated by resonant ultrasound spectroscopy with in situ applied magnetic field and stress as a function of temperature ranging from temperatures above the structural transition temperature T-S to below the antiferromagnetic ordering temperature T-N. One single crystal and two polycrystalline samples are investigated and compared to each other. Both paramagnetic and diamagnetic transducer carriers are used, allowing an examination of the effect of both stress and magnetic field on the behavior of the sample. The properties are reported in constant field/variable temperature and in constant temperature/variable field mode where substantial differences between both data sets are observed. In addition, elastic and magnetic poling at high fields and stresses at low temperature has been performed in order to trace the history dependence of the elastic constants. Four different temperature regions are identified, characterized by unusual elastic responses. The low-temperature phase diagram has been explored and found to exhibit rich complexity. The data evidence a considerable relaxation of elastic constants at high temperatures, but with little effect from magnetic field alone above 20 K, in addition to the known low-temperature coupling.

    DOI

  • Universal Behaviors of the Phonon Thermal Conductivity Associated with Charge/Orbital Ordering in Transition-Metal Oxides

    Takuro Katsufuji, Tetsuji Okuda, Ryuichi Murata, Tatsuya Kanzaki, Kota Takayama, Tomomasa Kajita

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   85 ( 1 ) 013703  2016年01月  [査読有り]

    担当区分:筆頭著者, 責任著者

     概要を見る

    We studied the thermal transport properties of transition-metal oxides, which have partially filled t(2g) orbitals of transition metals and exhibit intriguing phase transitions caused by orbital/charge ordering. We found that the temperature (T) dependence of the phonon thermal conductivity kappa(p) exhibits a universal behavior for all compounds; kappa(p)(T) decreases with decreasing T above the transition temperature T-c, whereas it is enhanced below T-c. This result implies that orbital/charge ordering in transition-metal oxides can be characterized by a unique T dependence of the phonon thermal conductivity.

    DOI CiNii

  • Orbital Glass State of the Nearly Metallic Spinel Cobalt Vanadate

    R. Koborinai, S. E. Dissanayake, M. Reehuis, M. Matsuda, T. Kajita, H. Kuwahara, S-H. Lee, T. Katsufuji

    PHYSICAL REVIEW LETTERS   116 ( 3 ) 037201  2016年01月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    Strain, magnetization, dielectric relaxation, and unpolarized and polarized neutron diffraction measurements were performed to study the magnetic and structural properties of spinel Co1-xV2+xO4. The strain measurement indicates that, upon cooling, Delta L/L in the order of similar to 10(-4) starts increasing below T-C, becomes maximum at T-max, and then decreases and changes its sign at T*. Neutron measurements indicate that a collinear ferrimagnetic order develops below T-C and upon further cooling noncollinear ferrimagnetic ordering occurs below T-max. At low temperatures, the dielectric constant exhibits a frequency dependence, indicating slow dynamics. These results indicate the existence of an orbital glassy state at low temperatures in this nearly metallic frustrated magnet.

    DOI

  • Electronic phase transition in hollandite titanates BaxTi8O16+δ

    R. Murata, T. Sato, T. Okuda, Y. Horibe, H. Tsukasaki, S. Mori, N. Yamaguchi, K. Sugimoto, S. Kawaguchi, M. Takata, T. Katsufuji

    Phys. Rev. B   92 ( 22 ) 220408(R)  2015年12月  [査読有り]

    担当区分:最終著者, 責任著者

    DOI

  • Coupling between Mott excitation and d−d transitions in CoV2O4

    H. Nishihara, A. Furuhashi, N. Yamaguchi, R. Koborinai, T. Katsufuji

    Phys. Rev. B   92 ( 14 ) 140401(R)  2015年10月  [査読有り]

    担当区分:最終著者, 責任著者

    DOI

  • Long-time relaxation of the magnetization in a pure crystal magnet La5Mo4O16

    T. Konishi, K. Kobayashi, T. Katsufuji

    PHYSICAL REVIEW B   92 ( 2 ) 020419(R)  2015年07月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We found that a layered magnet La5Mo4O16 exhibits a long-time relaxation of the magnetization from a ferromagnetic to an antiferromagnetic state. This unconventional relaxation in a pure crystal magnet far below the transition temperature arises from the extremely two-dimensional nature of the magnetic interactions, by which the Mo spins are strongly coupled and form a big spin within a layer.

    DOI

  • Magnetoelastic relaxations in EuTiO3

    J. Schiemer, L. J. Spalek, S. S. Saxena, C. Panagopoulos, T. Katsufuji, M. A. Carpenter

    EPL   109 ( 5 ) 57004  2015年03月  [査読有り]

     概要を見る

    The multiferroic properties of EuTiO3 are greatly enhanced when a sample is strained, signifying that coupling between strain and structural, magnetic or ferroelectric order parameters is extremely important. Here resonant ultrasound spectroscopy has been used to investigate strain coupling effects, as well as possible additional phase transitions, through their influence on elastic and anelastic relaxations that occur as a function of temperature between 2 and 300K and with applied magnetic field up to 14 T. Antiferromagnetic ordering is accompanied by acoustic loss and softening, and a weak magnetoelastic effect is also associated with the change in magnetization direction below similar to 2.8K. Changes in loss due to the influence of magnetic field suggest the existence of magnetic defects which couple with strain and may play a role in pinning of ferroelastic twin walls. open access Copyright (C) EPLA, 2015

    DOI

  • 23pBE-9 Ba_2Ti_<13>O_<22>の電子相転移とバナジウムドーピング

    小西 孝彦, 仁科 康祐, 桑原 英樹, 勝藤 拓郎

    日本物理学会講演概要集   70   2322 - 2322  2015年

    DOI CiNii

  • 24aBF-3 ペロブスカイト型BaSnO_3へのスピン・キャリアドーピング

    東出 唯志, 芝野 杏奈, 桑原 英樹, 梶原 凌, 勝藤 拓郎

    日本物理学会講演概要集   70   2354 - 2354  2015年

    DOI CiNii

  • 24aPS-81 軌道縮退系バナジウムスピネルCoV_2O_4における超音波音速測定

    山田 章悟, 小野 拓海, 石川 卓, 滝田 将太, 前田 穂, 高瀬 浩一, 高野 良紀, 渡辺 忠孝, 小堀内 類, 勝藤 拓郎

    日本物理学会講演概要集   70   2432 - 2432  2015年

    DOI CiNii

  • Elastic Anomalies in Orbital-Degenerate Frustrated Spinel CoV2O4

    Tadataka Watanabe, Shogo Yamada, Rui Koborinai, Takuro Katsufuji

    Physics Procedia   75   898 - 901  2015年  [査読有り]

     概要を見る

    Ultrasound velocity measurements were performed on a single crystal of the orbital-degenerate frustrated spinel CoV2O4 in all the symmetrically-independent elastic moduli of the cubic crystal. The measurements of temperature dependence of the elastic moduli observed discontinuous elastic anomalies due to a ferrimagnetic transition at TC = 165 K and another phase transition at T∗ = 50 K. Additionally, the measurements observed anomalous temperature dependence of the elastic moduli, specifically, non-monotonic temperature dependence in the magnetically-ordered phase below TC, and magnetic-field-sensitive elastic softening with decreasing temperature in the paramagnetic phase above TC. These anomalous temperature variations below and above TC should be driven by the coupling of lattice to magnetic excitations.

    DOI

  • Magnetoresistance in a doped Mott-Hubbard system: RTiO3

    T. Ito, Y. Shimada, T. Katsufuji

    PHYSICAL REVIEW B   91 ( 2 ) 024411  2015年01月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We studied the magnetoresistance of RTiO3 (R = Pr and Ce) with offstoichiometry as a doped Mott-Hubbard system and found positive magnetoresistance proportional to the absolute value of the magnetic field in the antiferromagnetic phase of both insulating and metallic samples. This result indicates that not only the coherent state that appears with doping and is responsible for the metallic conduction, but also the incoherent state of the Mott-Hubbard insulator is affected by the spin configuration.

    DOI

  • Comparative studies of the thermal conductivity of spinel oxides with orbital degrees of freedom

    Y. Ishitsuka, T. Ishikawa, R. Koborinai, T. Omura, T. Katsufuji

    PHYSICAL REVIEW B   90 ( 22 ) 224411  2014年12月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We studied the thermal conductivity of various transition-metal oxides with the spinel structure (MnV2O4, FeV2O4, CoV2O4, and Mn3O4) upon varying the temperature and magnetic field. We found that for the spinel oxides having V3+ ions (two electrons in the t(2g) states) at the octahedral site, the orbital ordering suppresses the thermal resistivity (inverse thermal conductivity) in contrast to purely magnetic ordering, indicating that the orbital fluctuation of the V t(2g) states is the main factor affecting the thermal conductivity. On the other hand, for Mn3O4, which has Mn3+ ions (one electron in the e(g) states) at the octahedral site, the thermal resistivity is suppressed in association with the successive magnetic phase transitions with decreasing temperature and increasing magnetic field. This can be explained by the frustration of Mn3+ spins and the fluctuation of Mn3+ e(g) orbitals.

    DOI

  • Elastic and anelastic relaxations associated with phase transitions in EuTiO3

    Leszek J. Spalek, Siddharth S. Saxena, Christos Panagopoulos, Takuro Katsufuji, Jason A. Schiemer, Michael A. Carpenter

    Physical Review B - Condensed Matter and Materials Physics   90 ( 5 )  2014年08月  [査読有り]

     概要を見る

    Elastic and anelastic properties of single crystal samples of EuTiO3 have been measured between 10 and 300 K by resonant ultrasound spectroscopy at frequencies in the vicinity of 1 MHz. Softening of the shear elastic constants C44 and 12(C11-C12) by ∼20-30% occurs with falling temperature in a narrow interval through the transition point, Tc=284 K, for the cubic-tetragonal transition. This is accounted for by classical coupling of macroscopic spontaneous strains with the tilt order parameter in the same manner as occurs in SrTiO3. A peak in the acoustic loss occurs a few degrees below Tc and is interpreted in terms of initially mobile ferroelastic twin walls, which rapidly become pinned with further lowering of temperature. This contrasts with the properties of twin walls in SrTiO3, which remain mobile down to at least 15 K. No further anomalies were observed that might be indicative of strain coupling to any additional phase transitions above 10 K. A slight anomaly in the shear elastic constants, independent of frequency and without any associated acoustic loss, was found at ∼140 K. It marks a change from elastic stiffening to softening with falling temperature and perhaps provides evidence for coupling between strain and local fluctuations of dipoles related to the incipient ferroelectric transition. An increase in acoustic loss below ∼80 K is attributed to the development of dynamical magnetic clustering ahead of the known antiferromagnetic ordering transition at ∼5.5 K. Detection of these elastic anomalies serves to emphasize that coupling of strain with tilting, ferroelectric, and magnetic order parameters is likely to be a permeating influence in determining the structure, stability, properties, and behavior of EuTiO3. © 2014 American Physical Society.

    DOI

  • Elastic and anelastic relaxations associated with phase transitions in EuTiO3

    Leszek J. Spalek, Siddharth S. Saxena, Christos Panagopoulos, Takuro Katsufuji, Jason A. Schiemer, Michael A. Carpenter

    PHYSICAL REVIEW B   90 ( 5 ) 054119  2014年08月  [査読有り]

     概要を見る

    Elastic and anelastic properties of single crystal samples of EuTiO3 have been measured between 10 and 300 K by resonant ultrasound spectroscopy at frequencies in the vicinity of 1 MHz. Softening of the shear elastic constants C-44 and 1/2 (C-11 - C-12) by similar to 20-30% occurs with falling temperature in a narrow interval through the transition point, T-c = 284 K, for the cubic-tetragonal transition. This is accounted for by classical coupling of macroscopic spontaneous strains with the tilt order parameter in the same manner as occurs in SrTiO3. A peak in the acoustic loss occurs a few degrees below Tc and is interpreted in terms of initially mobile ferroelastic twin walls, which rapidly become pinned with further lowering of temperature. This contrasts with the properties of twin walls in SrTiO3, which remain mobile down to at least 15 K. No further anomalies were observed that might be indicative of strain coupling to any additional phase transitions above 10 K. A slight anomaly in the shear elastic constants, independent of frequency and without any associated acoustic loss, was found at similar to 140 K. It marks a change from elastic stiffening to softening with falling temperature and perhaps provides evidence for coupling between strain and local fluctuations of dipoles related to the incipient ferroelectric transition. An increase in acoustic loss below similar to 80 K is attributed to the development of dynamical magnetic clustering ahead of the known antiferromagnetic ordering transition at similar to 5.5 K. Detection of these elastic anomalies serves to emphasize that coupling of strain with tilting, ferroelectric, and magnetic order parameters is likely to be a permeating influence in determining the structure, stability, properties, and behavior of EuTiO3.

    DOI

  • Anomalous metallic ground state in BaV13O18

    T. Kanzaki, J. Fujioka, Y. Tokura, H. Kuwahara, T. Katsufuji

    PHYSICAL REVIEW B   89 ( 14 ) 140401(R)  2014年04月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We studied single crystals of BaV13O18, in which V ions with an average valence of + 2.62 (3d(2.38)) form a quasi-fcc lattice. We found that the ground state of the stoichiometric crystal is insulating, whereas that of the off-stoichiometric crystal is metallic with a large effective mass. This metallic state can be explained by the Kondo coupling between the magnetic moment existing at the lone V ion and itinerant carriers in the V tetramers.

    DOI

  • Effect of Offstoichiometry on the Physical Properties of Sr2VO4

    Takashi Ueno, Jungeun Kim, Masaki Takata, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   83 ( 3 ) 034708  2014年03月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We studied the physical properties of Sr2VO4 with a K2NiF4 structure in polycrystalline forms synthesized under various different conditions. We found that stoichiometric samples or those with nominal hole doping into the d(1) state of the V ion exhibit insulating behavior with an antiferromagnetic interaction between V moments, whereas those with nominal electron doping exhibit metallic conduction and ferromagnetic interaction. A structural phase transition at T-c similar to 100 K, which is likely to be associated with the orbital ordering of the V4+ ion, is observed only in the former sample exhibiting a sharp drop in the magnetic susceptibility at the same temperature, whereas no structural anomaly was observed for the sample exhibiting a cusp in the magnetic susceptibility at similar to 50 K.

    DOI

  • Photoinduced dynamics of spinel MnV2O4

    K. Takubo, Y. Onishi, A. Furuhashi, A. Nogami, T. Katsufuji

    PHYSICAL REVIEW B   88 ( 21 ) 214416  2013年12月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    By pump-probe optical reflectivity measurements, we studied the photoinduced dynamics of MnV2O4, which is a d(2) Mott insulator and exhibits a simultaneous spin and orbital ordering accompanied by a structural phase transition at 57 K. We compared the present experimental results with the results of the steady-state optical measurement of the same compound and the steady-state and pump-probe optical measurements of Al-doped MnV2O4, in which the orbital ordering and structural phase transition are absent but the magnetic ordering exists. We found that the photoinduced dynamics of MnV2O4 can be separated into three components: (1) the suppression of the Mott-excitation peak, (2) the melting of the orbital ordering, and (3) the melting of the magnetic ordering. Furthermore, we found that the lattice distortion associated with the photoinduced structural phase transition from tetragonal to cubic induces a strain wave whose propagation velocity is discernibly smaller than that of a conventional shockwave.

    DOI

  • Dynamic spin correlations in the frustrated cubic phase of MnV 2O4

    Jae-Ho Chung, Young-Sang Song, J. H. Kim, T. Suzuki, T. Katsufuji, M. Matsuda, S. H. Lee

    Physical Review B - Condensed Matter and Materials Physics   88 ( 9 ) 094430  2013年09月

     概要を見る

    The ferrimagnetic spinel MnV2O4 undergoes an orbital-induced cubic-to-tetragonal distortion at TYK=58 K, below which noncollinear commensurate ferrimagnetic ordering occurs. Using inelastic neutron scattering, we investigated low-energy dynamics in its cubic phase above TYK. We observed two types of coexisting short-range magnetic excitations: the dispersive spin waves centered around the Brillouin-zone centers, k0 = (0, 0, 0), and quasielastic spin fluctuations centered at incommensurate wave vectors, kincom = (ξ, ξ, 0). The coexistence of the two distinct features can be understood as a dynamic realization of the conical spiral order observed in cubic spinels such as CoCr2O4. © 2013 American Physical Society.

    DOI

  • Photoinduced dynamics in spinel and pyrochlore vanadates as Mott-Hubbard insulators with magnetic ordering

    A. Furuhashi, H. Nishihara, K. Takubo, T. Katsufuji

    PHYSICAL REVIEW B   88 ( 6 ) 060410(R)  2013年08月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We studied the photoinduced dynamics of spinel MnV1.8Al0.2O4 and pyrochlore Yb2V2O7 as prototypical Mott-Hubbard insulators with magnetic ordering. We found that the Mott excitation spectrum is suppressed much sooner than 1 ps after photoirradiation, and subsequently it shows an energy shift over several or several tens of picoseconds, which is caused by the melting of magnetic ordering. This indicates that the Mott excitation spectrum is dominated not only by the Coulomb repulsion energy but also by magnetic interactions, and their different energy scales lead to the different time scales in the photoinduced dynamics.

    DOI

  • Electronic phase transition and an anomalous ordered phase in Ba 2Ti13O22 with 3d1 ions on a triangle-based lattice

    K. Takayama, T. Koyama, S. Mori, K. Kato, M. Takata, J. Fujioka, Y. Tokura, J. Miyazaki, T. Katsufuji

    Physical Review Letters   110 ( 19 ) 196405  2013年05月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    We found that Ba2Ti13O22 with Ti3 + (3d1) ions on a triangle-based lattice exhibits a phase transition at Tc∼200 K, below which the increase of electrical resistivity and decrease of magnetic susceptibility were observed. Transmission electron microscopy and optical reflectivity measurements indicate that the low-temperature phase of the present compound shares characteristics in common with a charge-density-wave state with remnant carriers, although a commensurate wave vector of the modulation and a linear temperature dependence of the magnetic susceptibility below Tc suggest an exotic ordered state. © 2013 American Physical Society.

    DOI

  • Temperature Dependence of the Photoinduced Dynamics on the Cleaved Surface of BaV10O15

    Ayaka Nogami, Tatsuya Kanzaki, Asuka Furuhashi, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   82 ( 3 ) 034713  2013年03月  [査読有り]

    担当区分:最終著者, 責任著者

     概要を見る

    The temperature dependence of the photoinduced dynamics by the irradiation of a femtosecond laser pulse was studied for the cleaved surface of BaV10O15, which shows V trimerization with orbital ordering at T-c = 123 K. For T &gt; T-c, the photoinduced reflectivity change exhibits oscillation with time, indicating that a photoinduced state propagates with time in the crystal. For T &lt; T-c, almost time-independent behavior after the instantaneous change of the reflectivity is observed over several tens of picoseconds, indicating the appearance of a stable photoinduced phase. The different photoinduced dynamics above and below T-c can be attributed to the absence or existence of V trimerization with orbital ordering. We also found that the absolute values of the photoinduced reflectivity change are several times larger on the cleaved surface than those on the polished surface, indicating that remnant stress on the polished surface affects the photoinduced dynamics.

    DOI

  • Moving Domain Boundary and the Spontaneous Flow of Thermal Current Controlled by Magnetic Field in Spinel MnV2O4

    Takuro Katsufuji, Takayuki Ishikawa, Yosuke Ishitsuka

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   82 ( 3 ) 034602  2013年03月  [査読有り]

    担当区分:筆頭著者, 責任著者

     概要を見る

    Heat conduction in the system that releases or absorbs latent heat caused by the first-order phase transition gives rise to an intriguing problem. A typical example is seen in the system showing melting and solidification, where the domain boundary between the liquid and solid phase moves with time, known as moving boundary problems. We report in the present paper that a similar moving domain boundary is observed in an all-solid-state system, spinel MnV2O4, where there is a first-order magnetic phase transition and the transition temperature changes with applied magnetic field. We found that in the sample attached to a heat bath with a constant temperature, thermal current flows (without external heat source) when the magnetic field is increased or decreased, associated with the release or absorption of the latent heat. We also found that the magnitude of the spontaneous heat current depends on the sweep rate of the magnetic field. We show that the experimental results, which have a nonlinear nature, can be reproduced by a simple model and simulation.

    DOI

  • Low Temperature Structural Instability of Tetragonal Spinel Mn3O4

    Jae-Ho Chung, Kee Hwan Lee, Young-Sang Song, Takehito Suzuki, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   82 ( 3 ) 034707  2013年03月  [査読有り]

     概要を見る

    Tetragonal ferrimagnetic spinel Mn3O4 shows complex magnetic phase transitions due to competing antiferromagnetic interactions involving Mn2+/3+ ions with difference valences. Using magnetization and neutron diffraction measurements, we propose that its tetragonal lattice possesses an instability towards orthorhombic distortion in zero field well below T-N. The direction of the observed instability suggests that the anomaly is related to lifting of the persistent geometrical frustration via spin-lattice coupling.

    DOI

  • Electric field modulation of the tetragonal domain orientation revealed in the magnetic ground state of quantum paraelectric EuTiO3

    A. P. Petrović, Y. Kato, S. S. Sunku, T. Ito, P. Sengupta, L. Spalek, M. Shimuta, T. Katsufuji, C. D. Batista, S. S. Saxena, C. Panagopoulos

    Physical Review B - Condensed Matter and Materials Physics   87 ( 6 )  2013年02月  [査読有り]

     概要を見る

    We present a study of the thermodynamic and magnetic properties of single-crystal EuTiO3. Signatures of metastability are visible in the heat capacity below the cubic-tetragonal phase transition at 283 K, supporting the evidence for a mismatch between long and short range structural order from previous x-ray diffraction studies. Employing the anisotropic magnetization as an indirect structural probe, we confirm the emergence of multiple orthogonal domains at low temperature. Torque magnetometry is capable of revealing the nature and temperature dependence of the magnetic anisotropy in spite of the domain misalignment
    we hence deduce that tetragonal EuTiO3 enters an easy-axis antiferromagnetic phase at 5.6 K, with a first-order phase transition to an easy-plane ground state below 3 K. Our experimentally determined magnetic phase diagram is accurately reproduced by a three-dimensional (3D) anisotropic Heisenberg spin model. Furthermore, we demonstrate that electric field cooling acts to suppress this orientational disorder by realigning the domains due to the strong coupling between electric fields and lattice dipoles characteristic of paraelectric materials. © 2013 American Physical Society.

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  • Electric field modulation of the tetragonal domain orientation revealed in the magnetic ground state of quantum paraelectric EuTiO3

    A. P. Petrovic, Y. Kato, S. S. Sunku, T. Ito, P. Sengupta, L. Spalek, M. Shimuta, T. Katsufuji, C. D. Batista, S. S. Saxena, C. Panagopoulos

    PHYSICAL REVIEW B   87 ( 6 ) 064103  2013年02月  [査読有り]

     概要を見る

    We present a study of the thermodynamic and magnetic properties of single-crystal EuTiO3. Signatures of metastability are visible in the heat capacity below the cubic-tetragonal phase transition at 283 K, supporting the evidence for a mismatch between long and short range structural order from previous x-ray diffraction studies. Employing the anisotropic magnetization as an indirect structural probe, we confirm the emergence of multiple orthogonal domains at low temperature. Torque magnetometry is capable of revealing the nature and temperature dependence of the magnetic anisotropy in spite of the domain misalignment; we hence deduce that tetragonal EuTiO3 enters an easy-axis antiferromagnetic phase at 5.6 K, with a first-order phase transition to an easy-plane ground state below 3 K. Our experimentally determined magnetic phase diagram is accurately reproduced by a three-dimensional (3D) anisotropic Heisenberg spin model. Furthermore, we demonstrate that electric field cooling acts to suppress this orientational disorder by realigning the domains due to the strong coupling between electric fields and lattice dipoles characteristic of paraelectric materials. DOI: 10.1103/PhysRevB.87.064103

    DOI

  • Change of the optical conductivity spectra with orbital and spin ordering in spinel MnV2O4

    T. Katsufuji, T. Takubo, T. Suzuki

    PHYSICAL REVIEW B   87 ( 5 ) 054424  2013年02月  [査読有り]

    担当区分:筆頭著者, 責任著者

     概要を見る

    Spinel MnV2O4 exhibits orbital ordering at T-c = 57 K accompanied by a structural phase transition from cubic to tetragonal and ferrimagnetic ordering. We measured the temperature dependence of the optical reflectivity spectra on the cleaved surface of a MnV2O4 single crystal with controlled twin structures belowT(c). We found that there are two peaks at 1.6 and 2.3 eV, corresponding to the Mott excitations, in the optical conductivity spectra obtained by the Kramers-Kronig transformation of the reflectivity, and that the two peaks exhibit a similar anisotropic change below T-c. Furthermore, we found that these two peaks exhibit a different temperature dependence above T-c. We analyzed these results by a model in which the multiplet structures of the d state for the Mott excitation are fully taken into account, and discussed the occupation of the triply degenerate t(2g) orbitals both below and above T-c. DOI: 10.1103/PhysRevB.87.054424

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  • Giant magnetic anisotropy in Mn3O4 investigated by Mn-55(2+) and Mn-55(3+) NMR

    Changsoo Kim, Euna Jo, Byeongki Kang, Sangil Kwon, Soonchil Lee, Jeong Hyun Shim, Takehiko Suzuki, Takuro Katsufuji

    PHYSICAL REVIEW B   86 ( 22 ) 224420  2012年12月  [査読有り]

     概要を見る

    In Mn3O4, the magnetization along the c axis is different from that along the ab plane even in the strong field of 30 T. To investigate the origin of the huge magnetic anisotropy, Mn2+ and Mn3+ nuclear magnetic resonance spectra were measured in the 7-T magnetic field. The canting angle of the magnetic moments was estimated for various directions of field by rotating a single-crystalline Mn3O4 sample. One of the main results is that Mn3+ moments lie nearly in the ab plane in the external field perpendicular to the plane, meaning that the macroscopic magnetic anisotropy of Mn3O4 originates from the magnetic anisotropy of Mn3+ in the ab plane. The anisotropy field is estimated to be about 65 T. It is obvious that the Yafet-Kittel structure made of Mn2+ and Mn3+ spins lies in the ab plane due to this huge magnetic anisotropy, contrary to the previous reports. By the least-squares fit of the canting angle data for various field directions to a simple model, we obtained that J(BB) = 1.88J(AB) - 0.09 meV and K-A = -14.7J(AB) + 2.0 meV, where J(AB), J(BB), and K-A are the exchange interaction constants between Mn2+ moments, Mn2+ and Mn3+ moments, and an anisotropy constant of Mn2+, respectively. DOI: 10.1103/PhysRevB.86.224420

    DOI

  • EuTiO 3 magnetic structure studied by neutron powder diffraction and resonant x-ray scattering

    Valerio Scagnoli, Mattia Allieta, Helen Walker, Marco Scavini, Takuro Katsufuji, Leyre Sagarna, Oksana Zaharko, Claudio Mazzoli

    Physical Review B - Condensed Matter and Materials Physics   86 ( 9 )  2012年09月  [査読有り]

     概要を見る

    We combine neutron powder diffraction and x-ray single-crystal magnetic diffraction at the Eu L 2 edge to scrutinize the magnetic motif of the Eu ions in magnetoelectric EuTiO 3. Our measurements are consistent with an antiferromagnetic G-type pattern with the Eu magnetic moments ordering along the a,b-plane diagonal. Recent reports of a novel transition at 2.75 K with a flop of magnetic moments upon poling the sample in an electric field cannot be confirmed for a nonpoled sample. Our neutron diffraction data do not show any significant change of the structure below the Néel temperature. Magnetoelastic coupling, if present, is therefore expected to be negligible. © 2012 American Physical Society.

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  • EuTiO3 magnetic structure studied by neutron powder diffraction and resonant x-ray scattering

    Valerio Scagnoli, Mattia Allieta, Helen Walker, Marco Scavini, Takuro Katsufuji, Leyre Sagarna, Oksana Zaharko, Claudio Mazzoli

    PHYSICAL REVIEW B   86 ( 9 ) 094432  2012年09月  [査読有り]

     概要を見る

    We combine neutron powder diffraction and x-ray single-crystal magnetic diffraction at the Eu L-2 edge to scrutinize the magnetic motif of the Eu ions in magnetoelectric EuTiO3. Our measurements are consistent with an antiferromagnetic G-type pattern with the Eu magnetic moments ordering along the a, b-plane diagonal. Recent reports of a novel transition at 2.75 K with a flop of magnetic moments upon poling the sample in an electric field cannot be confirmed for a nonpoled sample. Our neutron diffraction data do not show any significant change of the structure below the Neel temperature. Magnetoelastic coupling, if present, is therefore expected to be negligible.

    DOI

  • Orbital fluctuations in spinel Mn(V1-xAlx)(2)O-4 studied by thermal conductivity measurements

    T. Omura, T. Ishikawa, Y. Ishitsuka, T. Katsufuji

    PHYSICAL REVIEW B   86 ( 5 ) 054436  2012年08月  [査読有り]

     概要を見る

    We measured the thermal conductivity of spinel MnV2O4 and Al-doped samples with an applied magnetic field, and they exhibited orbital and spin ordering at low temperatures. We found that the thermal resistivity (reciprocal thermal conductivity) in the orbital-disordered phase is enhanced when the system approaches the critical point of the orbital ordering under the variation of both temperature and magnetic field, but it is suppressed when it crosses the critical point and goes into the orbital-ordered phase. This result indicates that the thermal conductivity of these compounds is dominated by the fluctuation of V (t2g) orbitals.

    DOI

  • Orbital states of V trimers in BaV10O15 detected by resonant x-ray scattering

    K. Takubo, T. Kanzaki, Y. Yamasaki, H. Nakao, Y. Murakami, T. Oguchi, T. Katsufuji

    PHYSICAL REVIEW B   86 ( 8 ) 085141  2012年08月  [査読有り]

     概要を見る

    BaV10O15 shows a structural phase transition associated with the trimerization of V ions at T-s = 123 K. We investigated the orbital states of V trimers using the technique of resonant x-ray scattering (RXS) near the V K-edge absorption energy. The ratio of the intensity between the pre-edge structure (1s to 3d transition) and the main-edge structure (1s to 4p transition) strongly depends on the reciprocal Q position of the scattering. We found that the Q position dependence is consistent with the model of orbital ordering proposed by Pen et al., [Phys. Rev. Lett. 78, 1323 (1997)].

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  • Infrared phonon anomalies and orbital ordering in single-crystalline MnV2O4 spinel

    K. Myung-Whun, S. Y. Jang, T. Katsufuji, A. V. Boris

    PHYSICAL REVIEW B   85 ( 22 ) 224423  2012年06月  [査読有り]

     概要を見る

    We used infrared ellipsometry to study phonons in a spinel MnV2O4 single crystal. At 300 K, the imaginary part of the complex dielectric function spectra show four phonon peaks. Below the cubic-tetragonal structural transition temperature at 58 K, small peaks appear around the cubic modes and the number of phonons becomes more than 12. The group analysis predicted 12 infrared modes for the I4(1/a) symmetry. The observed modes support the I4(1/a) or lower crystal symmetry, which reflects the antiferro ordering of V3+ t(2g) orbitals.

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  • Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO 3

    Mattia Allieta, Marco Scavini, Leszek J. Spalek, Valerio Scagnoli, Helen C. Walker, Christos Panagopoulos, Siddharth S. Saxena, Takuro Katsufuji, Claudio Mazzoli

    Physical Review B - Condensed Matter and Materials Physics   85 ( 18 )  2012年05月  [査読有り]

     概要を見る

    Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO 3 has been considered to remain cubic down to low temperature. Here we present high-resolution synchrotron x-ray powder-diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I4/mcm, involving TiO 6 octahedra tilting, in analogy to the case of SrTiO 3. The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature T c = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at T A ∼ 282 K. By performing atomic pair distribution function analysis on diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated T c, the average model symmetry is fully compatible with the local environment distortion, but the former is characterized by a reduced value of the tilting angle compared to the latter. At T = 240 K, data show the presence of local octahedra tilting identical to the low-temperature one, while the average crystallographic structure remains cubic. On this basis, we propose that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiO 3 properties. © 2012 American Physical Society.

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  • Role of intrinsic disorder in the structural phase transition of magnetoelectric EuTiO3

    Mattia Allieta, Marco Scavini, Leszek J. Spalek, Valerio Scagnoli, Helen C. Walker, Christos Panagopoulos, Siddharth S. Saxena, Takuro Katsufuji, Claudio Mazzoli

    PHYSICAL REVIEW B   85 ( 18 ) 184107  2012年05月  [査読有り]

     概要を見る

    Up to now, the crystallographic structure of the magnetoelectric perovskite EuTiO3 has been considered to remain cubic down to low temperature. Here we present high-resolution synchrotron x-ray powder-diffraction data showing the existence of a structural phase transition, from cubic Pm-3m to tetragonal I 4/mcm, involving TiO6 octahedra tilting, in analogy to the case of SrTiO3. The temperature evolution of the tilting angle and of the full width at half maximum of the (200) cubic reflection family indicate a critical temperature T-c = 235 K. This critical temperature is well below the recent anomaly reported by specific-heat measurement at T-A similar to 282 K. By performing atomic pair distribution function analysis on diffraction data, we provide evidence of a mismatch between the local (short-range) and the average crystallographic structures in this material. Below the estimated T-c, the average model symmetry is fully compatible with the local environment distortion, but the former is characterized by a reduced value of the tilting angle compared to the latter. At T = 240 K, data show the presence of local octahedra tilting identical to the low-temperature one, while the average crystallographic structure remains cubic. On this basis, we propose that intrinsic lattice disorder is of fundamental importance in the understanding of EuTiO3 properties.

    DOI

  • Change of phonon Raman spectra with V trimerization in BaV10O15

    T. Kanzaki, R. Kubota, T. Katsufuji

    PHYSICAL REVIEW B   85 ( 14 ) 144410  2012年04月  [査読有り]

     概要を見る

    We investigated Raman scattering of BaV10O15, which exhibits a structural phase transition with V trimerization at T-c = 123 K possibly dominated by the orbital ordering of V t(2g) states. It was found that phonon Raman spectra drastically change at T-c. According to the analysis of the phonon spectra based on the bond-polarizability model, the force constant of the V-V bond increases by 75% and the peak width is reduced by half below T-c. This result can be explained by the existence of disorder originating from orbital degrees of freedom above T-c, which is suppressed by the long-range orbital ordering below T-c.

    DOI

  • Thermal Rectification in the Vicinity of a Structural Phase Transition

    Wataru Kobayashi, Daisuke Sawaki, Tsubasa Omura, Takuro Katsufuji, Yutaka Moritomo, Ichiro Terasaki

    APPLIED PHYSICS EXPRESS   5 ( 2 ) 027302  2012年02月  [査読有り]

     概要を見る

    We have fabricated an oxide thermal rectifier made of La1.98Nd0.02CuO4 and MnV2O4. By utilizing a jump of a thermal conductivity originated in the structural phase transition accompanied by the orbital ordering in MnV2O4, a rectifying coefficient of 1.14 has been achieved in the presence of a small temperature difference of 2 K. A thermal rectifier operating under a small temperature difference will play an important role for realizing heat-current control in electronic devices. (C) 2012 The Japan Society of Applied Physics

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  • Charge-gap formation in the insulating states of AV(10)O(15) (A = Ba, Sr)

    M. Hoshino, T. Kajita, T. Kanzaki, M. Uchida, Y. Tokura, T. Katsufuji

    PHYSICAL REVIEW B   85 ( 8 ) 085106  2012年02月  [査読有り]

     概要を見る

    We measured the optical conductivity spectra of SrV10O15 with V2.8+, which exhibits an insulating behavior without any phase transition, and compared the results with those of BaV10O15, which shows orbital ordering below T-c = 123 K. We found that a soft gap opens at the lowest temperature but it collapses with increasing temperature in SrV10O15, in contrast to a finite charge gap (similar to 0.3 eV) in BaV10O15 below T-c. We also found that the difference of the conductivity spectra between SrV10O15 and BaV10O15 evolves even above T-c of Ba, indicating that the fluctuation of orbital ordering produces a pseudogap in BaV10O15.

    DOI

  • Orbital ordering and spin-singlet clusters in triangular-based vanadates

    Yasuhiro Shimizu, Kenichiro Matsudaira, Masayuki Itoh, Takamasa Kajita, Miho Ikeda, Jun Miyazaki, Takuro Katsufuji

    26TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT26), PTS 1-5   400  2012年  [査読有り]

     概要を見る

    We report V-51 NMR spectra in the triangular-lattice vanadates, AV(10)O(15) (3d(2.2)), AV(13)O(18) (3d(2.38)), and A(2)V(13)O(22) (3d(1.92)) (A = Ba, Sr). These compounds have orbital degrees of freedom and itinerant electrons arising from the partially occupied t(2g) bands. We show microscopic observations of spin-singlet trimers and antiferromagnetic tertramers with orbital orders in BaV10O15, but no indication of the orbital order and the spin singlet in SrV10O15. The paramagnetic and nonmagnetic spins coexist in BaV13O22 but only the nonmagnetic spins are present in AV(13)O(18) (A = Ba, Sr). The spin singlet formation in the incommensurate band filling is understood as the incommensurate charge-density wave induced by the 3d orbital order.

    DOI

  • Dynamics of photoinduced phenomena in AV 10O 15 (A = Ba, Sr)

    A. Nogami, K. Takubo, T. Kajita, M. Hoshino, T. Katsufuji

    Physical Review B - Condensed Matter and Materials Physics   84 ( 21 )  2011年12月  [査読有り]

     概要を見る

    We studied the dynamics of photoinduced phenomena in AV 10O 15 (A=Ba, Sr), where the ground state changes from an insulating state with V trimerization (Ba) to another insulating state with no structural anomaly (Sr). We found that the time dependence of photoinduced reflectivity change is similar up to several picoseconds, but is critically different in the range of several tens of picoseconds between the two compounds. These different behaviors can be attributed to the coupled mode of the photoexcited electronic state and the lattice distortion, which propagates in the crystal of SrV 10O 15 but is trapped in the local minimum of the free energy in BaV 10O 15. © 2011 American Physical Society.

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  • Dynamics of photoinduced phenomena in AV(10)O(15) (A = Ba, Sr)

    A. Nogami, K. Takubo, T. Kajita, M. Hoshino, T. Katsufuji

    PHYSICAL REVIEW B   84 ( 21 ) 214442  2011年12月  [査読有り]

     概要を見る

    We studied the dynamics of photoinduced phenomena in AV(10)O(15) (A = Ba, Sr), where the ground state changes from an insulating state with V trimerization (Ba) to another insulating state with no structural anomaly (Sr). We found that the time dependence of photoinduced reflectivity change is similar up to several picoseconds, but is critically different in the range of several tens of picoseconds between the two compounds. These different behaviors can be attributed to the coupled mode of the photoexcited electronic state and the lattice distortion, which propagates in the crystal of SrV10O15 but is trapped in the local minimum of the free energy in BaV10O15.

    DOI

  • Evolution of phonon Raman spectra with orbital ordering in spinel MnV2O4

    K. Takubo, R. Kubota, T. Suzuki, T. Kanzaki, S. Miyahara, N. Furukawa, T. Katsufuji

    PHYSICAL REVIEW B   84 ( 9 ) 094406  2011年09月  [査読有り]

     概要を見る

    We investigated Raman scattering of spinel MnV2O4, for which orbital ordering of Vt(2g) states below T-c = 57K was proposed. Several peaks evolve in the Raman spectra below T-c, and they show a peculiar polarization dependence, which cannot be explained only by a group theory analysis. We found that it can be explained by considering Mott excitations in the presence of orbital ordering as the dominant intermediate states in the Raman process. This result gives the evidence of orbital ordering in this compound from the viewpoint of electronic structure.

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  • Spin-singlet trimer state induced by competing orbital order in triangular-lattice BaV10O15

    Yasuhiro Shimizu, Keniichiro Matsudaira, Masayuki Itoh, Tomomasa Kajita, Takuro Katsufuji

    PHYSICAL REVIEW B   84 ( 6 ) 064421  2011年08月  [査読有り]

     概要を見る

    Local spin and orbital textures are investigated by V-51 NMR measurements on a triangular-lattice compound BaV10O15 with orbital degrees of freedom and itinerant electrons. The Knight shift shows the spin-singlet V trimer formation in a itinerant phase above the semiconductor-insulator transition temperature T-SI = 140 K. Below T-SI, the observation of the asymmetric electric hyperfine coupling tensor agrees with the orbital order giving direct d-d bonds of the trimer. The remaining paramagnetic spins on a V tetramer exhibit an antiferromagnetic long-range order accompanied by a ferro-type order. The coexistence of the trimer and tetramer with the two opposite magnetism is ascribed to the orbitally induced Peierls instability on a non-half-filling triangular lattice with nonuniform V-V separation.

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  • Spin-doping effect on the electronic structure of Sr1-(x plus y)Lax+yTi1-xCrxO3

    T. Saitoh, H. Iwasawa, K. Kurahashi, Y. Nakano, T. Katsufuji, I. Hase, K. Shimada, H. Namatame, M. Taniguchi

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   184 ( 3-6 ) 232 - 235  2011年04月  [査読有り]

     概要を見る

    We have investigated spin-doping effects on the valence-band electronic structure of Sr1-(x+y)Lax+yTi1-xCrxO3 at y = 0.2 using high-resolution photoemission spectroscopy and first-principles band structure calculations. The Cr 3d photoemission spectral weight located between 2.0 and 3.5 eV increased with x, indicating that x works as a rough measure of the spin density. However, the increase in the spectral weight was actually more rapid than the band theory prediction. The near-Fermi level (E-F) weight due to the Ti 3d states showed significant reduction with x. Considering that the band theory predicted nearly constant density of states near E-F, this reduction of the near-E-F weight should be interpreted as a manifestation of strong Ti-Cr site disorder effects. (C) 2011 Elsevier B.V. All rights reserved.

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  • Competition between vanadium tetramerization and trimerization in Ba1-xSrxV13O18

    M. Ikeda, T. Okuda, K. Kato, M. Takata, T. Katsufuji

    PHYSICAL REVIEW B   83 ( 13 ) 134417  2011年04月  [査読有り]

     概要を見る

    We studied the transport, magnetic, and structural properties of Ba1-xSrxV13O18, in which V and O ions form a sodium-chloride lattice with periodically missing sites. We found that the high-temperature insulating phase is characterized by spin-singlet V tetramers, whereas the low-temperature conducting phase is characterized by V trimers, and these two phases are competing in this series of compounds. Various properties substantially change at the phase transition between the two, indicating a drastic reconstruction of the electronic structure.

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  • High-temperature interlayer magnetoresistance in La5Mo4O16

    K. Kobayashi, T. Katsufuji

    PHYSICAL REVIEW B   83 ( 10 ) 100411(R)  2011年03月  [査読有り]

     概要を見る

    We found that La5Mo4O16 with Mo4+ and Mo5+ ions (S = 1 and S = 1/2 spins) on a quasisquare lattice exhibits a distinct magnetoresistance for the current perpendicular to the square-lattice layers below the antiferromagnetic ordering temperature T-AF = 190 K. This magnetoresistance occurs well below 1 T, and can be attributed to a metamagnetic transition from antiferromagnetically aligned moments between the layers to ferromagnetically aligned ones. The magnetoresistance changes its characteristic with the change of the magnetic state below T-F = 70 K, where spontaneous magnetization appears.

    DOI

  • Ferroelectric and structural antiphase domains in hexagonal RMnO 3

    K. Kobayashi, T. Koyama, Y. Horibe, T. Katsufuji, S. W. Cheong, S. Mori

    Materials Research Society Symposium Proceedings   1397   20 - 25  2011年  [査読有り]

     概要を見る

    We have investigated characteristic ferroelectric and structural antiphase domain structures in single crystals of hexagonal RMnO3 (R=Y, Ho, Lu, and Yb) by obtaining various electron diffraction patterns, dark-filed images and high-resolution lattice images. In the ferroelectric phase RMnO3 characteristic domain structures consisting of six ferroelectric and structural antiphase domains, which can be identified as the "cloverleaf" pattern, is found in the (110) plane, in addition to the (001) plane, and are inherent to the ferroelectric phase of hexagonal RMnO3. In domain configuration with the cloverleaf pattern in the (110) plane, the structural antiphase boundaries are inclined to be parallel to the [001] direction. © 2012 Materials Research Society.

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  • Local spin susceptibility in the orbital-frustrated vanadium oxide BaV 10O15

    Yasuhiro Shimizu, Ken-Ichiro Matsudaira, Masayuki Itoh, Tomomasa Kajita, Takuro Katsufuji

    Journal of Physics: Conference Series   320  2011年  [査読有り]

     概要を見る

    BaV10O15 having a bilayer triangular lattice exhibits a semiconductor-insulator transition accompanied by the structural phase transition at Tc ∼130 K. We show the microscopic properties of the phase transition investigated by 51V NMR measurements on a single crystal. We have observed 51V NMR spectra from two of the three inequivalent V sites at high temperatures. The Knight shift tensors are determined at 300 K for the two V sites. The local spin susceptibility strongly depends on the V sites and temperature despite the nearly independent magnetic susceptibility. Each spectrum is split into two below 200 K far above T c, suggesting a charge/order ordering transition. The local spin susceptibilities of the three sites gradually vanish with decreasing temperature, consistent with the V trimer formation. The microscopic evidence for the spin singlet state is clearly observed below Tc. © 2011 Published under licence by IOP Publishing Ltd.

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  • Magnetic susceptibility and specific heat of a spinel MnV2O4 single crystal

    Kim Myung-Whun, J. S. Kim, T. Katsufuji, R. K. Kremer

    PHYSICAL REVIEW B   83 ( 2 ) 024403  2011年01月  [査読有り]

     概要を見る

    We investigated the magnetic and the thermodynamic properties of a spinel MnV2O4 single crystal. The magnetization and susceptibility show the signature of ferromagnetic ordering at similar to 61 K. A sharp peak in the magnetization and the specific heat indicate the structural phase transition at 58 K, consistent with the scattering experiments. The specific-heat data exhibits the signature of double transitions. In addition to the higher-temperature transition at T-S, the second transition occurs at a lower temperature T*, which divides the phase below T-S into a low-magnetic-field and a high-magnetic-field phase. Based on the magnetic susceptibility and the specific heat, a wide-range temperature and magnetic-field phase diagram was constructed. The low-magnetic-field phase shows the characteristics of a spin-glass-like state. A comparison with the magnetic and thermodynamic characteristics of similar materials suggests that the low temperature and low-magnetic-field, spin-glass-like phase in MnV2O4 may originate from the competition between the interchain and the intrachain exchange interaction owing to the fluctuating V-ion orbitals.

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  • Chemical doping-induced flop of ferroelectric polarization in multiferroic Mn0.9Co0.1WO4

    Y. -S. Song, Li Qin Yan, Bumsung Lee, Sae Hwan Chun, Kee Hoon Kim, Sung Baek Kim, A. Nogami, T. Katsufuji, J. Schefer, J. -H. Chung

    PHYSICAL REVIEW B   82 ( 21 ) 214418  2010年12月  [査読有り]

     概要を見る

    We studied the multiferroic properties of the incommensurate spiral phase of Mn1-xCoxWO4 using single-crystal neutron-diffraction and ferroelectric measurements. Whereas the rotational plane of the spin spiral is parallel to the b axis for x &lt;= 0.05, it flops by 90 degrees for x=0.10. As a result, the direction of the ferroelectric polarization also flops from the b to the a axis. These results show that the orientation of the ferroelectric polarization in MnWO4 can be controlled via chemical doping.

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  • Phase transitions and the role of vanadium t(2g) states in AV(13)O(18) (A = Sr, Ba)

    M. Ikeda, Y. Nagamine, S. Mori, J. E. Kim, K. Kato, M. Takata, T. Katsufuji

    PHYSICAL REVIEW B   82 ( 10 ) 104415(R)  2010年09月  [査読有り]

     概要を見る

    We studied AV(13)O(18) (A = Sr, Ba), in which V ions form a face-centered-cubic lattice with periodically missing ions. It was found that there is a phase transition in both SrV13O18 (T-c = 270 K) and BaV13O18 (T-c = 200 K), where anomalies are observed in resistivity and magnetic susceptibility. Resistivity shows a contrasting behavior in the low-temperature phase between SrV13O18 (a metallic behavior) and BaV13O18 (an insulating behavior). By synchrotron x-ray powder-diffraction measurement, it was found that V trimerization occurs at the phase transition of SrV13O18 . On the other hand, BaV13O18 does not show a large change in the x-ray powder diffraction but superlattice peaks at (01/21/2) was observed in the electron diffraction. Possible orbital states of the t(2g) electrons in the V ions are discussed.

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  • Crossover behavior of the crystal structure and the relation to magnetism in perovskite RTiO3

    K. Takubo, M. Shimuta, J. E. Kim, K. Kato, M. Takata, T. Katsufuji

    PHYSICAL REVIEW B   82 ( 2 ) 020401(R)  2010年07月  [査読有り]

     概要を見る

    We found that a crossover behavior of the crystal structure at high temperatures dominates the magnetic state at low temperatures in the series of RTiO3 (R is rare earth). At the crossover temperature, which exist only in the samples that become antiferromagnetic at lower temperatures, the temperature derivative of lattice constants changes its sign, and the crossover temperature approaches zero as the Neel temperature approaches zero in the phase diagram of the ionic radius of R vs temperature. This result indicates that the orbital state of the Ti ion dominates the low-temperature magnetic state in RTiO3.

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  • Formation of a Three-Dimensional Network of V Trimers in A(2)V(13)O(22) (A = Ba, Sr)

    J. Miyazaki, K. Matsudaira, Y. Shimizu, M. Itoh, Y. Nagamine, S. Mori, J. E. Kim, K. Kato, M. Takata, T. Katsufuji

    PHYSICAL REVIEW LETTERS   104 ( 20 ) 207201  2010年05月  [査読有り]

     概要を見る

    We found that in A(2)V(13)O(22) (A = Ba, Sr), which contains a trilayer slab of VO in the sodium-chloride structure with periodically missing ions, the trimerization of V ions occurs at 290 K (A = Ba) and 380 K (A = Sr). V trimers form a three-dimensional network, but some V ions remain untrimerized in these compounds. The suppression of magnetic susceptibility with trimerization and the existence of a Curie tail at low temperatures, together with the result of NMR measurement, indicate that the V trimers are spin singlet, whereas the untrimerized V ions have a magnetic moment; i.e., there is a spontaneous separation between nonmagnetic and magnetic ions in the crystal.

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  • Large off-diagonal magnetoelectric coupling in the quantum paraelectric antiferromagnet EuTiO3

    V. V. Shvartsman, P. Borisov, W. Kleemann, S. Kamba, T. Katsufuji

    Physical Review B - Condensed Matter and Materials Physics   81 ( 6 )  2010年02月  [査読有り]

     概要を見る

    The third-order E2 H2 -type magnetoelectric (ME) response of polycrystalline EuTiO3 changes sign under magnetic bias and shows a large anomaly at the antiferromagnetic (AF)-paramagnetic phase boundary below TN ≈5.3 K. It is attributed to critical fluctuations of the AF order parameter reinforced by quantum paraelectric polar correlations. The underlying biquadratic spin-lattice coupling involves electric field induced Dzyaloshinskii-Moriya interaction as described within mean-field approximation. Single domaining by ME annealing (or cooling) significantly enhances the response by additional EH and E H2 effects. © 2010 The American Physical Society.

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  • Large off-diagonal magnetoelectric coupling in the quantum paraelectric antiferromagnet EuTiO3

    V. V. Shvartsman, P. Borisov, W. Kleemann, S. Kamba, T. Katsufuji

    PHYSICAL REVIEW B   81 ( 6 ) 064426  2010年02月  [査読有り]

     概要を見る

    The third-order (EH2)-H-2-type magnetoelectric (ME) response of polycrystalline EuTiO3 changes sign under magnetic bias and shows a large anomaly at the antiferromagnetic (AF)-paramagnetic phase boundary below T-N approximate to 5.3 K. It is attributed to critical fluctuations of the AF order parameter reinforced by quantum paraelectric polar correlations. The underlying biquadratic spin-lattice coupling involves electric field induced Dzyaloshinskii-Moriya interaction as described within mean-field approximation. Single domaining by ME annealing (or cooling) significantly enhances the response by additional EH and EH2 effects.

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  • Opening of a charge gap with V trimerization in BaV10O15

    T. Kajita, T. Kanzaki, T. Suzuki, J. E. Kim, K. Kato, M. Takata, T. Katsufuji

    PHYSICAL REVIEW B   81 ( 6 ) 060405(R)  2010年02月  [査読有り]

     概要を見る

    A structural phase transition at T-c similar to 130 K in BaV10O15 containing modified V3+ (3d(2)) triangular lattices was studied. It was found that the electrical resistivity jumps by three orders of magnitude and a gap opens up in the optical conductivity spectrum at T-c. It was also found from synchrotron x-ray powder-diffraction measurement that the trimerization of the V ions occurs below T-c. These results indicate that the orbital ordering of V t(2g) states occurs at T-c and that induces a charge gap near the Fermi level in the electronic states of BaV10O15.

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  • Study of Dynamics in Inhomogeneous States by Temperature-Modulated Optical Reflectivity Measurement: Application to Perovskite Manganites

    Junichi Masuzawa, Yuki Saiki, Takehito Suzuki, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   79 ( 1 ) 014704  2010年01月  [査読有り]

     概要を見る

    We developed an experimental system for measuring temperature-modulated optical reflectivity with a frequency up to 20 kHz to Study the dynamics of inhomogeneity in a crystal. By this system, we studied doped perovskite manganites, in which a ferromagnetic metallic phase and a (short-ranged) charge-ordered phase coexist near the Curie temperature (T-C). We found an appreciable frequency dependence of the temperature-modulated reflectivity around T-C. This result can be explained by the dynamics of the domains in the two coexisting phases, i.e., by the relaxation time to reach a new equilibrium state of the two-phase coexistence when the temperature is modulated.

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  • Frustrated Magnetism and Cooperative Phase Transitions in Spinels

    Seung-Hun Lee, Hidenori Takagi, Despina Louca, Masaaki Matsuda, Sungdae Ji, Hiroaki Ueda, Yutaka Ueda, Takuro Katsufuji, Jae-Ho Chung, Sungil Park, Sang-Wook Cheong, Collin Broholm

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   79 ( 1 ) 011004  2010年01月  [査読有り]

     概要を見る

    The spinel crystal system, AB(2)O(4), has been fertile ground in studying the effects of magnetic and orbital frustration. The experimental findings, with primary focus on neutron and synchrotron x-ray scattering techniques, are hereby reviewed in spinels with magnetic B ions. Highlighted are novel collective phenomena, such as the zero-energy excitation mode in the spin liquid phase, zero-field and field-induced novel phase transitions, the emergence of complex local spin entities, and heavy fermionic behaviors. Such a diversity in the exotic properties stems from their close proximity to critical points among degenerate states and a delicate balance among different degrees of freedom such as spin, orbital, and lattice.

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  • Optical Study of the Accumulated Charge in the Amorphous InGaZnO4 Thin-Film Transistor

    Akira Kato, Yoshiaki Uesu, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   78 ( 9 ) 093706  2009年09月  [査読有り]

     概要を見る

    Thin-film transistors (TFTs) with amorphous InGaZnO4 as a channel layer are known to exhibit high field-effect mobility. We studied the characteristic of the accumulated carriers in this TFT by optical spectroscopy and observed Drude absorption and oscillation in the transmittance spectrum of the channel region appearing with an applied gate voltage. The scattering rate of the carriers in the accumulation layers estimated from the optical spectrum is (h) over bar/tau(AL) = 0.19eV. and this small value is the origin of the high mobility of the TFT with amorphous InGaZnO4.

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  • Electronic structure of Sr0.8-yLa0.2+yTi0.8Cr0.2O3 studied by photoemission spectroscopy and first-principles band structure calculations

    H. Iwasawa, S. Kaneyoshi, K. Kurahashi, T. Saitoh, I. Hase, T. Katsufuji, K. Shimada, H. Namatame, M. Taniguchi

    PHYSICAL REVIEW B   80 ( 12 ) 125122  2009年09月  [査読有り]

     概要を見る

    We have investigated electron-doping effects on the electronic structure of Sr0.8-yLa0.2+yTi0.8Cr0.2O3 using high-resolution photoemission spectroscopy and first-principles band structure calculations. Photoemission results showed that the Cr 3d peak intensity is independent of the carrier concentration (y), while the spectral weight near the Fermi level, composed of the Ti 3d states, is proportional to y at a fixed spin concentration. Detailed comparisons between the photoemission spectra and the band structure calculations including magnetic-moment calculations reveal that the large energy separation between the Cr 3d states and the Ti 3d states plays an important role to control the electric and magnetic properties in this system.

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  • Impact of orbital degrees of freedom on geometrical frustration in the kagome-like magnet SrVxGa12-xO19

    J. Miyazaki, T. Sonehara, D. Akahoshi, H. Kuwahara, J. E. Kim, K. Kato, M. Takata, T. Katsufuji

    PHYSICAL REVIEW B   79 ( 18 ) 180410(R)  2009年05月  [査読有り]

     概要を見る

    We studied kagome-like magnets, SrVxGa12-xO19 with V3+ (3d(2) in the triply degenerate t(2g) states), as a geometrically frustrated spin system with orbital degrees of freedom. We observed a suppression of low-temperature spin entropy and an anomalous contraction of the V triangles on the kagome lattice, which can be regarded as the V trimerization with spin-singlet formation. This result, which is in clear contrast to their Cr3+ (3d(3)) analogs exhibiting a spin-liquid behavior, indicates a substantial impact of orbitals on geometrically frustrated magnets.

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  • Direct observation of a repeatable change in electronic states with applied electric voltage pulses in the metal-insulator-metal structure

    K. Azumi, K. Aoyama, S. Asanuma, Y. Uesu, T. Katsufuji

    PHYSICAL REVIEW B   79 ( 12 ) 121101(R)  2009年03月  [査読有り]

     概要を見る

    By the microspectroscopic imaging of optical reflectivity for the La1-xSrxFeO3-delta thin film with indium-tin-oxide electrodes, we observed that the shape of the conducting area between two electrodes is reversibly varied with applied electric voltage pulses and that causes a repeatable change in the electrical resistance between the electrodes. These results can be explained by the oxygen-ion migration with applied electric field, which induces a variation in the distribution of the Fe4+ ions in the thin film.

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  • Magnetodielectric and magnetoelastic properties of spinel Mn(3)O(4)

    T. Suzuki, T. Katsufuji

    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4   150  2009年  [査読有り]

     概要を見る

    The magnetodielectric and magnetoelastic properties of spinel Mn(3)O(4) with the orbital degree of freedom in the e(g) state of the Mn(3+) site were investigated. We found that strain (Delta L/L) shows anomalies at the magnetic phase transition temperatures and is anisotropic against the crystallographic axis. We also found that this compound exhibits large magnetic field (H) dependences of Delta L/L, details of which are different at three different magnetic phases. Similar anomalies and H dependence are observed in dielectric measurements. These behaviors can be explained by the inverse effect of single-ion spin anisotropy.

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  • Microscopic Observation of the Spin-Singlet Cluster in the Frustrated Spinel AlV2O4

    Yasuhiro Shimizu, Moe Tanaka, Masayuki Itoh, Takuro Katsufuji

    25TH INTERNATIONAL CONFERENCE ON LOW TEMPERATURE PHYSICS (LT25), PART 4   150  2009年  [査読有り]

     概要を見る

    The spinel oxide AlV2O4 exhibits a structural phase transition from cubic to rhombohedral lattice at T-c = 700 K, accompanied by V valence skipping. We have investigated the microscopic magnetic properties by the V-51 and Al-27 NAIR measurements across T-c. in a temperature range from 2 K to 850 K. The Knight shifts at the 51V and 27Al sites selectively monitor the local spin susceptibilities with different temperature dependence. We reveal that there are two V sites having a nonmagnetic ground state and a frustrated paramagnetic spin. The results are consistent with the spin-singlet heptamer formation consisting of seven vanadium atoms with 18 electrons. This transition uniquely releases the spin and charge frustration on the original V pyrochlore lattice with an aid of the orbital order and the structural distortion. The spin frustration is also highlighted in the anomalous magnetic correlations detected by the nuclear spin-lattice relaxation rate at the V and Al sites.

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  • Second Harmonic Generation from Multiferroic MnWO4

    Ayaka Nogami, Takehito Suzuki, Takuro Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   77 ( 11 ) 115001  2008年11月  [査読有り]

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  • Spin-singlet formation in the geometrically frustrated spinel oxide AlV2O4: V-51 and Al-27 NMR measurements

    Yasuhiro Shimizu, Moe Tanaka, Masayuki Itoh, Takuro Katsufuji

    PHYSICAL REVIEW B   78 ( 14 ) 144423  2008年10月  [査読有り]

     概要を見る

    We report V-51 and Al-27 NMR studies of the spin-frustrated spinel oxide AlV2O4 in a temperature range from 850 to 2 K across a structural phase transition at 700 K. Knight shifts and nuclear spin-lattice relaxation rates at the V and Al sites show that the local spin susceptibility at two V sites diminishes at low temperatures in the insulating state, giving direct evidence for the spin-singlet formation. In addition to the singlet spins, one of the two Al sites probes paramagnetic spins with the Curie-Weiss behavior and the spin-glass transition. The results demonstrate coexistence of the spin-singlet cluster and the nearly free spins, anticipated from a theoretical model and the bulk magnetic-susceptibility measurements.

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  • Magnetodielectric properties of spin-orbital coupled system Mn3O4

    Takehito Suzuki, Takuro Katsufuji

    PHYSICAL REVIEW B   77 ( 22 ) 220402(R)  2008年06月

     概要を見る

    We studied the dielectric properties and strain of spinel Mn3O4 with applied magnetic field. It was found that the dielectric constant (epsilon(1)) exhibits a large magnetic-field (H) dependence, which is anisotropic both against the direction of the magnetic field and the electric field in the magnetically ordered phases below T-N=43 K. It was also found that the epsilon(1)(H) exhibits distinctively different behaviors at three different magnetic phases. These results can be explained by the change of the orbital state of Mn3+ with the applied magnetic field due to the inverse process of single-ion spin anisotropy.

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  • Structural and magnetic properties of spinel FeV2O4 with two ions having orbital degrees of freedom

    Takuro Katsufuji, Takehito Suzuki, Haruki Takei, Masao Shingu, Kenichi Kato, Keiichi Osaka, Masaki Takata, Hajime Sagayama, Taka-hisa Arima

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   77 ( 5 ) 53708  2008年05月  [査読有り]

     概要を見る

    FeV2O4 with orbital degrees of freedom both at the Fe2+ and V3+ ions exhibits successive structural phase transitions. We studied this structural and magnetic properties as well as the correlation between the two. We found that the crystalline domains in the tetragonal or orthorhombic phases can be aligned by the applied magnetic field, inducing a large magnetostriction (similar to 1%). The competition and cooperation of Fe2+ and V3+ orbitals is the possible origin of these intriguing properties in FeV2O4.

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  • Magnetic impurities and itinerant carriers in doped SrTiO(3): Anomalous Hall resistivity

    D. Satoh, K. Okamoto, T. Katsufuji

    PHYSICAL REVIEW B   77 ( 12 ) 121201(R)  2008年03月

     概要を見る

    We studied perovskite SrTiO(3) into which magnetic impurities (Cr or V) and itinerant carriers (electrons in the Ti 3d band) are introduced. We found that the itinerancy of the electrons on the magnetic impurities and magnetic properties of the compounds depend on the species of the transition metal as impurities. Anomalous Hall resistivity was observed in Cr-doped SrTiO(3), and it was found that its coefficient is inversely proportional to the number of itinerant carriers.

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  • Magnetic excitations and orbital physics in the ferrimagnetic spinels Mn B2 O4 (B=Mn,V)

    J. H. Chung, J. H. Kim, S. H. Lee, T. J. Sato, T. Suzuki, M. Katsumura, T. Katsufuji

    Physical Review B - Condensed Matter and Materials Physics   77 ( 5 ) 54412  2008年02月  [査読有り]

     概要を見る

    Using neutron scattering techniques, we have investigated spin wave excitations in noncollinear ferrimagnetic spinels Mn B2 O4 (B=Mn,V) with eg and t2g orbital degeneracies, respectively, that lead to tetragonal distortions along opposite directions. Linear spin wave analysis of the excitations yields spatially inhomogeneous nearest neighbor interactions in both tetragonal spinels. We find the ratio Jc Jab ≈-0.06 (4) for Mn3 O4 (c>a=b) and ≈0.3 (1) for Mn V2 O4 (c<a=b). Resulting exchange couplings of Mn3 O4 can be qualitatively explained in terms of possible overlaps of t2g and eg electrons of Mn2+ and Mn3+ ions. On the other hand, those of Mn V2 O4, in particular, the strong Jc, seem to contradict with the antiferro-orbital state of V3+ (t 2g 2) ions that was proposed by a recent synchrotron x-ray study. Theoretical implications to the orbital physics are also discussed. © 2008 The American Physical Society.

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  • Nonvolatile memory effect of capacitance in polycrystalline spinel vanadate

    H. Takei, T. Suzuki, T. Katsufuji

    APPLIED PHYSICS LETTERS   91 ( 7 ) 72506  2007年08月  [査読有り]

     概要を見る

    The authors found that capacitance of polycrystalline spinel FeV2O4 exhibits a magnetic-field dependence with hysteresis and takes two values at zero field depending on the direction of a small magnetic field (similar to 1000 G) applied prior to measurement. This behavior can be potentially used as a nonvolatile memory device in which the data are stored as a difference of capacitance through the change of magnetic-field directions. (C) 2007 American Institute of Physics.

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  • Orbital ordering and magnetic field effect in MnV2O4

    T. Suzuki, M. Katsumura, K. Taniguchi, T. Arima, T. Katsufuji

    PHYSICAL REVIEW LETTERS   98 ( 12 ) 127203  2007年03月  [査読有り]

     概要を見る

    We studied the structural properties of an orbital-spin-coupled spinel oxide, MnV2O4, mainly by single-crystal x-ray diffraction measurement. It was found that a structural phase transition from cubic to tetragonal and ferrimagnetic ordering occur at the same temperature (T-s,T-N=57 K). The structural phase transition was induced also by magnetic field above T-s. In addition, magnetic-field-induced alignment of tetragonal domains results in large magnetostriction below T-s. We also found that the structural phase transition is caused by the antiferro-type ordering of the V t(2g) orbitals.

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  • Ab initio calculation of charge- and spin-controlled Sr1-x-yLax+yTi1-xCrxO3

    I. Hase, T. Saitoh, T. Katsufuji

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   310 ( 2 ) E281 - E282  2007年03月  [査読有り]

     概要を見る

    Perovskite SrTiO3 is a wide-gap band insulator. Because of the stability of its crystal structure, various nonmagnetic and magnetic atoms can be doped into the A-site and the B-site, respectively. A-site substitution, namely La3+ for Sr2+, introduces free electrons in the conduction band, while B-site substitution, namely Cr3+ for Ti4+, introduces not only localized spins but also free holes in the valence band. Therefore, we can independently introduce x localized spins and y free electrons (or holes) in SrTiO3 by co-doping La and Cr, by the composition Sri(-x-y)La(x+y)Ti(1-x)Cr(x)O(3) In this paper, we present an ab initio calculation of the electronic structure of Sr1-x,La--y(x),Ti1-x,CrxO3 by the KKR-CPA-LDA method, changing the doping concentrations x and y systematically within 0 &lt; x &lt; 0.2 and -0.2 &lt; y &lt; 0.2. We found that the Cr t(2g)-states are well localized, and due to the large exchange splitting, only the majority spin states are occupied. However, the energy of the occupied Cr t(2g)-states are considerably shallow compared to the photoemission experiment, suggesting the importance of the electron correlation for these localized states. The total magnetic moment is almost proportional to the Cr concentration as expected; however, it considerably changes with La concentration. (c) 2006 Elsevier B.V. All rights reserved.

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  • Independent control of charge and spin density in Sr(1-(xplusy))La(x+y)Ti(1-x)Cr(x)O(3) probed by photoemission spectroscopy

    S. Kaneyoshi, H. Iwasawa, T. Saitoh, Y. Aiura, D. Satoh, T. Katsufuji, I. Hase, M. Higashiguchi, K. Shimada, H. Namatame, M. Taniguchi, M. Kubota, K. Ono

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   310 ( 2 ) E278 - E280  2007年03月  [査読有り]

     概要を見る

    Using photoemission (PES) spectroscopy and a first-principles band-structure calculation, we have investigated the electronic structure of perovskite-type Ti-Cr oxides Sr(1-(x+y))La(x+y)Ti(1-x)Cr(x)O(3), in which the charge and spin density can be controlled. It was found that the near-Fermi level (E(F)) electronic structure consisted of a Cr 3d localized state and a Ti 3d itinerant state that were well separated in energy. The spectral weight of the Cr 3d state was proportional to x, demonstrating a controllable spin density. In contrast, the near-E(F) spectral weight exhibited different behavior. The band theory has reproduced the essential features of the Cr and Ti 3d states as well as the O 2p ones, but failed to predict the location of the Cr 3d and O 2p states. Implications of this discrepancy have been discussed in connection with calculated magnetic moments. (c) 2006 Elsevier B.V. All rights reserved.

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  • Correlation between magnetic, dielectric properties and crystal structure in MnT2O4 (T = V, Cr, Mn)

    T. Suzuki, K. Adachi, T. Katsufuji

    JOURNAL OF MAGNETISM AND MAGNETIC MATERIALS   310 ( 2 ) 780 - 781  2007年03月  [査読有り]

     概要を見る

    We measured the temperature dependence of dielectric constant (epsilon(1)) and lattice striction (Delta L/L) for MnV2O4, Mn3O4, and MnCr2O4. For MnV2O4 and Mn3O4, which have the orbital degree of freedom in the t(2g) or e(g) orbitals of the transition metal on the octahedral site, a critical decrease of epsilon(1) and Delta L/L at the ferrimagnetic transition temperature (T-N) was observed. On the other hand, MnCr2O4 which has no orbital degree of freedom, anomalies in epsilon(1) and Delta L/L at T-N are barely observed. These results indicate that the orbital degree of freedom is a key factor for the correlation between magnetic, dielectric properties and crystal structure in the series of MnT2O4. (c) 2006 Elsevier B.V. All rights reserved.

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  • Doping effect on ferroelectric microstructure in YMn1-xTixO3

    S. Mori, J. Tokunaga, Y. Horibe, T. Asada, Y. Koyama, T. Katsufuji

    FERROELECTRICS   348   572 - 578  2007年  [査読有り]

     概要を見る

    We have investigated microstructures related to anomalous ferroelectric propel-ties such as the magnetocapacitance effect in magnetic ferroelectric compounds YMn1-xTixO3 and (Y,A)Mn0.825Ti0.175O3 (A = La, Ca, Zr) mainly by the transmission electron microscopy. It is found that the size of the ferroelectric domains in YMn1-xTixO3 depends strongly on the Ti concentration (x) and the size is estimated to be about 10-20 nm in the x = 0.175 compound. In addition, structural change from the ferroelectric P6(3) cm phase to the rhombohedral one was found in the x = 0.40 compound. On the other hand, characteristic diffuse scattering elongating along both the [001] and [110] direction was found in (Y,Zr)Mn0.825Ti0.175O3, which is similar to that found in the x = 0.175 compound. It is revealed that the partial substitution in YMnO3 breaks the long-ranged FE ordering and structural fluctuation is induced, which should be responsible for the anomalous FE behaviors in these compounds.

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  • Dynamical specific heat of two-phase coexistence in RMnO3

    T. Nomura, J. Masuzawa, T. Katsufuji

    PHYSICAL REVIEW B   74 ( 22 ) 220403(R)  2006年12月  [査読有り]

     概要を見る

    We studied the dynamics of two-phase coexistence in perovskite manganite RMnO3 (R=rare earth) by means of dynamical specific heat measurement. We found that specific heat exhibits a Debye-type frequency dependence around the transition temperature between two different magnetic phases, which can be attributed to the relaxational domain motion between the two phases. We also found that relaxation time (tau) systematically varies with the change of R as well as with an applied magnetic field, and dispersion frequency (1/tau) divergently increases as the system approaches the critical point of the magnetic phases.

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  • Transport, magnetic, and structural properties of spinel MnTi2O4 and the effect of V doping

    T. Sonehara, K. Kato, K. Osaka, M. Takata, T. Katsufuji

    PHYSICAL REVIEW B   74 ( 10 ) 104424  2006年09月  [査読有り]

     概要を見る

    We studied the transport, magnetic, and structural properties of spinel MnTi2O4 and V-doped samples, MnTi2-xVxO4, to clarify the relation between charge, spin, and orbital degrees of freedom of 3d electrons in this series of compounds. A structural phase transition occurs in MnTi2O4 at 180 K, which is presumably dominated by the ordering of t(2g) orbitals, but it can be easily suppressed by the V doping to the Ti site. Associated with this suppression of the structural anomaly, a ferrimagnetic and electrically conducting state appears in MnTi2-xVxO4 with finite values of x. Fairly large magnetoresistance was observed in this state, whose sign changes from positive to negative with an increase in the V concentration x. With approaching the other end of this series, MnV2O4, the electrical resistivity increases again and another type of structural phase transition occurs at 53 K. Large magnetostriction was observed in MnV2O4, the only compound in this series to have both a ferrimagnetic state and tetragonal lattice distortion.

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  • Magnetic-field switching of crystal structure in spinel MnV2O4

    T. Katsufuji, K. Adachi, T. Suzuki, M. Katsumura

    PHYSICA B-CONDENSED MATTER   383 ( 1 ) 13 - 15  2006年08月  [査読有り]

     概要を見る

    We demonstrate that switching of crystal structure as well as domain rotation occur in spinel MnV2O4 with applied magnetic field. Such large magnetoelastic effects arise from a strong coupling between spin and orbital degrees of freedom for the V ions in this compound. (c) 2006 Elsevier B.V. All rights reserved.

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  • Spin-dependent charge dynamics of an orbital-spin-coupled system: Yb2V2O7

    K. Waku, T. Suzuki, T. Katsufuji

    PHYSICAL REVIEW B   74 ( 2 ) 024402  2006年07月  [査読有り]

     概要を見る

    Optical conductivity spectra of ferromagnetic pyrochlore vanadate Yb2V2O7 were investigated under a variation of both temperature and magnetic field. It was found that the spectrum of a Mott-gap excitation, a d-d transition between neighboring V sites, exhibits spectral weight transfer to lower energies with ferromagnetic ordering as well as an applied magnetic field. This result indicates that charge dynamics is influenced by the spin state in this prototypical orbital-spin-coupled system.

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  • Erratum:Spontaneous formation of vanadium "molecules" in a geometrically frustrated crystal: AlV2O4 (vol 96, pg 086406, 2006)

    Y. Horibe, M. Shingu, K. Kurushima, H. Ishibashi, N. Ikeda, K. Kato, Y. Motome, N. Furukawa, S. Mori, T. katsufuji

    Phys. Rev. Lett.   96 ( 16 ) 169901  2006年04月  [査読有り]

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  • Electric-field-induced resistance switching universally observed in transition-metal-oxide thin films

    M Hamaguchi, K Aoyama, S Asanuma, Y Uesu, T Katsufuji

    APPLIED PHYSICS LETTERS   88 ( 14 ) 142508  2006年04月  [査読有り]

     概要を見る

    We show that polarity-dependent, nonvolatile resistance switching by electric field occurs in the thin film of various transition-metal oxides in almost the same manner. This result indicates that, contrary to the general acceptance, perovskite manganite is by no means a special compound for this phenomenon. It is also suggested that the resistance switching is not dominated by a detailed electronic structure of each sample, but dominated by a more general origin, e.g., crystalline defect. (c) 2006 American Institute of Physics.

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  • Spontaneous formation of vanadium "molecules" in a geometrically frustrated crystal: AlV2O4

    Y Horibe, M Shingu, K Kurushima, H Ishibashi, N Ikeda, K Kato, Y Motome, N Furukawa, S Mori, T Katsufuji

    PHYSICAL REVIEW LETTERS   96 ( 8 ) 086406  2006年03月  [査読有り]

     概要を見る

    We investigated the charge-ordered (CO) state in spinel AlV2O4 by electron diffraction, synchrotron x-ray diffraction, and magnetic measurements. It was found that the CO structure appearing below 700 K is characterized by the formation of V clusters (heptamers), each of which is consisting of 7 vanadium atoms and is in a spin-singlet state as a total. Theoretical consideration indicates that this unique molecularlike V heptamer is stabilized by a strong bonding of vanadium t(2g) orbitals.

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  • Electronic structure of charge- and spin-controlled Sr1-(xplusy)Lax+yTi1-xCrxO3

    H Iwasawa, K Yamakawa, T Saitoh, J Inaba, T Katsufuji, M Higashiguchi, K Shimada, H Namatame, M Taniguchi

    PHYSICAL REVIEW LETTERS   96 ( 6 ) 067203  2006年02月  [査読有り]

     概要を見る

    We present the electronic structure of Sr1-(x+y)Lax+yTi1-xCrxO3 investigated by high-resolution photoemission spectroscopy. In the vicinity of the Fermi level, it was found that the electronic structure was composed of a Cr 3d local state with the t(2g)(3) configuration and a Ti 3d itinerant state. The energy levels of these Cr and Ti 3d states are well interpreted by the difference of the charge-transfer energy of both ions. The spectral weight of the Cr 3d state is completely proportional to the spin concentration x irrespective of the carrier concentration y, indicating that the spin density can be controlled by x as desired. In contrast, the spectral weight of the Ti 3d state is not proportional to y, depending on the amount of Cr doping.

    DOI

  • Coupling between magnetic, dielectric properties and crystal structure in MnT2O4 (T = V, Cr, Mn)

    T. Suzuki, K. Adachi, T. Katsufuji

    THIRD 21COE SYMPOSIUM: ASTROPHYSICS AS INTERDISCIPLINARY SCIENCE   31   235 - +  2006年  [査読有り]

     概要を見る

    We measured the temperature dependence of dielectric constant and striction for spinel MnT2O4 (T = V, Cr, Mn) under magnetic field. We found critical changes of the dielectric constant and striction with ferrimagnetic ordering as well as applied magnetic field in MnV2O4 and Mn3O4, which have orbital degree of freedom in the T3+ ion. This result indicates the importance of the orbital degree of freedom for the coupling between dielectric, magnetic properties and crystal structure in these spinel compounds.

    DOI

  • Magnetocapacitance effect and related microstrucuture in Ti-doped YMnO3

    S. Mori, J. Tokunaga, Y. Horibe, Y. Aikawa, T. Katsufuji

    Physical Review B - Condensed Matter and Materials Physics   72 ( 22 )  2005年12月  [査読有り]

     概要を見る

    We have investigated microstructures giving rise to magnetocapacitance effect in Ti-doped YMnO3 (YMn1-xTixO3) by both electron diffraction and real-space imaging techniques. Diffuse scattering elongating along the [110] direction due to the instability of the Mn trimerization were found in the x=0.175 compound, which exhibits the enhanced magnetocapacitance effect. With increasing Ti concentration, the centrosymmetric (paraelectric) R3̄c phase grows up at the expense of the ferroelectric P63cm phase around x=0.20 and the R3̄c phase is dominant above x=0.30. Our present results indicate that the Mn trimers with the short-range correlation inside the ferroelectric nanodomains with the 10-20nm size should be responsible for the enhancement of the magnetocapacitance and, on the other hand, the appearance of the R3̄c phase suppressed the magnetocapacitance effect. © 2005 The American Physical Society.

    DOI

  • Magnetocapacitance effect and related microstrucuture in Ti-doped YMnO3

    S Mori, J Tokunaga, Y Horibe, Y Aikawa, T Katsufuji

    PHYSICAL REVIEW B   72 ( 22 ) 224434  2005年12月  [査読有り]

     概要を見る

    We have investigated microstructures giving rise to magnetocapacitance effect in Ti-doped YMnO3 (YMn1-xTixO3) by both electron diffraction and real-space imaging techniques. Diffuse scattering elongating along the [110] direction due to the instability of the Mn trimerization were found in the x=0.175 compound, which exhibits the enhanced magnetocapacitance effect. With increasing Ti concentration, the centrosymmetric (paraelectric) R3c phase grows up at the expense of the ferroelectric P6(3)cm phase around x=0.20 and the R3c phase is dominant above x=0.30. Our present results indicate that the Mn trimers with the short-range correlation inside the ferroelectric nanodomains with the 10-20 nm size should be responsible for the enhancement of the magnetocapacitance and, on the other hand, the appearance of the R3c phase suppressed the magnetocapacitance effect.

    DOI

  • Magnetic-field switching of crystal structure in an orbital-spin-coupled system: MnV2O4

    K Adachi, T Suzuki, K Kato, K Osaka, M Takata, T Katsufuji

    PHYSICAL REVIEW LETTERS   95 ( 19 ) 197202  2005年11月  [査読有り]

     概要を見る

    We studied the magnetic and structural properties of spinel MnV2O4, which has S=5/2 spin with no orbital degrees of freedom on the Mn2+ site and S=1 spin and three orbital degrees of freedom on the V3+ site. We found that the ferrimagnetic ordering at T-N=56.5 K and the structural phase transition at T-s=53.5 K are closely correlated in this compound and found a switching of crystal structure between cubic and tetragonal phases by the magnetic field. This phenomenon can be explained by the coupling between orbital and spin degrees of freedom in the t(2g) states of the V site.

    DOI

  • Structural phase transition and domain structures in YMn1-xTixO3

    S Mori, J Tokunaga, Y Horibe, Y Aikawa, T Katsufuji

    JAPANESE JOURNAL OF APPLIED PHYSICS PART 1-REGULAR PAPERS BRIEF COMMUNICATIONS & REVIEW PAPERS   44 ( 9B ) 7174 - 7176  2005年09月  [査読有り]

     概要を見る

    We investigated changes in domain structures induced by a partial substitution of Ti for Mn in multiferroic YMnO3 by transmission electron microscopy (TEM). In the ferroelectric phase of YMnO3 at room temperature, there exist 180-degree ferroelectric (FE) domain structures of a micrometer size. On substituting Ti for Mn, the size of FE domains becomes fragile and, at approximately x = 0.175 in YMn1-xTixO3, the size can be estimated to be 10-20nm. On further substitution, it is found that a structural phase transition from a hexagonal P6(3) cm structure to a new rhombohedral R3c one takes place at approximately x = 0.20. In YMn1-xTixO3 with 0.20 &lt; x &lt; 0.40, these two distinct structures (the hexagonal P6(3) cm and rhombohedral R3c structures) coexist and this coexistence should have some strong effect on the ferroelectric properties such as the magnetocapacitance effect found in Ti-substituted YMnO3.

    DOI

  • Large magnetoresistance in spin- and carrier-doped SrTiO3

    J Inaba, T Katsufuji

    PHYSICAL REVIEW B   72 ( 5 ) 052408  2005年08月  [査読有り]

     概要を見る

    We studied SrTiO3 into which spins and carriers are doped by Cr substitution for Ti and La substitution for Sr, respectively. It was found that Weiss temperatures vary from negative to positive values with carrier doping, indicating the appearance of a ferromagnetic interaction between Cr spins via the conduction carriers. We also found negative magnetoresistance in these compounds, whose magnitude amounts to 70% at low temperatures. These results demonstrate that magnetic semiconductors with large magnetoresistance can be obtained based on conventional oxides, like SrTiO3, with independent doping of spins and carriers.

    DOI

  • Dynamics of the charge-spin interplay in a perovskite manganite

    K Waku, T Suzuki, T Nomura, T Katsufuji

    PHYSICAL REVIEW B   72 ( 1 ) 012418  2005年07月  [査読有り]

     概要を見る

    The interplay between charge and spin degrees of freedom is a key ingredient of the colossal magnetoresistance in perovskite manganites. However, our knowledge of such an interplay was limited to its static aspect, and very little was known regarding its dynamics. Here, we show that the ac (optical) conductivity of perovskite manganite Pr0.6Sr0.4MnO3 (T-C=297 K) under magnetic field does not show such a universal M-2 scaling (M is the magnetization) as observed in the dc conductivity, but shows a marginal parallel to M parallel to(1) dependence in the critical regime. This result indicates a unique interplay between fluctuating spin and charge in this series of compounds.

    DOI

  • Soft X-ray photoemission study of CaB6

    A Sekiyama, T Sasabayashi, A Higashiya, H Fujiwara, S Imada, K Taniguchi, H Takagi, T Katsufuji, K Kitazawa, S Suga

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   144   659 - 661  2005年06月  [査読有り]

     概要を見る

    We have performed the soft X-ray photoemission and absorption studies of CaB6. The crystal field splitting in the Ca 3d levels is estimated as 0.65 eV from the analysis of the Ca 2p X-ray absorption spectrum. It is found that the surface Ca 2p contribution is seen at the lower binding energy side of the bulk Ca 2p peak in the core-level spectra. The Ca 2p-3d resonance photoemission shows the clear enhancement of the spectral weight in the vicinity of the Fermi level. (c) 2005 Elsevier B.V. All rights reserved.

    DOI

  • Effect of Mn trimerization on the magnetic and dielectric properties of hexagonal YMnO3

    Y Aikawa, T Katsufuji, T Arima, K Kato

    PHYSICAL REVIEW B   71 ( 18 ) 184418  2005年05月  [査読有り]

     概要を見る

    We have studied the crystal structure, magnetic properties, and dielectric properties of Ti-doped hexagonal YMnO3 composed of a Mn triangular lattice with trimerization. It was found that a structural phase transition occurring in YMn1-xTixO3 around x=0.2, which is correlated with a suppression of Mn trimerization by Ti doping, has a substantial effect on the magnetic and dielectric properties of this series of compounds. This correlation between the crystal structure and magnetic and dielectric properties indicates that Mn trimerization plays an important role for the large magnetodielectric coupling unique to hexagonal RMnO3.

    DOI

  • Magnetocapacitance in geometrically frustrated magnet ZnFe2O4

    T Suzuki, Y Aikawa, T Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   74 ( 3 ) 863 - 866  2005年03月  [査読有り]

     概要を見る

    We report the measurement of dielectric constant in ZnFe2O4 as a function of temperature and magnetic field. The behavior of dielectric constant for single crystals, which is dominated by the pair correlation of the neighboring Fe spins, indicates that ferromagnetic correlation is enhanced below 30 K, but is suppressed below 10 K, where positive magnetocapacitance is enhanced. It is also found that the magnetocapacitance of polycrystalline samples is much larger (similar to 10% at 14T) than that of single crystals, because of the additional dielectric relaxation coupled with a uniform magnetization.

    DOI

  • Doping effect on the charge ordering in AlV2O4

    Y Horibe, K Kurushima, S Mori, T Asada, Y Koyama, M Shingu, T Katsufuji

    PHYSICAL REVIEW B   71 ( 5 ) 052411  2005年02月  [査読有り]

     概要を見る

    Effect of the Cr doping on the stable charge ordering (CO) in the spinal compound AlV2O4 was investigated by transmission electron microscopy. It is found that the stable CO is destabilized drastically by substituting Cr ions for V ones and the long-ranged CO in AlV2O4 becomes short ranged. The size of the CO microdomains in AlV2-xCrxO4 with x=0.125 was estimated to be about 10-20 nm. Furthermore, high-resolution lattice images revealed fine mixture of the CO and charge disordered microdomains in AlV2-xCrxO4 at room temperature. In addition, we found the presence of the characteristic diffuse scattering due to local lattice distortion induced by the Cr substitution in AlV2-xCrxO4. Our results suggested that doped Cr 31 ions should serve as a quenched "charge" impurity in the CO structure of AlV2O4, which is characterized as the charge frustration system.

    DOI

  • Dynamics of the charge-spin interplay in a perovskite manganite

    K. Waku, T. Suzuki, T. Nomura, T. Katsufuji

    Physical Review B - Condensed Matter and Materials Physics   72 ( 1 )  2005年  [査読有り]

     概要を見る

    The interplay between charge and spin degrees of freedom is a key ingredient of the colossal magnetoresistance in perovskite manganites. However, our knowledge of such an interplay was limited to its static aspect, and very little was known regarding its dynamics. Here, we show that the ac (optical) conductivity of perovskite manganite Pr0.6Sr0.4MnO3 (TC=297K) under magnetic field does not show such a universal M2 scaling (M is the magnetization) as observed in the dc conductivity, but shows a marginal £ M £1 dependence in the critical regime. This result indicates a unique interplay between fluctuating spin and charge in this series of compounds. © 2005 The American Physical Society.

    DOI

  • Ferroelectric domains in hexagonal YMnO3

    S Mori, Y Horibe, T Katsufuji

    JOURNAL OF THE KOREAN PHYSICAL SOCIETY   46 ( 1 ) 37 - 39  2005年01月  [査読有り]

     概要を見る

    In order to clarify the microstructure related to the ferroelectric properties in multi-ferroic YMnO3, we carefully examined the ferroelectric-domain configuration by transmission electron microscopy (TEM). It was found that ferroelectric domains in YMnO3 consist of six anti-phase domains at room temperature. On warming from room temperature, the ferroelectric-domain configuration changes into one consisting of three anti-phase domains around 738 K. In addition, the ferroelectric domains in YMnO3 change drastically into microdomains with a size of 10 similar to 20 nm by substitution of Ti ions for some of the Mn ones.

  • Charge dynamics of Ca2-xNaxCuO2Cl 2 as a correlated electron system with the ideal tetragonal lattice

    K. Waku, T. Katsufuji, Y. Kohsaka, T. Sasagawa, H. Takagi, H. Kishida, H. Okamoto, M. Azuma, M. Takano

    Physical Review B - Condensed Matter and Materials Physics   70 ( 13 ) 1 - 134501  2004年10月  [査読有り]

     概要を見る

    We report the reflectivity and the resistivity measurement of Ca 2-xNaxCuO2Cl2 (CNCOC), which has a single-CuO2-plane lattice with no orthorhombic distortion. The doping dependence of the in-plane optical conductivity spectra for CNCOC is qualitatively the same to those of other cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe instability in CNCOC. The temperature dependence of the optical conductivity spectra of CNCOC has been analyzed both by the two-component model (Drude+Lorentzian) and by the one-component model (extended-Drude analysis). The latter analysis gives a universal trend of the scattering rate Γ(ω) with doping. It was also found that Γ(ω) shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.

    DOI

  • Charge dynamics of Ca2-xNaxCuO2Cl2 as a correlated electron system with the ideal tetragonal lattice

    K Waku, T Katsufuji, Y Kohsaka, T Sasagawa, H Takagi, H Kishida, H Okamoto, M Azuma, M Takano

    PHYSICAL REVIEW B   70 ( 13 ) 134501  2004年10月  [査読有り]

     概要を見る

    We report the reflectivity and the resistivity measurement of Ca2-xNaxCuO2Cl2 (CNCOC), which has a single-CuO2-plane lattice with no orthorhombic distortion. The doping dependence of the in-plane optical conductivity spectra for CNCOC is qualitatively the same to those of other cuprates, but a slight difference between CNCOC and LSCO, i.e., the absence of the 1.5 eV peak in CNCOC, can be attributed to the smaller charge-stripe instability in CNCOC. The temperature dependence of the optical conductivity spectra of CNCOC has been analyzed both by the two-component model (Drude+Lorentzian) and by the one-component model (extended-Drude analysis). The latter analysis gives a universal trend of the scattering rate Gamma(omega) with doping. It was also found that Gamma(omega) shows a saturation behavior at high frequencies, whose origin is the same as that of resistivity saturation at high temperatures.

    DOI

  • Raman study of the metal-insulator transition in pyroclilore Mo oxides

    K. Taniguchi, T. Katsufuji, S. Iguchi, Y. Taguchi, H. Takagi, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   70 ( 10 ) 1 - 100401  2004年09月  [査読有り]

     概要を見る

    Raman scattering spectra have been investigated for the correlated 4d-electron system, R2Mo2O7 (Tf=Nd, Sm, Gd, Tb,Dy,Nd1-xDyx), which undergoes a metal-insulator transition with changing the rare-earth ion R, or equivalently the one-electron bandwidth. It is found that several phonon peaks modulating the Mo-O-Mo bond angle appear in the metallic phase (R=Nd, Sm, Gd, Nd1-xJDy x), whereas they are remarkably suppressed in intensity in the insulating phase (R=Tb,Dy). This result indicates that the phonon modes of R2Mo2O7 are coupled with the electron-hole excitation across the Fermi level, thus probing sensitively the low-energy charge dynamics in the vicinity of the bandwidth-control Mott transition.

    DOI

  • Raman study of the metal-insulator transition in pyrochlore Mo oxides

    K Taniguchi, T Katsufuji, S Iguchi, Y Taguchi, H Takagi, Y Tokura

    PHYSICAL REVIEW B   70 ( 10 ) 100401  2004年09月  [査読有り]

     概要を見る

    Raman scattering spectra have been investigated for the correlated 4d-electron system, R2Mo2O7 (R=Nd, Sm, Gd, Tb,Dy,Nd1-xDyx), which undergoes a metal-insulator transition with changing the rare-earth ion R, or equivalently the one-electron bandwidth. It is found that several phonon peaks modulating the Mo-O-Mo bond angle appear in the metallic phase (R=Nd, Sm, Gd, Nd1-xDyx), whereas they are remarkably suppressed in intensity in the insulating phase (R=Tb,Dy). This result indicates that the phonon modes of R2Mo2O7 are coupled with the electron-hole excitation across the Fermi level, thus probing sensitively the low-energy charge dynamics in the vicinity of the bandwidth-control Mott transition.

    DOI

  • Electric-pulse-induced reflectance change in the thin film of perovskite manganite

    K Aoyama, K Waku, A Asanuma, Y Uesu, T Katsufuji

    APPLIED PHYSICS LETTERS   85 ( 7 ) 1208 - 1210  2004年08月  [査読有り]

     概要を見る

    We demonstrate a nonvolatile, reversible change of infrared reflectance from the thin film of perovskite manganite (Pr1-xCaxMnO3) by applying electric pulse. The result provides a possibility to use the electric-pulse-induced phenomena of this compound in optical devices. (C) 2004 American Institute of Physics.

    DOI

  • Universal charge transport of the Mn oxides in the high temperature limit

    W Kobayashi, Terasaki, I, M Mikami, R Funahashi, T Nomura, T Katsufuji

    JOURNAL OF APPLIED PHYSICS   95 ( 11 ) 6825 - 6827  2004年06月  [査読有り]

     概要を見る

    We have found that various Mn oxides have the universal resistivity and thermopower in the high temperature limit. The resistivities and thermopowers of all the samples go toward constant values of 7+/-1 mOmega cm and -79+/-3 muV/K, which are independent of carrier density and crystal structures. We propose that the electric conduction occurs in a highly localized way in the high temperature limit, where the exchange of entropy and charge occurs in the neighboring Mn3+ and Mn4+ ions. (C) 2004 American Institute of Physics.

    DOI

  • Magnetocapacitance and spin fluctuations in the geometrically frustrated magnets R2Ti2O7 (R = rare earth)

    T Katsufuji, H Takagi

    PHYSICAL REVIEW B   69 ( 6 ) 064422  2004年02月  [査読有り]

     概要を見る

    Magnetocapacitance has been measured for the geometrically frustrated magnets R2Ti2O7 (R=rare earth). The temperature dependence of magnetocapacitance, which largely deviates from the square of magnetization, indicates the evolution of strong ferromagnetic (R=Ho) or antiferromagnetic (R=Gd) fluctuation at low temperatures. This result demonstrates that the measurement of mangetocapacitance is a powerful technique to detect the magnetic fluctuation of exotic magnets.

    DOI

  • Unconventional spin fluctuations in the hexagonal antiferromagnet YMnO <inf>3</inf>

    T. J. Sato, S. H. Lee, T. Katsufuji, M. Masaki, S. Park, J. R.D. Copley, H. Takagi

    Physical Review B - Condensed Matter and Materials Physics   68 ( 1 ) 144321 - 144325  2003年07月  [査読有り]

     概要を見る

    We used inelastic neutron scattering to show that well below its Néel temperature, TN, the two-dimensional (2D) XY nearly triangular antiferromagnet YMnO3 has a prominent central peak associated with 2D antiferromagnetic fluctuations with a characteristic lifetime of 0.55(5) ps, coexisting with the conventional long-lived spin waves. Existence of the two time scales suggests competition between the Néel phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system.

  • Neutron scattering study of the charge and the magnetic ordering in La2−xSrxNiO4

    Y. Oohara, R. Kajimoto, T. Kakeshita, H. Yoshizawa, T. Tanabe, T. Katsufuji, K. Ishizaka, Y. Taguchi, Y. Tokura

    Physica B   329   725 - 726  2003年05月  [査読有り]

    DOI

  • Charge ordering and spin frustration in AIV<inf>2-×</inf>Cr<inf>×</inf>O<inf>4</inf>

    K. Matsuno, T. Katsufuji, S. Mori, M. Nohara, A. Machida, Y. Moritomo, K. Kato, E. Nishibori, M. Takata, M. Sakata, K. Kitazawa, H. Takagi

    Physical Review Letters   90 ( 9 )  2003年03月

     概要を見る

    It was found that the charge-ordered phase of hte AlV2O4 spinel was rapidly suppressed and a charge-disorder phase grows up instead with Cr doping into the V site, yielding a two-phase-coexisting state in modestly Cr-doped samples. The magnetic ground state is a spin-glass state dominated by geometrical frustrtion low temperatures in the charge-ordered phase. Such a difference in magnetic properties can be attributed to the difference in dimensionality as a spin system etween the charge-ordered phase and the charge-disordered phase.

  • Charge ordering and spin frustration in AlV2-xCrxO4

    K Matsuno, T Katsufuji, S Mori, M Nohara, A Machida, Y Moritomo, K Kato, E Nishibori, M Takata, M Sakata, K Kitazawa, H Takagi

    PHYSICAL REVIEW LETTERS   90 ( 9 ) 096404  2003年03月  [査読有り]

     概要を見る

    The relationship between charge and spin degrees of freedom in a geometrically frustrated system, AlV2-xCrxO4 spinel, is investigated. Upon Cr doping, the charge-ordered phase of AlV2O4 is rapidly suppressed and a charge-disordered phase grows up instead. It is found that the magnetic ground state is a spin-glass state dominated by geometrical frustration for both phases, but larger spin entropy remains down to low temperatures in the charge-ordered phase, possibly owing to its two-dimensional character.

    DOI

  • Optical probe of charge-spin-lattice dynamics in half-doped (formula presented)

    K. Yamamoto, K. Ishizaka, E. Saitoh, S. Shinomori, T. Tanabe, T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   67 ( 1 )  2003年  [査読有り]

     概要を見る

    We have investigated the charge-spin-ordering transition in (formula presented) by measurements of Raman and optical conductivity spectra. The checkerboard-type charge ordering below (formula presented) can be probed by activated phonon (formula presented) and (formula presented) and two-magnon (formula presented) modes in the infrared and Raman spectra as well as by formation of pseudogaps in the optical conductivity spectra. Besides, another (formula presented) Raman mode emerges below 150 K together with reduction of the optical gap, which can be attributed to another ordering process, i.e., the stripe formation. © 2003 The American Physical Society.

    DOI

  • Charge Ordering and Spin Frustration in [Formula presented]

    K. Matsuno, T. Katsufuji, S. Mori, M. Nohara, A. Machida, Y. Moritomo, K. Kato, E. Nishibori, M. Takata, M. Sakata, K. Kitazawa, H. Takagi

    Physical Review Letters   90 ( 9 ) 4  2003年  [査読有り]

     概要を見る

    The relationship between charge and spin degrees of freedom in a geometrically frustrated system, [Formula presented] spinel, is investigated. Upon Cr doping, the charge-ordered phase of [Formula presented] is rapidly suppressed and a charge-disordered phase grows up instead. It is found that the magnetic ground state is a spin-glass state dominated by geometrical frustration for both phases, but larger spin entropy remains down to low temperatures in the charge-ordered phase, possibly owing to its two-dimensional character. © 2003 The American Physical Society.

    DOI

  • Unconventional spin fluctuations in the hexagonal antiferromagnet YMnO3

    J. Sato, H. Lee, T. Katsufuji, M. Masaki, S. Park, D. Copley, H. Takagi

    Physical Review B - Condensed Matter and Materials Physics   68 ( 1 )  2003年  [査読有り]

     概要を見る

    We used inelastic neutron scattering to show that well below its Néel temperature, TN, the two-dimensional (2D) XY nearly triangular antiferromagnet YMnO3 has a prominent central peak associated with 2D antiferromagnetic fluctuations with a characteristic lifetime of 0.55(5) ps, coexisting with the conventional long-lived spin waves. Existence of the two time scales suggests competition between the Néel phase favored by weak interplane interactions, and the Kosterlitz-Thouless phase intrinsic to the 2D XY spin system. © 2003 The American Physical Society.

    DOI

  • Optical probe of charge-spin-lattice dynamics in half-doped La1.5Sr0.5NiO4

    K Yamamoto, K Ishizaka, E Saitoh, S Shinomori, T Tanabe, T Katsufuji, Y Tokura

    PHYSICAL REVIEW B   67 ( 1 ) 014414  2003年01月  [査読有り]

     概要を見る

    We have investigated the charge-spin-ordering transition in La1.5Sr0.5NiO4 by measurements of Raman and optical conductivity spectra. The checkerboard-type charge ordering below similar to400 K can be probed by activated phonon (A(1g) and E-u) and two-magnon (B-1g) modes in the infrared and Raman spectra as well as by formation of pseudogaps in the optical conductivity spectra. Besides, another B-2g Raman mode emerges below 150 K together with reduction of the optical gap, which can be attributed to another ordering process, i.e., the stripe formation.

    DOI

  • Microscopic phase separation and ferromagnetic microdomains in Cr-doped Nd0.5Ca0.5MnO3

    S Mori, R Shoji, N Yamamoto, T Asaka, Y Matsui, A Machida, Y Moritomo, T Katsufuji

    PHYSICAL REVIEW B   67 ( 1 ) 012403  2003年01月  [査読有り]

     概要を見る

    We succeeded in observing ferromagnetic (FM) microdomains in the phase-separated state found in Cr-doped Nd0.5Ca0.5MnO3 by low-temperature Lorentz microscopy. The presence of the FM microdomains with the size of 20-30 nm were found in Nd0.5Ca0.5Mn1-yCryO3 for y=0.03. In addition, it is clearly shown that the low-temperature phase characterized as the phase-separated state consists of a fine mixture of the two competing ground states, the FM metallic state and the charge/orbital ordered insulator one. Our experimental findings clearly demonstrate the occurrence of microscopic and static phase separation in the Cr-doped manganites.

    DOI

  • Stripe order in highly doped La2-xSrxNiO4

    R Kajimoto, T Kakeshita, H Yoshizawa, T Tanabe, T Katsufuji, Y Tokura

    APPLIED PHYSICS A-MATERIALS SCIENCE & PROCESSING   74   S1765 - S1769  2002年12月  [査読有り]

     概要を見る

    Stripe order in La2-xSrxNiO4 with 0.289less than or similar toxless than or similar to1/2 was studied with the neutron-diffraction technique. The stripe order of holes and spins was observed at low temperatures in all samples. The incommensurability E of the stripe order is approximately linear with the hole concentration n(h) up to x = 1/2. We found that the stripe is most stable at n(h) = 1/3 with epsilon = 1/3 from the analyses of the onset temperatures of the charge and spin orderings and the n(h) dependence of the correlation length of the stripe order. The stability of an 8 = 1/3 stripe induces a systematic deviation of E from the epsilon = n(h) law for samples with n(h) not equal 1/3. This deviation affects the stripe order on both sides of the hole concentration with n(h)&lt;1/3 as well as n(h)&gt;1/3.

    DOI

  • Crystal structure and magnetic properties of hexagonal RMnO<inf>3</inf> (R = Y, Lu, and Sc) and the effect of doping

    T. Katsufuji, M. Masaki, A. Machida, M. Moritomo, K. Kato, E. Nishibori, M. Takata, M. Sakata, K. Ohoyama, K. Kitazawa, H. Takagi

    Physical Review B - Condensed Matter and Materials Physics   66 ( 13 ) 1344341 - 1344348  2002年10月

     概要を見る

    A ferroelectricity-magnetism-coexisting system, hexagonal RMnO3 (R = Y, Lu, and Sc), has been investigated by synchrotron x-ray and neutron powder diffraction measurements. It is found from x-ray diffraction measurements that the ferroelectric polarization originates from the tilting of MnO5 polyhedra and the buckling of R layers, which persists up to 1000 K. Neutron diffraction measurements have revealed the reduction of ordered moments from the expected value, as well as strong magnetic diffuse scattering existing far above TN, both of which are caused by geometrical frustration of the triangular lattice of Mn ions. We have also investigated the effects of Zr doping into the R site and have found that Zr doping drastically suppresses both ferroelectric distortion and magnetic ordering.

  • Crystal structure and magnetic properties of hexagonal RMnO3 (R=Y, Lu, and Sc) and the effect of doping

    T Katsufuji, M Masaki, A Machida, M Moritomo, K Kato, E Nishibori, M Takata, M Sakata, K Ohoyama, K Kitazawa, H Takagi

    PHYSICAL REVIEW B   66 ( 13 ) 134434  2002年10月  [査読有り]

     概要を見る

    A ferroelectricity-magnetism-coexisting system, hexagonal RMnO3 (R=Y, Lu, and Sc), has been investigated by synchrotron x-ray and neutron powder diffraction measurements. It is found from x-ray diffraction measurements that the ferroelectric polarization originates from the tilting of MnO5 polyhedra and the buckling of R layers, which persists up to 1000 K. Neutron diffraction measurements have revealed the reduction of ordered moments from the expected value, as well as strong magnetic diffuse scattering existing far above T-N, both of which are caused by geometrical frustration of the triangular lattice of Mn ions. We have also investigated the effects of Zr doping into the R site and have found that Zr doping drastically suppresses both ferroelectric distortion and magnetic ordering.

    DOI

  • Charge dynamics and possibility of ferromagnetism in A(1-x)La(x)B(6) (A = Ca and Sr)

    K Taniguchi, T Katsufuji, F Sakai, H Ueda, K Kitazawa, H Takagi

    PHYSICAL REVIEW B   66 ( 6 ) 064407  2002年08月  [査読有り]

     概要を見る

    Ferromagnetism was recently reported in La-doped alkaline-earth hexaborides A(1-x)La(x)B(6) (A=Ca, Sr, and Ba). We have performed reflectivity, Hall resistivity, and magnetization measurements of A(1-x)La(x)B(6). The results indicate that A(1-x)La(x)B(6) can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (similar to10 mum in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of A(1-x)La(x)B(6) reported so far is not intrinsic.

    DOI

  • Photoemission spectral weight transfer and mass renormalization in the Fermi-liquid system La1-xSrxTiO3+y/2

    T Yoshida, A Ino, T Mizokawa, A Fujimori, Y Taguchi, T Katsufuji, Y Tokura

    EUROPHYSICS LETTERS   59 ( 2 ) 258 - 264  2002年07月  [査読有り]

     概要を見る

    We have performed a photoemission study of La1-xSrxTiO3+y/2 near the filling-control metal-insulator transition (MIT) as a function of hole doping. The spectral intensity at the chemical potential (mu) and the mass enhancement factor deduced from the bandwidth show qualitatively the same doping dependence as the specific coefficient gamma except for near the MIT, where additional mass renormalization may occur in the vicinity of mu. Upon antiferromagnetic ordering, spectral weight transfer occurs from the coherent to the incoherent parts, which we associate with the partial gap opening at mu

  • Charge ordered state in the impurity-doped manganites

    S Mori, R Shoji, N Yamamoto, A Machida, Y Moritomo, T Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   71 ( 5 ) 1280 - 1283  2002年05月  [査読有り]

     概要を見る

    Impurity effect on the charge and orbital ordered (CO/OO) state in doped manganites has been studied by the transmission electron microscopy (TEM), in addition to the electric and magnetic measurements. In the Cr-doped manganites (Nd0.5Ca0.5Mn1-yCryO3), the CO/OO state characterized by the incommensurate wave vector is still realized as the microdomains with the size of 10-30 nm even at lower temperature of 20 K. On the other hand, the Sc-doped manganites (Nd0.5Ca0.5Mn1-yScyO3) clearly show an incommensurate to commensurate structural phase transition accompanying the formation of the lon-ranged CO/OO state around 60 K. From our experimental findings, it is suggested that the substituted Cr3+ and Sc3+ ions serve as the spin impurity and hole vacancy, respectively.

    DOI

  • Strong self-phase modulation in planar chalcogenide glass waveguides

    S Spalter, HY Hwang, J Zimmermann, G Lenz, T Katsufuji, SW Cheong, RE Slusher

    OPTICS LETTERS   27 ( 5 ) 363 - 365  2002年03月  [査読有り]

     概要を見る

    Single-mode planar waveguides were fabricated from chalcogenide glass compounds with large Kerr nonlinearities. Strong self-phase modulation of subpicosecond pulses along with low linear and nonlinear absorption losses demonstrates the potential for ultrafast, low-power, all-optical processing applications. (C) 2002 Optical Society of America.

  • Phase separation and ferromagnetic transition in B-site substituted Nd1/2Ca1/2MnO3

    A Machida, Y Moritomo, K Ohoyama, T Katsufuji, A Nakamura

    PHYSICAL REVIEW B   65 ( 6 ) 064435  2002年02月  [査読有り]

     概要を見る

    B-site substitution effects on the structural and magnetic properties are investigated in Nd1/2Ca1/2(Mn0.97M0.03)O-3 (M = Al, Zn, Sc, Fe, Ni, Co, and Cr). With decreases of temperature below similar to200 K, these compounds transform into a two-phase state. With further decrease of temperature below similar to100 K, the Cr-, Co-, and Ni-doped compounds show the ferromagnetic transition, while the other compounds show only the antiferromagnetic component. We discuss these dopant-dependent ferromagnetic transition in terms of the effective one-electron bandwidth of the e(g) band.

    DOI

  • Charge dynamics and possibility of ferromagnetism in (formula presented) (formula presented) and Sr)

    K. Taniguchi, T. Katsufuji, F. Sakai, H. Ueda, K. Kitazawa, H. Takagi

    Physical Review B - Condensed Matter and Materials Physics   66 ( 6 ) 1 - 7  2002年  [査読有り]

     概要を見る

    Ferromagnetism was recently reported in La-doped alkaline-earth hexaborides (formula presented) (formula presented) Sr, and Ba). We have performed reflectivity, Hall resistivity, and magnetization measurements of (formula presented) The results indicate that (formula presented) can be regarded as a simple doped semimetal, with no signature of an excitonic state as suggested by several theories. It is also found that the surface of as-grown samples (formula presented) in thickness) has a different electronic structure from a bulk one, and a fairly large number of paramagnetic moments are confined in this region. After eliminating these paramagnetic moments at the surface, we could not find any evidence of an intrinsic ferromagnetic moment in our samples, implying the possibility that the ferromagnetism of (formula presented) reported so far is not intrinsic. © 2002 The American Physical Society.

    DOI

  • Phase separation and ferromagnetic transition in B-site substituted (formula presented)

    A. Machida, Y. Moritomo, K. Ohoyama, T. Katsufuji, A. Nakamura

    Physical Review B - Condensed Matter and Materials Physics   65 ( 6 ) 1 - 6  2002年  [査読有り]

     概要を見る

    (formula presented) -site substitution effects on the structural and magnetic properties are investigated in (formula presented) (formula presented) (formula presented) (formula presented) Zn, Sc, Fe, Ni, Co, and Cr). With decreases of temperature below (formula presented) these compounds transform into a two-phase state. With further decrease of temperature below (formula presented) the Cr-, Co-, and Ni-doped compounds show the ferromagnetic transition, while the other compounds show only the antiferromagnetic component. We discuss these dopant-dependent ferromagnetic transition in terms of the effective one-electron bandwidth of the (formula presented) band. © 2002 The American Physical Society.

    DOI

  • Impurity effect on the charge ordered state in manganites-In case of Sc doped manganites

    S. Mori, R. Shoji, N. Yamamoto, T. Katsufuji

    Journal of Physics and Chemistry of Solids   63 ( 6-8 ) 929 - 933  2002年  [査読有り]

     概要を見る

    We investigated an influence of substituting Sc for Mn sites on the charge and orbital ordered (CO/OO) state in Nd0.5Ca0.5MnO3. In the 3% Sc-doped sample, it is found that a magnetic susceptibility curve shows an anomalous cusp around 60 K on cooling. Electron diffraction measurement revealed that an incommensurate (IC) to commensurate (C) structural phase transition takes place around 60 K on cooling and it has a large thermal hysteresis. Furthermore, real space image in the IC phase by electron microscopy shows the presence of the discommensurastions with a phase slip of π. © 2002 Published by Elsevier Science Ltd.

    DOI

  • Far-infrared resonance in Sr2RuO4

    MG Hildebrand, M Reedyk, T Katsufuji, Y Tokura

    PHYSICAL REVIEW LETTERS   87 ( 22 ) art. no. - 227002  2001年11月  [査読有り]

     概要を見る

    The far-infrared c-axis reflectance of Sr2RuO4 has been measured above and below the 2.5 K superconducting transition temperature of the sample. A plasma edge develops near 70 cm(-1) as the temperature is lowered, which corresponds to a Drude peak in the real optical conductivity associated with the onset of the coherent motion of the carriers. A gaplike suppression of the frequency-dependent scattering rate at low temperatures indicates that the coherent transport is related to a loss of scattering below 60 cm(-1). A strong resonance near 9 meV is responsible for the scattering that destroys coherence.

  • Hole concentration dependence of the ordering process of the stripe order in La2-xSrxNiO4

    R. Kajimoto, T. Kakeshita, H. Yoshizawa, T. Tanabe, T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   64 ( 14 ) 1444321 - 1444324  2001年10月  [査読有り]

     概要を見る

    Ordering process of stripe order in La2-xSrxNiO4 with x being around 1/3 was investigated by neutron-diffraction experiments. When the stripe order is formed at high temperature, incommensurability ε of the stripe order has a tendency to show the value close to 1/3 for the samples with x at both sides of 1/3. With decreasing temperature, however, ε becomes close to the value determined by the linear relation of ε=nh, where nh is a hole concentration. This variation of the ε strongly affects the character of the stripe order through the change of the carrier densities in stripes and antiferromagnetic domains.

    DOI

  • Hole concentration dependence of the ordering process of the stripe order in La2-xSrxNiO4

    R Kajimoto, T Kakeshita, H Yoshizawa, T Tanabe, T Katsufuji, Y Tokura

    PHYSICAL REVIEW B   64 ( 14 ) 144432  2001年10月  [査読有り]

     概要を見る

    Ordering process of stripe order in La2-xSrxNiO4 with x being around 1/3 was investigated by neutron-diffraction experiments. When the stripe order is formed at high temperature, incommensurability epsilon of the stripe order has a tendency to show the value close to 1/3 for the samples with x at both sides of 1/3. With decreasing temperature, however, epsilon becomes close to the value determined by the linear relation of epsilon = n(h) where n(h) is a hole concentration. This variation of the epsilon strongly affects the character of the stripe order through the change of the carrier densities in stripes and anti ferromagnetic domains.

  • Dielectric and magnetic anomalies and spin frustration in hexagonal RMnO3 (R = Y, Yb, and Lu)

    T Katsufuji, S Mori, M Masaki, Y Moritomo, N Yamamoto, H Takagi

    PHYSICAL REVIEW B   64 ( 10 ) 104419  2001年09月  [査読有り]

     概要を見る

    Single crystals of hexagonal RMnO3 (R = Y, Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration. It is found that the ratio of a Weiss temperature to T-N is similar to 10 in RMnO3, indicating the dominant role of strong geometrical frustration. The effect of geometrical frustration also appears in specific heat, which shows a presence of a substantial amount of residual magnetic contribution below T-N, indicating that a part of the spins are still fluctuating at T much less thanT(N). It is also found that the dielectric anomaly at T-N is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds.

  • Coupling between magnetism and dielectric properties in quantum paraelectric EuTiO3

    T Katsufuji, H Takagi

    PHYSICAL REVIEW B   64 ( 5 ) 054415  2001年08月  [査読有り]

     概要を見る

    The dielectric constant of quantum paraelectric EuTiO3, which contains Eu2+ with S=7/2 spin and Ti4+, has been measured under a magnetic field. The dielectric constant shows a critical decrease at the antiferromagnetic ordering of the Eu spins at 5.5 K, as well as a substantial chance under a magnetic field (by similar to7% with 1.5 T), indicating a strong coupling between the Eu spins and dielectric properties. We show that the variation of the dielectric constant is dominated by the pair correlation of the nearest-neighbor Eu spins, likely via the variation of the soft-phonon-mode frequency.

  • Electronic structure of Mott-Hubbard-type transition-metal oxides

    A Fujimori, T Yoshida, K Okazaki, T Tsujioka, K Kobayashi, T Mizokawa, M Onoda, T Katsufuji, Y Taguchi, Y Tokura

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   117   277 - 286  2001年06月  [査読有り]

     概要を見る

    Oxides of Ti and V belong to the Mott-Hubbard regime of the Zaanen-Sawatzky-Allen classification scheme of transition-metal compounds and have simple electronic structures which allow us to study the effect of electron correlation in a transparent way. In this article, we make an overview of our recent photoemission studies on Ti and V oxides. with special emphasis on metal-insulator transitions induced by the control of the width and the filling of the Ti and V 3d bands. Spectroscopic data yield spectral weight transfer between the coherent and incoherent parts of the d band, the spectral intensities at the Fermi level and the chemical potential shifts as functions of band filling. We show that such spectroscopic information well corresponds to the thermodynamic and transport properties and is necessary to understand electron correlation phenomena from a fundamental viewpoint. (C) 2001 Elsevier Science B.V. All rights reserved.

  • Charge ordering in the geometrically frustrated spinel AlV2O4

    K Matsuno, T Katsufuji, S Mori, Y Moritomo, A Machida, E Nishibori, M Takata, M Sakata, N Yamamoto, H Takagi

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   70 ( 6 ) 1456 - 1459  2001年06月  [査読有り]

     概要を見る

    We have found that spinel AlV2O4 shows a phase transition at similar to 700 K with anomalies of transport and magnetic properties. It is clarified that this corresponds to a charge-ordering transition, where the V2.5+ ions are separated into three V2.5-delta ions and one V2.5+3 delta loll, This three-one type of charge ordering, which is coupled with rhombohedral distortion along the (111) axis, reduces the high (111) axis, reduces the high degeneracy of ground states inherent in this spinel structure, and thus can be understood in terms of "change frustration." thus can be understood in terms of "change frustration."

  • Chemical pressure control of exchange interaction in Mo pyrochlore

    Y Moritomo, S Xu, A Machida, T Katsufuji, E Nishibori, M Takata, M Sakata, SW Cheong

    PHYSICAL REVIEW B   63 ( 14 ) 144425  2001年04月  [査読有り]

     概要を見る

    Structural parameters are systematically investigated for Mo pyrochlore R2Mo2O7 (R = Dy, Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius r(R) of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with r(R) from 129.7 degrees for Dy2Mo2O7 (SG) to 131.5 degrees for Nd2Mo2O7 (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.

  • Nanoscale anisotropic structural correlations in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5MnO3

    Kiryukhin, V, BG Kim, T Katsufuji, JP Hill, SW Cheong

    PHYSICAL REVIEW B   63 ( 14 ) 144406  2001年04月  [査読有り]

     概要を見る

    We report x-ray scattering studies of short-range structural correlations and diffuse scattering in Nd0.5Sr0.5MnO3. On cooling, this material undergoes a series of transitions, first from a paramagnetic insulating (PI) to a ferromagnetic metallic (FM) phase, and then to a charge-ordered (CO) insulating state. Highly anisotropic structural correlations were found in both the PI and FM states of this compound. The correlations increase with decreasing temperature, reaching a maximum at the CO transition temperature. Below this temperature, they abruptly collapsed. Single-polaron diffuse scattering was also observed in both the PI and FM states suggesting that substantial local lattice distortions are present in these phases. We argue that our measurements indicate that nanoscale regions exhibiting layered orbital order exist in the paramagnetic and ferromagnetic phases of Nd0.5Sr0.5MnO3.

  • Temperature dependence of stripe order in La2-xSrxNiO4

    R. Kajimoto, T. Kakeshita, H. Yoshizawa, T. Tanabe, T. Katsufuji, Y. Tokura

    Journal of the Physical Society of Japan   70 ( Supplement A ) 55 - 57  2001年03月  [査読有り]

  • Chemical pressure control of exchange interaction in Mo pyrochlore

    Y. Moritomo, Sh Xu, A. Machida, T. Katsufuji, E. Nishibori, M. Takata, M. Sakata, S. W. Cheong

    Physical Review B - Condensed Matter and Materials Physics   63 ( 14 ) 1444251 - 1444255  2001年

     概要を見る

    Structural parameters are systematically investigated for Mo pyrochlore R2Mo2O7 (R = Dy, Gd, Sm, and Nd) by means of synchrotron radiation x-ray powder diffraction. This system shows a crossover from the spin-glass (SG) state to the ferromagnetic metallic (FM) state as the averaged ionic radius rR of the rare-earth ion increases. We have found the Mo-O-Mo bond angle increases with rR from 129.7° for Dy2Mo2O7 (SG) to 131.5° for Nd2Mo2O7 (FM). This behavior is opposite to the so-called Kanamori-Goodenough rule.

    DOI

  • Dielectric and magnetic anomalies and spin frustration in hexagonal (formula presented) (formula presented) Yb, and Lu)

    N. Yamamoto, H. Takagi, T. Katsufuji, M. Masaki, S. Mori, Y. Moritomo

    Physical Review B - Condensed Matter and Materials Physics   64 ( 10 )  2001年  [査読有り]

     概要を見る

    Single crystals of hexagonal (formula presented) (formula presented) Yb, and Lu), where Mn ions form the triangular lattice, were investigated, focusing on their dielectric/magnetic anomalies as well as geometrical spin frustration. It is found that the ratio of a Weiss temperature to (formula presented) is (formula presented) in (formula presented) indicating the dominant role of strong geometrical frustration. The effect of geometrical frustration also appears in specific heat, which shows a presence of a substantial amount of residual magnetic contribution below (formula presented) indicating that a part of the spins are still fluctuating at (formula presented) It is also found that the dielectric anomaly at (formula presented) is strongly anisotropic, suggesting a unique correlation between magnetism and dielectric properties in these compounds. © 2001 The American Physical Society.

    DOI

  • Coupling between magnetism and dielectric properties in quantum paraelectric (formula presented)

    T. Katsufuji, H. Takagi

    Physical Review B - Condensed Matter and Materials Physics   64 ( 5 )  2001年  [査読有り]

     概要を見る

    The dielectric constant of quantum paraelectric (formula presented) which contains (formula presented) with (formula presented) spin and (formula presented) has been measured under a magnetic field. The dielectric constant shows a critical decrease at the antiferromagnetic ordering of the Eu spins at 5.5 K, as well as a substantial change under a magnetic field (by (formula presented) with 1.5 T), indicating a strong coupling between the Eu spins and dielectric properties. We show that the variation of the dielectric constant is dominated by the pair correlation of the nearest-neighbor Eu spins, likely via the variation of the soft-phonon-mode frequency. © 2001 The American Physical Society.

    DOI

  • Phase separation and destabilization of the charge-ordered state in Cr-doped manganites

    R Shoji, S Mori, N Yamamoto, A Machida, Y Moritomo, T Katsufuji

    JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN   70 ( 1 ) 267 - 271  2001年01月  [査読有り]

     概要を見る

    Destabilization of the charge-ordered (CO) state in half-doped manganites by chromium (Cr) doping in the: manganese (Mn) sites was investigated. The Cr doping: in the half-doped manganites makes the CO state short-ranged and induces new ferromagnetic metallic (FMM) phase at low temperature. Real space images taken in the CO state by electron microscope clearly show coexistence of the short-ranged CO and FMM states, which is characterized as a microscopic scale phase separation. Furthermore, the CO state in the Cr-doped manganites has a superlattice structure with the incommensurate wave vector q = (1/2 - epsilon )a(o)(*); where epsilon depends on the Cr concentration. These results suggest that the Cr doping suppresses the incommensurate to commensurate structural transition found in the half-doped manganites and produces new FMM phase.

  • Optical study on the doping and temperature dependence of the anisotropic electronic structure in bilayered manganites: La2-2xSr1+2xMn2O7 (0.3≤x≤0.5)

    T. Ishikawa, K. Tobe, T. Kimura, T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   62 ( 18 ) 12354 - 12362  2000年11月  [査読有り]

     概要を見る

    The electronic structure of bilayered manganite (La2-2xSr1+2xMn2O7) has been investigated by optical measurements. In the paramagnetic phase, the in-plane and out-of-plane optical spectra show a doping dependence reflecting the change in the orbital character of conduction electrons. A drastic electronic structural change has been observed with the evolution of in-plane magnetic ordering irrespective of the ferromagnetic (e.g., x=0.4) or antiferromagnetic (x=0.3) interbilayer magnetic coupling. The optical conductivity spectra of the ferromagnetic state show a strongly incoherent feature, such as a minimal Drude component followed by a dominant broad peak centered around 0.4 eV. Anomalies in the phonon spectra are found in the antiferromagnetic state of the x=0.45 and 0.5 compounds, indicating a different structural change accompanied by the A-type antiferromagnetic (and perhaps x2-y2-type orbital) ordering. The relevant charge ordering and lattice distortion may be responsible for the nonmetallic nature of the A-type antiferromagnetic phase.

    DOI

  • Optical study on the doping and temperature dependence of the anisotropic electronic structure in bilayered manganites: La2-2xSr1+2xMn2O7 (0.3 &lt;= x &lt;= 0.5)

    T Ishikawa, K Tobe, T Timura, T Katsufuji, Y Tokura

    PHYSICAL REVIEW B   62 ( 18 ) 12354 - 12362  2000年11月  [査読有り]

     概要を見る

    The electronic structure of bilayered manganite (La2-2xSr1+2xMn2O7) has been investigated by optical measurements. In the paramagnetic phase, the in-plane and out-of-plane optical spectra show a doping dependence reflecting the change in the orbital character of conduction electrons. A drastic electronic structural change has been observed with the evolution of in-plane magnetic ordering irrespective of the ferromagnetic (e.g., x = 0.4) or antiferromagnetic (x = 0.3) interbilayer magnetic coupling. The optical conductivity spec tra of the ferromagnetic state show a strongly incoherent feature, such as a minimal Dr de component followed by a dominant broad peak centered around 0.4 eV. Anomalies in the phonon spectra are found in the antiferromagnetic state of the x=0.45 and 0.5 compounds, indicating a different structural change accompanied by the A-type antiferromagnetic (and perhaps x(2)-y(2)-type orbital) ordering. The relevant charge ordering and lattice distortion may be responsible for the nonmetallic nature of the A-type antiferromagnetic phase.

  • Coexistence of two components in the optical spectra of CeTiO3.04 in the antiferromagnetic metallic phase

    T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   62 ( 16 ) 10797 - 10801  2000年10月  [査読有り]

     概要を見る

    The optical spectra of CeTiO3.04, which undergoes a phase transition from a paramagnetic metal to an antiferromagnetic metal, are investigated with changing temperature (T). In the paramagnetic phase, optical conductivity shows a T-dependent feature up to 1 eV, which can be attributed to the T-dependent scattering rate. In the antiferromagnetic phase, on the other hand, optical conductivity reduces its low-energy weight below 0.4 eV, but no gaplike structure appears. A two-component model, where the spectrum is a sum of a metallic and an insulating component, gives a simple explanation for the change of the optical spectra in the antiferromagnetic metallic phase.

    DOI

  • Coexistence of two components in the optical spectra of CeTiO3.04 in the antiferromagnetic metallic phase

    T Katsufuji, Y Tokura

    PHYSICAL REVIEW B   62 ( 16 ) 10797 - 10801  2000年10月  [査読有り]

     概要を見る

    The optical spectra of CeTiO3.04. which undergoes a phase transition from a paramagnetic metal to an antiferromagnetic metal, are investigated with changing temperature (T). In the paramagnetic phase, optical conductivity shows a T-dependent feature up to 1 eV, which can be attributed to the T-dependent scattering rate. In the antiferromagnetic phase, on the other hand, optical conductivity reduces its low-energy weight below 0.4 eV, but no gaplike structure appears. A two-component model, where the spectrum is a sum of a metallic and an insulating component, gives a simple explanation for the change of the optical spectra in the antiferromagnetic metallic phase.

  • Raman spectroscopy of the charge-orbital ordering in layered manganites

    K Yamamoto, T Kimura, T Ishikawa, T Katsufuji, Y Tokura

    PHYSICAL REVIEW B   61 ( 21 ) 14706 - 14715  2000年06月  [査読有り]

     概要を見る

    The lattice and electronic response in the course of concomitant charge and orbital-ordering transition has been investigated for single crystals of layered manganites, single-layered La0.5Sr1.5MnO4, and bilayered LaSr2Mn2O7, by Raman-scattering spectroscopy. In the charge-orbital-ordered state (T&lt;217 K for La0.5Sr1.5MnO4 and similar to 100 K&lt;T&lt;210 K for LaSr2Mn2O7), four phonon modes are activated in the MnO2 plane, indicating the existence of both Jahn-Teller type and breathing mode type lattice distortions. The diffuse scattering response with B-1g symmetry is suppressed in the charge-orbital-ordered state, both for La0.5Sr1.5MnO4 and LaSr2Mn2O7, signaling that the fluctuation caused by the dynamical charge-orbital correlation is suppressed in the charge-orbital-ordered state. These features reveal the unique nature of the charge-orbital-ordering transition realized by the complex interplay of charge and orbital degrees of freedom.

  • Charge ordering and chemical potential shift in La2-xSrxNiO4 studied by photoemission spectroscopy

    M Satake, K Kobayashi, T Mizokawa, A Fujimori, T Tanabe, T Katsufuji, Y Tokura

    PHYSICAL REVIEW B   61 ( 23 ) 15515 - 15518  2000年06月  [査読有り]

     概要を見る

    We have studied the chemical potential shift in La2-xSrxNiO4 and the charge ordering transition in La1.67Sr0.33NiO4 by photoemission spectroscopy. The result shows a large (similar to 1 eV/hole) downward shift of the chemical potential with hole doping in the high-doping regime (delta greater than or similar to 0.33) while the shift is suppressed in the low-doping regime (delta less than or similar to 0.33). This suppression is attributed to a segregation of doped holes on a microscopic scale when the hole concentration is lower than delta similar or equal to 1/3. In the delta=1/3 sample, the photoemission intensity at the chemical potential vanishes below the charge ordering transition temperature T-CO=240 K.

  • Anomalous magnetotransport properties of R2Mo2O7 near the magnetic phase boundary

    T Katsufuji, HY Hwang, SW Cheong

    PHYSICAL REVIEW LETTERS   84 ( 9 ) 1998 - 2001  2000年02月  [査読有り]

     概要を見る

    The magnetic and transport properties of pyrochlore R2Mo2O7 have been studied with variation of the rare earth (R). The change of the mean ionic radius of R, which induces change of the lattice structure, determines the magnetic ground state (ferromagnetic or spin glass), and the magnetic phase boundary is correlated with the metal-insulator crossover. Furthermore, we found enhanced magnetoresistance and unusual residual anomalous Hall effect at low temperatures near the phase boundary, which can be attributed to the coexistence of both phases.

  • Stripe order at low temperatures in with

    H. Yoshizawa, T. Kakeshita, R. Kajimoto, T. Tanabe, T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   61 ( 2 ) R854 - R857  2000年  [査読有り]

     概要を見る

    The stripe order in (Formula presented) with (Formula presented) was studied with neutron-scattering technique. At low temperatures, all samples exhibit hole stripe order. Incommensurability (Formula presented) of the stripe order is approximately linear in the hole concentration (Formula presented) up to (Formula presented) where (Formula presented) denotes the off stoichiometry of oxygen atoms. The charge and spin ordering temperatures exhibit maxima at (Formula presented) and both decrease beyond (Formula presented) For (Formula presented) the stripe ordering consists of the mixture of the (Formula presented) stripe order and the (Formula presented) charge/spin order. © 2000 The American Physical Society.

    DOI

  • Transport and magnetic properties of a ferromagnetic metal: (formula presented)

    T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   60 ( 22 ) R15021 - R15023  1999年  [査読有り]

     概要を見る

    We report the transport and magnetic properties of a series of ferromagnetic metallic compounds, (Formula presented) where the maximum (Formula presented) is 8 K at (Formula presented) Magnetic interaction between Eu (Formula presented) spins mediated by itinerant Ti (Formula presented) electrons is essential for the ferromagnetism of this compound, and a clear correlation between transport properties and magnetism exists in this series. Possible roles of the electron-correlation effect in Ti (Formula presented) electrons are also discussed. © 1999 The American Physical Society.

    DOI

  • Probing charge/orbital correlation in La1.2Sr1.8Mn2O7 by Raman spectroscopy

    Ki-Ichi Yamamoto, Tsuyoshi Kimura, Tadahiko Ishikawa, Takuro Katsufuji, Yoshinori Tokura

    Journal of the Physical Society of Japan   68 ( 8 ) 2538 - 2541  1999年  [査読有り]

     概要を見る

    Raman spectra have been investigated for a single crystal of La1.2Sr1.8Mn2O7 that shows colossal magnetoresistance around the Curie temperature Tc = 126K. Several broad phonon peaks grow with decrease of temperature down to TC but disappear below TC, which can be clearly ascribed to a change in the charge/orbital correlation by comparison with the spectra for the charge/orbital ordered states of the layered manganites (La0.5Sr1.5MnO4 and LaSr2Mn2O7). Such a real-space charge density fluctuation is also observed to cause the low-energy diffuse Raman response, the B1g component of which is critically suppressed below TC as well.

    DOI

  • Frequency and temperature dependence of conductivity for perovskite titanates

    T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   60 ( 11 ) 7673 - 7676  1999年  [査読有り]

     概要を見る

    Frequency- and temperature-dependent conductivity has been studied by optical spectroscopy for perovskite titanates, which are typical three-dimensional correlated metals. Extended Drude analysis of the data indicates that only one parameter, the (Formula presented) coefficient of the scattering rate, dominates the temperature dependence of conductivity over a wide energy range (from dc to (Formula presented) This result is consistent with the Fermi-liquid theory in the low-frequency limit, and can be regarded as a prototypical behavior of correlated metals. © 1999 The American Physical Society.

    DOI

  • Coherent-incoherent crossover of charge dynamics in the ferromagnetic ground state of manganites: R0.6Sr0.4MnO3(R = La - Sm)

    E. Saitoh, Y. Okimoto, Y. Tomioka, T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   60 ( 14 ) 10362 - 10366  1999年  [査読有り]

     概要を見る

    Low-energy optical spectra and their temperature dependence have been investigated for single crystals of perovskite-type manganese oxides, R1-xSrxMnO3, at a fixed doping level (x=0.4) but with varying perovskite R species or one-electron bandwidth (W). Variation of the optical conductivity spectra with a change of R, such as a diminishment of the Drude peak and the emergence of a nearly ω-independent component, indicates a coherent-incoherent crossover of charge dynamics in the ferromagnetic metallic phase. This cannot be accounted for in terms of simple change in carrier mass or scattering rate, but indicates the importance of competitive instabilities triggered by decreasing W against the double-exchange interaction near the charge/orbital-ordering instability. © 1999 The American Physical Society.

    DOI

  • Fermi-Liquid Versus Pseudogap Behaviors in Filling-Control Transition-Metal Oxides

    A. Fujimori, T. Yoshida, A. Ino, T. Mizokawa, C. Kim, Z.-X. Shen, Y. Taguchi, T. Katsufuji, Y. Tokura, H. Eisaki, S. Uchida, K. Kishio

    Springer Series in Solid-State Sciences     111  1999年  [査読有り]

    DOI

  • Commensurability effect on the charge ordering of La2-xSrxNiO4

    T. Katsufuji, T. Tanabe, T. Ishikawa, S. Yamanouchi, Y. Tokura, T. Kakeshita, R. Kajimoto, H. Yoshizawa

    Physical Review B - Condensed Matter and Materials Physics   60 ( 8 ) R5097 - R5100  1999年  [査読有り]

     概要を見る

    Measurements of Hall coefficients, thermoelectric power, and optical conductivity spectra were done for single crystals of La2-xSrxNiO4 (x=0.27-0.50) showing prototypical charge ordering. We found that the character of the carriers in the charge-ordered state changes from electronlike to holelike when the hole concentration p crosses 1/3, and that the correlation gap in the optical spectrum for p=1/3 is collapsed when p is away from 1/3. These results indicate that the deviation of p from 1/3 can be regarded as electron or hole doping into the commensurate charge-ordered state at p=1/3. © 1999 The American Physical Society.

    DOI

  • Optical spectroscopy of charge-ordering transition in La1/3Sr2/3FeO3

    Ishikawa T, Park SK, Katsufuji T, Arima T, Tokura Y

    PHYSICAL REVIEW B   58 ( 20 ) 13326 - 13329  1998年11月  [査読有り]

    DOI

  • Chemical potential shifts in correlated electron systems studied by photoemission spectroscopy

    A Fujimori, A Ino, T Mizokawa, T Tsujioka, T Yoshida, M Satake, K Okazaki, H Eisaki, S Uchida, K Kishio, Y Taguchi, T Katsufuji, Y Tokura, Y Yasui, T Nishikawa, M Sato

    JOURNAL OF ELECTRON SPECTROSCOPY AND RELATED PHENOMENA   92 ( 1-3 ) 59 - 63  1998年05月  [査読有り]

     概要を見る

    We have studied chemical potential shifts in filling-control transition-metal compounds from the shifts of photoemission and inverse photoemission spectra. Implications of the results for the electronic structure and electron correlation in these systems are discussed. In addition to the conventional Fermi-liquid behavior we find unusual shifts caused by a pseudo-gap formation and gap-opening resulting from stripe order in two-dimensional systems. (C) 1998 Elsevier Science B.V. All rights reserved.

    DOI

  • Raman scattering of the charge-spin stripes in La1.67Sr0.33NiO4

    K Yamamoto, T Katsufuji, T Tanabe, Y Tokura

    PHYSICAL REVIEW LETTERS   80 ( 7 ) 1493 - 1496  1998年02月  [査読有り]

     概要を見る

    Raman scattering spectra have been investigated for a single crystal of La1.67Sr0.33NiO4 which shows stripe ordering of charge (T-c = 230 K) and spin (T-s = 180 K). Upon the charge ordering, the activated phonon peaks grow with the evolution of the charge modulation amplitude, while the electronic Raman continuum of the metallic state abruptly disappears. The two broad peaks with the B-1g, symmetry arising from the two-magnon excitations emerge immediately below T-c,, indicating the evolution of the two-dimensional antiferromagnetic spin correlation in the charge-ordered state even above T-s,. [S0031-9007(98)05303-4].

  • Optical probe of anisotropic and incoherent charge dynamics in a layered ferromagnet

    T. Ishikawa, T. Kimura, T. Katsufuji, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   57 ( 14 ) R8079 - R8082  1998年  [査読有り]

     概要を見る

    Highly anisotropic optical conductivity spectra and their temperature variation have been investigated for a layered ferromagnetic (Formula presented)=121 K) crystal of La(Formula presented)Sr(Formula presented)Mn(Formula presented)O(Formula presented)=0.4). The in-plane spectra show conspicuous spectral-weight transfer over an energy region of 0-3 eV with evolution of spin polarization below (Formula presented) and in the ferromagnetic, highly conducting ground state form a prominent peak structure around 0.4 eV with minimal Drude weight. By contrast, the (Formula presented)-axis spectra show little change with temperature and remain insulatorlike. The results indicate highly incoherent dynamics of the fully quasi-two dimensional spin-polarized carriers. © 1998 The American Physical Society.

    DOI

  • Transport and magnetic properties of a Mott-Hubbard system whose bandwidth and band filling are both controllable

    T. Katsufuji, Y. Taguchi, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   56 ( 16 ) 10145 - 10153  1997年  [査読有り]

     概要を見る

    Transport and magnetic properties of (Formula presented) have been systematically investigated varying the one-electron bandwidth (Formula presented) and the band filling (Formula presented) which can be controlled by the (Formula presented)-dependent lattice distortion and by the Ca content (Formula presented) and/or oxygen offstoichiometry (Formula presented) respectively. The end compound (Formula presented) is a (Formula presented) Mott-Hubbard insulator and its charge-gap magnitude increases with decreasing ionic radius of (Formula presented) i.e., an increase of electron correlation (Formula presented) in proportion with (Formula presented) where (Formula presented) is the critical value for the (hypothetical) (Formula presented) Mott transition. Such a Mott insulator is transformed to a correlated metal by substitution of (Formula presented) with Ca (hole doping), and the nominal hole concentration required for the insulator-metal transition (Formula presented) increases in proportion with (Formula presented) Concerning magnetism, (Formula presented) with (Formula presented) Pr, Nd, and Sm, shows the antiferromagnetic ordering and its Néel temperature (Formula presented) decreases with smaller (Formula presented) also decreases with Ca doping, but remains finite up to the metal-insulator phase boundary. On the basis of these results, electronic phase diagrams are derived for a series of titanates as an electron-correlated system with changes of two parameters, i.e., the strength of electron correlation and band filling. Possible origins of the insulating state with finite hole doping are also discussed in terms of the kinetic energy of doped carriers in the Mott-Hubbard insulator. © 1997 The American Physical Society.

    DOI

  • Variation of electronic structure f(0⩽x⩽0.3) as investigated by optical conductivity spectra

    Y. Okimoto, T. Katsufuji, T. Ishikawa, T. Arima, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   55 ( 7 ) 4206 - 4214  1997年  [査読有り]

     概要を見る

    Optical conductivity spectra and their variation with temperature and doping level x have been investigated for single crystals of (Formula presented)(Formula presented)(Formula presented)n(0⩽x⩽0.3). For the low-doped insulating crystal (x=0.1) which shows a ferromagnetic insulating state at low temperature, the spectral weight of the optical conductivity increases only in the inner-gap region around 0.5 eV, but no Drude part emerges due to carrier localization effect. For x⩾0.17, where the low-temperature ferromagnetic metallic state shows up, the optical conductivity spectrum above (Formula presented) is characterized by interband transitions between the exchange-split conduction bands, and it gradually changes into that of intraband excitations below (Formula presented). The energy scale (up to ≈2 eV) of the spectral weight transfer is determined by the effective Hundșs-rule coupling energy. In the metallic phase, low-energy spectra arising from intraband excitations can be sorted into two parts: One is a nearly ω-independent broad structure (incoherent part), and the other a sharp coherent Drude peak with anomalously low spectral weight. This can hardly be reconciled with the simple double-exchange theory, but indicates that another degree of freedom (e.g., the orbital ordering and/or electron-lattice interactions) should be taken into account. © 1997 The American Physical Society.

    DOI

  • Spin and charge ordering in La2-xSrxNiO4 with 0.27 ≤ x ≤ 0.5

    H. Yoshizawa, T. Kakeshita, R. Kajimoto, T. Tanabe, T. Katsufuji, Y. Tokura

    Physica B: Condensed Matter   241-243   880 - 882  1997年  [査読有り]

     概要を見る

    We report the Sr-concentration dependence of the spin and charge ordering in La2-xSrxNiO4. with 0.27 ≤ x ≤ 0.5. In this range of the Sr concentration x, the characteristic wave vector ε for the magnetic and charge order varies approximately linear in x, i.e. ε ∼ x. The transition temperature of spin order is always lower than that of stripe order. Two transition temperatures increase for x &lt
    xc where xc = 1/3, and then tend to decrease as x further increases. © 1998 Elsevier Science B.V. All rights reserved.

    DOI J-GLOBAL

  • Optical spectroscopy of the charge-ordering transition in La1.67Sr0.33NiO4

    T Katsufuji, T Tanabe, T Ishikawa, Y Fukuda, T Arima, Y Tokura

    PHYSICAL REVIEW B   54 ( 20 ) 14230 - 14233  1996年11月  [査読有り]

     概要を見る

    Optical spectra of a single crystal of La1.67Sr0.33NiO4, which undergoes a charge-ordering transition at T(CO)similar to 240 K, have been investigated for wide ranges of photon energy (0.008-3 eV) and temperature (10-480 K). The opening of a charge gap as well as change of the gap magnitude (2 Delta) with temperature is clearly observed below T-CO, with an anomalously large ratio of 2 Delta(T=0)/k(B)T(CO) (similar to 13). Conspicuous spectral change with temperature is also observed above T-CO over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above T-CO.

  • Optical spectroscopy of the charge-ordering transition

    T. Katsufuji, T. Tanabe, T. Ishikawa, Y. Fukuda, T. Arima, Y. Tokura

    Physical Review B - Condensed Matter and Materials Physics   54 ( 20 ) R14230 - R14233  1996年  [査読有り]

     概要を見る

    Optical spectra of a single crystal of (Formula presented)(Formula presented)(Formula presented), which undergoes a charge-ordering transition at (Formula presented)∼240 K, have been investigated for wide ranges of photon energy (0.008-3 eV) and temperature (10-480 K). The opening of a charge gap as well as change of the gap magnitude (2Δ) with temperature is clearly observed below (Formula presented), with an anomalously large ratio of 2Δ(T=0)/(Formula presented)(Formula presented) (∼13). Conspicuous spectral change with temperature is also observed above (Formula presented) over the energy region up to 2 eV, which suggests persistent fluctuation of the charge ordering or formation of small polarons above (Formula presented). © 1996 The American Physical Society.

    DOI

  • In-plane and out-of-plane optical spectra of Sr2RuO4

    T Katsufuji, M Kasai, Y Tokura

    PHYSICAL REVIEW LETTERS   76 ( 1 ) 126 - 129  1996年01月  [査読有り]

     概要を見る

    In-plane (ab-plane) and out-of-plane (c-axis) optical spectra were investigated st various temperatures for Sr2RuO4 which is isostructural with the cuprate superconductor La2-xSrxCuO4 and becomes superconducting below 1 K. The ratio of the in-plane to out-of-plane low-energy spectral weight of this compound is approximately 10(2), an anisotropy comparable to that of the cuprate. The temperature dependence of the c-axis spectrum suggests coherent motion of conduction carriers with strongly renormalized mass and scattering rate along the c axis at low temperatures.

  • SPECTRAL WEIGHT TRANSFER OF THE OPTICAL CONDUCTIVITY IN DOPED MOTT INSULATORS

    T KATSUFUJI, Y OKIMOTO, Y TOKURA

    PHYSICAL REVIEW LETTERS   75 ( 19 ) 3497 - 3500  1995年11月  [査読有り]

     概要を見る

    Optical spectra have been systematically investigated for R(1-x)Ca(x)TiO(3) with various rare earth ions R (= La, Pr, Nd, Sm, and Y) as a typical three-dimensional correlated electron system, in which both the electon-correlation strength (U/W) and the band filling (n) can be varied. With a decrease of n from 1, the spectrum of the Mott-gap excitation collapses and the Drude part evolves. We found that the rate of the Drude-part evolution with the nominal hole-doping level delta (= 1 - n) is critically enhanced as U/W approaches the critical value (U/W)(c) for the bandwidth-controlled Mott transition.

  • ANOMALOUS VARIATION OF OPTICAL-SPECTRA WITH SPIN POLARIZATION IN DOUBLE-EXCHANGE FERROMAGNET - LA1-XSRXMNO3

    Y OKIMOTO, T KATSUFUJI, T ISHIKAWA, A URUSHIBARA, T ARIMA, Y TOKURA

    PHYSICAL REVIEW LETTERS   75 ( 1 ) 109 - 112  1995年07月  [査読有り]

  • CHANGE OF ELECTRONIC-PROPERTIES ON THE DOPING-INDUCED INSULATOR-METAL TRANSITION IN LA1-XSRXVO3

    F INABA, T ARIMA, T ISHIKAWA, T KATSUFUJI, Y TOKURA

    PHYSICAL REVIEW B   52 ( 4 ) R2221 - R2224  1995年07月  [査読有り]

     概要を見る

    Changes in electrical, magnetic, and optical properties have been investigated for the doping-induced insulator-metal (I-M) transition in hole-doped Mott insulators, La1-xSrxVO3. In the optical spectra, the I-M transition is characterized by doping-induced transfer of spectral weight into the in-gap region. Resistivity in the metallic phase near the critical magnetic phase boundary shows such a temperature dependence as expressed by the relation rho=rho(0)+AT(1.5) over a wide temperature range (2-200 K) with the enhanced coefficient A towards the phase boundary. The results are argued in terms of effect of the antiferromagnetic spin fluctuation on the charge transport.

  • OPTICAL-SPECTRA IN (LA,Y)TIO3 - VARIATION OF MOTT-HUBBARD GAP FEATURES WITH CHANGE OF ELECTRON CORRELATION AND BAND FILLING

    Y OKIMOTO, T KATSUFUJI, Y OKADA, T ARIMA, Y TOKURA

    PHYSICAL REVIEW B   51 ( 15 ) 9581 - 9588  1995年04月  [査読有り]

  • OPTICAL SPECTROSCOPY OF THE METAL-INSULATOR-TRANSITION IN NDNIO3

    T KATSUFUJI, Y OKIMOTO, T ARIMA, Y TOKURA, JB TORRANCE

    PHYSICAL REVIEW B   51 ( 8 ) 4830 - 4835  1995年02月  [査読有り]

  • Change of electronic properties on the doping-induced insulator-metal transition in La1-xSrxVO3

    F. Inaba, T. Arima, T. Ishikawa, T. Katsufuji, Y. Tokura

    Physical Review B   52 ( 4 ) R2221 - R2224  1995年  [査読有り]

     概要を見る

    Changes in electrical, magnetic, and optical properties have been investigated for the doping-induced insulator-metal (I-M) transition in hole-doped Mott insulators, La1-xSrxVO3. In the optical spectra, the I-M transition is characterized by doping-induced transfer of spectral weight into the in-gap region. Resistivity in the metallic phase near the critical magnetic phase boundary shows such a temperature dependence as expressed by the relation =0+AT1.5 over a wide temperature range (2200 K) with the enhanced coefficient A towards the phase boundary. The results are argued in terms of effect of the antiferromagnetic spin fluctuation on the charge transport. © 1995 The American Physical Society.

    DOI

  • ELECTRONIC RAMAN-SCATTERING SPECTRA IN STRONGLY CORRELATED METALS - CUPRATE SUPERCONDUCTORS AND TITANIUM-OXIDES

    T KATSUFUJI, Y TOKURA

    PHYSICA C   235   1147 - 1148  1994年12月  [査読有り]

     概要を見る

    Electronic Raman scattering spectra for cuprate superconductors, La2-xSrxCuO4, and prototypical strongly correlated metals, Sr1-xLaxTiO3, were investigated and compared with each other. All the spectra can be reproduced by ''extended'' relaxational function with omega-dependent Gamma. Electron correlation effect leads to strong omega dependence of Gamma, which produces the spectral weight of chi''(omega) up to the high energy region. We also found anomalous symmetry dependence of the spectral intensity for La2-xSrxCuO4 in the lightly doped region.

  • ANOMALOUS VARIATION OF PHONON RAMAN INTENSITIES NEAR THE METAL-TO-MOTT-INSULATOR TRANSITION IN TITANIUM-OXIDE SYSTEMS

    T KATSUFUJI, Y TOKURA

    PHYSICAL REVIEW B   50 ( 4 ) 2704 - 2707  1994年07月  [査読有り]

     概要を見る

    Raman-scattering spectra were investigated for strongly correlated electron systems, La1-xSrxTiO3 and Y1-xCaxTiO3, which undergo a metal-to-Mott-insulator transition under variation of the band filling. The phonon spectra decrease in their intensity as the system approaches the metal-insulator phase boundary, accompanied by a critical enhancement of the value of the effective mass, and almost disappear in the insulating phase. We found that the phonon spectral intensity in the metallic region approximately scales with (n/m*)2, n and m* being the density and effective mass of conduction carriers.

  • ELECTRONIC RAMAN-SCATTERING IN FILLING-CONTROLLED METALS - SR1-XLAXTIO3

    T KATSUFUJI, Y TOKURA

    PHYSICAL REVIEW B   49 ( 6 ) 4372 - 4375  1994年02月  [査読有り]

     概要を見る

    Electronic Raman scattering has been investigated for metallic Sr1-xLaxTiO3 in which the 3d band filling (x) can be systematically changed from x = 0 (a band insulator) to x = 1 (a Mott-Hubbard insulator). The symmetry dependence of the scattering intensity can be accounted for in terms of the neutral carrier density fluctuation model. However, a systematic change of the spectral shape with the carrier density (congruent-to x) is observed. All the spectra can be reproduced by a relaxational function with an omega-dependent scattering rate (GAMMA). The omega dependence of GAMMA gets stronger with x, which is in accord with the x-dependent enhancement of the effective mass, due to the electron correlation effect.

  • Symmetry-dependent electronic Raman scattering in La2-x SrxCuO4: Evidence for doping-induced change in the κ-space anisotropy of charge dynamics

    T. Katsufuji, Y. Tokura, T. Ido, S. Uchida

    Phys. Rev. B   48 ( 21 ) 16131 - 16134  1993年12月  [査読有り]

  • CHANGE OF ELECTRONIC-STRUCTURES WITH HOLE DOPING IN STRONGLY CORRELATED ELECTRON-SYSTEMS - TITANIUM-OXIDES

    Y TOKURA, Y TAGUCHI, Y OKADA, T KATSUFUJI, T ARIMA, K KUMAGAI

    PHYSICA B-CONDENSED MATTER   186-88   992 - 994  1993年05月  [査読有り]

     概要を見る

    Crossover behavior of electronic structures from Mott insulator to Pauli metal has been investigated as a function of filling in titanium oxide compounds with perovskite-like structures. The role of the electron correlation manifests itself in filling-dependent enhancement of the density of states at the Fermi level, carrier scattering rate and spin susceptibility on the verge of the metal-insulator transition.

  • DIELECTRIC PHASE-TRANSITION AND SYMMETRY CHANGE OF CONSTITUENT MOLECULES IN PROTON DEUTERON MIXED-CRYSTALS OF SQUARIC ACID

    Y MORITOMO, T KATSUFUJI, Y TOKURA

    JOURNAL OF CHEMICAL PHYSICS   95 ( 4 ) 2244 - 2251  1991年08月  [査読有り]

     概要を見る

    We have investigated effects of random substitution of proton sites with deuterons on the temperature- and pressure-induced phase transitions in squaric acid crystal, which consists of ferroelectrically ordered molecular sheets of [C4O4]2- (SQ2-) skeletons and intervening protons or deuterons. In the isotopically mixed crystals (H1-xD(x))2SQ, the critical temperature T(c) for the dielectric phase transition was found to increase linearly with the deuterium concentration x, while the critical pressure P(c) increases nonlinearly with x at 300 K. Around the pressure approximately 20 kbar above P(c), the constituent SQ2- skeletons in the paraelectric phase show a symmetry change from the asymmetric (C1h) to centrosymmetric (C4h) form, which is likely caused by the deformation of the correlated proton (deuteron) potential.

  • 電荷と幾何学的フラストレーションが生み出す新しい物性

    勝藤拓郎

    日本物理学会誌/日本物理学会   57 ( 2 )

  • 外場に応答するチタン酸化物

    勝藤拓郎

    セラミックス   41 ( 3 ) 163 - 166

  • 電場誘起抵抗変化型メモリー材料

    勝藤拓郎

    まてりあ   46 ( 6 ) 395 - 401

  • スピネル型バナジウム酸化物の軌道整列

    勝藤拓郎

    日本結晶学会誌   50 ( 2 ) 124 - 129

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    CiNii

  • 19pAG-4 六方晶系マンガン酸化物RMnO_3における強誘電ドメイン構造の走査透過型電子顕微鏡観察(誘電体(招待講演,マルチフェロイック),19pAG 領域10,領域8合同チュートリアル講演,領域10(誘電体,格子欠陥,X線・粒子線,フォノン))

    小林 敬介, 久留島 康輔, 小山 司, 加茂 秀基, 勝藤 拓郎, 堀部 陽一, 森 茂生

    日本物理学会講演概要集   67 ( 2 ) 859 - 859  2012年08月

    CiNii

  • 19aHC-3 スピネル型V酸化物の光誘起ダイナミクス(19aHC 光誘起相転移(超伝導体・強相関系・磁性体),領域5(光物性))

    古橋 明日香, 田久保 耕, 神崎 達也, 勝藤 拓郎

    日本物理学会講演概要集   67 ( 2 ) 690 - 690  2012年08月

    CiNii

  • 26pYH-6 三量体相転移を起こすA_2V_<13>O_<22> (A=Ba, Sr)の不純物効果(26pYH V酸化物・秩序型ペロブスカイト,領域8(強相関系:高温超伝導,強相関f電子系など))

    佐藤 拓也, 小山 司, 森 茂生, 勝藤 拓郎

    日本物理学会講演概要集   67 ( 1 ) 676 - 676  2012年03月

    CiNii

  • 27pYK-14 La_<2-x>Sr_xNiO_4の電荷秩序と磁気秩序(27pYK 電荷・軌道整列と関連物質II,領域8(強相関系分野-高温超伝導,強相関f電子系など))

    大原 泰明, 梶本 亮一, 掛下 照久, 吉沢 英樹, 田邊 隆哉, 勝藤 拓郎, 石坂 香子, 田口 康二郎, 十倉 好紀

    日本物理学会講演概要集   57 ( 1 ) 602 - 602  2002年03月

    CiNii

  • 26pYH-4 六方晶Y_<1-x>Zr_xMnO_3単結晶の磁性と誘電性(26pYH Mn系(置換効果・新物質),領域8(強相関系分野-高温超伝導,強相関f電子系など))

    正木 元基, 勝藤 拓郎, 守友 浩, 大山 研司, 高木 英典

    日本物理学会講演概要集   57 ( 1 ) 571 - 571  2002年03月

    CiNii

  • 電荷・軌道秩序状態に対する不純物効果

    東海林 隆一, 森 茂生, 山本 直紀, 勝藤 拓郎, 守友 浩

    電子顕微鏡   35   331 - 331  2000年05月

    CiNii

  • 2p-YC-10 La_<2-x>Sr_xNiO_4単結晶のx〜1/3における電荷整列相転移の物性I : 輸送現象と磁性(低温(高温超伝導・非銅系))

    有馬 孝尚, 勝藤 拓郎, 田邊 隆哉, 福田 征正, 十倉 好紀

    日本物理学会講演概要集. 年会   51 ( 3 ) 460 - 460  1996年03月

    CiNii

  • 2p-YC-11 La_<2-x>Sr_xNiO_4単結晶のx〜1/3における電荷整列相転移の物性II : 光学スペクトル(低温(高温超伝導・非銅系))

    勝藤 拓郎, 有馬 孝尚, 田邊 隆哉, 福田 征正, 石川 忠彦, 山本 貴一, 十倉 好紀

    日本物理学会講演概要集. 年会   51 ( 3 ) 460 - 460  1996年03月

    CiNii

  • 3p-YC-2 La_<1-x>Sr_xMnO_3結晶のラマン散乱(低温(高温超伝導・マンガン酸化物))

    山本 貴一, 勝藤 拓郎, 守友 浩, 十倉 好紀

    日本物理学会講演概要集. 年会   51 ( 3 ) 518 - 518  1996年03月

    CiNii

▼全件表示

共同研究・競争的資金等の研究課題

  • 強相関電子系における軌道/電荷揺らぎと新奇物性

    研究期間:

    2019年04月
    -
    2022年03月
     

     概要を見る

    強相関電子系における軌道や電荷自由度に関して、秩序状態ではなく「揺らぎ」がもたらす新奇な物理を実験的に明らかにする。このために、「研究に適した新奇物質の開発」と「目的のために最適な物性測定」を両輪として行い、「軌道/電荷揺らぎのもたらす新しい物理」を見出すことを目標とする。(1) BaV10O15の軌道秩序の相転移ダイナミクスについて、電気抵抗率、磁化率、歪における時間依存性を測定した結果、それが核生成-核成長モデルに従うこと、また転移温度以下において変態時間が低温と転移温度に向かって長くなるU型のカーブを描くこと、その温度依存性から軌道秩序相と無秩序相の間の界面エネルギーが求まることを見出した。このような相転移ダイナミクスは、無機物における電子相転移においては、初めて見出されたものである。(2) 130Kで軌道秩序を起こすBaV10O15について、一軸歪下の光学反射率測定を室温で行った結果、b軸方向への歪によって等方的なスペクトル変化が生じ、圧縮の場合は低温への変化に、引っ張りの場合は高温への変化にそれぞれ対応したスペクトル変化となることを見出した。さらにこのb軸は、転移温度以上で温度変化が最も大きな軸であることも見出した。これらの結果は、軌道自由度と歪が結合したモデルにおいて、軌道自由度の揺らぎを考慮することで定性的に理解できる。(3) Vのダイマー構造をとるSr2VO4とVの一部をNbで置換した物質について、熱伝導度を測定した。その結果、母物質のSr2VO4のフォノン熱伝導度は低温にむけて増大し、50K付近で最大になった後に減少するが、Vサイトのうち3%をNbで置換した物質においては、低温でのフォノン熱伝導度は増大しなかった。このことは、母物質では軌道が秩序化しているが、Nbを導入することにより電子の遍歴性が高まり、軌道が無秩序になって、それがフォノンを散乱するためであると考えられる。BaV10O15の相転移ダイナミクスが核生成-核成長モデルで記述できることの発見と、それに伴う界面エネルギーの見積もりなど、当初の計画以上に進展している。当初の予定からさらに加速して進めてい

  • 多自由度相関系における巨大負熱膨張現象の理論研究と物質探索

    研究期間:

    2019年06月
    -
    2021年03月
     

     概要を見る

    通常の物質と異なり,温度降下により体積が膨張する「負熱膨張物質」は、体積変化が致命的となる光学・精密機器の発展に伴い,近年重要度が増している。しかし、その物理的起源には謎が多く、系統的かつ戦略的な物質開拓が行われていない状況である。本研究では、固体中の磁性と格子、軌道自由度の結合に由来する巨大な熱体積効果を示す物質を理論的に設計し、実験と連携して新物質の開拓と熱体積機能の劇的な向上を目指す。研究代表者(望月)が昨年提唱した「逆ペロフスカイト型マンガン窒化物における磁性誘起の負熱膨張現象」の理論をさらに深化・発展させ、この物質群における「様々な磁気秩序」と「負熱膨張発現の可否」を、系統的に説明することに成功した。これにより、研究代表者が構築した理論による包括的な理解の整理が完成したと言える。これらの考察や検証に基づき、「現代化学誌」や「日本物理学会誌」に解説記事を執筆した。同時に、この重要で奥の深い現象を本当に理解するために、検証が必要な様々な問題点や謎を洗い出した。特に重要な問題として、研究代表者の理論研究では、金属磁性体である本物質群を、局在スピンモデル(古典ハイゼンベルグ模型)で取り扱っている。これはこの物質の金属伝導はマンガンの4s軌道電子が担っているのに対し、Mn3d軌道の電子スピンは局在電子として振る舞っているという素朴な仮説に基づいている。この仮説は、非共線的磁性を取り扱える第一原理計算手法等による、電子構造および磁気構造の計算から検証されるべきものである。また、研究代表者の理論では特異な反強磁性秩序の発現は「負熱膨張現象」が起こるための必要条件であり、Mnスピン間の複数の交換相互作用がある条件を満たさないと負熱膨張が起こらないことが予測されている。この理論予測を、同じく非共線的磁性を取り扱える第一原理計算手法による「交換相互作用係数の微視的な計算」と「実験研究」との連携により検証していく必要がある。今年度は、第一原理計算の研究者とそのような研究計画を詰めるところに時間を割いたが、来年度はいよいよ、それが成果として結実させる。また、研究代表者の理論から予測される「新しい磁性誘起の負熱膨張物質」の探索のための指針を、物質合成と結晶構造測定を担当する研究分担者の勝藤と共有し、いくつかの遷移金属酸化物系で物質探索を行ったが、新物質の発見には至らなかった。研究代表者(望月)が構築した基礎理論を用いて、逆ペロフスカイト型マンガン窒化物が示す「Γ5g型反強磁性秩序」、「Γ4g型反強磁性秩序」、「強磁性秩序」などの多彩な磁気秩序の発現や、Γ5g型反強磁性秩序の存在下で、マンガンスピン間に働く交換相互作用ある条件を満たした場合に「負の熱膨張」が引き起こされることを系統的に説明・解明したことは、本年度の研究の重要な成果である。また、この理論を用いて、これらの物質群や固溶体で実験的に観測・報告されている実験事実や実験結果を余すところなく説明し、包括的な理解に至ったことも特筆に値する成果と言える。さらに、半世紀以上もの長い歴史を持つこの重要な研究分野におけるブレイクスルーを二本の解説記事として発表したことも、分野を牽引・開拓する重要な意義を持つと考えている。このように、本年度の研究では、基礎学理の構築という点では大きな成果を上げ、一定の進捗を得ることができたと自負している。しかし一方で、新物質探索・開発は、なかなかうまくいかずに苦労をしている。理論から予測される「新しい磁性誘起負熱膨張物質」の探索のための指針を、物質合成と結晶構造測定を担当する研究分担者の勝藤と共有し、実際にいくつかの遷移金属酸化物系の磁性体で結晶構造体積の温度依存の精密測定を行ったが、いまだに新物質の発見には至っていない。物質開発や物質探索はなかば運や賭けに近い側面もあることは否定できないが、負の熱膨張研究の発展のためには「新しい物質」の発見は必要不可欠であり、昨年度の1年間の研究ではその点が達成できなかったことには、少々忸怩たる思いがある。「磁性が誘起する負の熱膨張現象」の研究にブレイクスルーをもたらした研究代表者の基礎理論をさらに深化・発展させるべく、この理論が立脚した「仮説」や「モデル」の正当性や、導き出された条件や結論の妥当性を、第一原理計算などを用いたバイアスのない電子状態計算で検証・立証していくことが次の重要な課題となる。また、「d-p軌道混成強度」や「電荷移動エネルギー」、「結晶構造・対称性」など、負の熱膨張を引き起こす新しい条件やファクターを、電子状態計算や微視的なモデル計算で明らかにしていくことも次に取り組むべき重要な課題である。特に、「物質固有の結晶構造、電子構造、磁気構造を真面目に考慮し、磁性・電荷・軌道と結晶格子の結合を微視的に捉えることで、磁性誘起の負熱膨張、さらには、「軌道秩序」や「電荷秩序」など磁気秩序以外の長距離秩序が「格子との結合」を通じて負熱膨張を引き起こす可能性も検証・考察していく。そして、本研究課題において「物質合成」と「結晶構造測定」、その他の実験・測定を担当する研究分担者(勝藤)と緊密に連携し、遷移金属酸化物系のみならず、様々な磁性体を対象に広く物質探索を行って、新しい負の熱膨張物質の発見を目指していく

  • 非整合な軌道/電荷秩序状態における新奇物性

    研究期間:

    2016年04月
    -
    2019年03月
     

     概要を見る

    遷移金属酸化物におけるd電子の軌道/電荷秩序に由来する非整合な秩序と、それがもたらすエキゾチックな物性について実験的に調べた。その結果、Tiの二重鎖を持つホランダイト型Ti酸化物、Vの擬三角格子をとるV酸化物などにおいて、特徴的な非整合超格子構造とその温度、ドーピング依存性を明らかにした。さらに、そのような非整合秩序の形成に伴う輸送現象、磁性、熱物性の特異な振舞を見出した。物性物理の研究分野の中でも、現在最も活発な研究が進んでいる強相関電子系の研究において、非整合な軌道/電荷秩序という新たな研究分野を作り上げることができた。さらに本研究で扱ったホランダイト型Ti酸化物においては、大きな熱起電力、高い電気伝導度、低い熱伝導度を持つことから、新たなn型熱電材料としても期待されている

  • 擬三角格子上の新奇な秩序状態と特異な外場応答の探索

    研究期間:

    2013年04月
    -
    2016年03月
     

     概要を見る

    擬三角格子上の新奇な秩序状態として、(1) Ba1-xSrxV13O18の電荷整列と三量体相の競合と有効質量の重い金属相(2) BaxTi8O16におけるTi1次元鎖上のd電子の変調構造を見出した。また擬三角格子系の特異な外場応答として(c) 擬三角格子系Ti,V酸化物フォノン熱伝導度が軌道/電荷整列転移温度前後で普遍的な振舞を示すこと(d) 擬三角格子系Ti酸化物のポーラロン伝導における光誘起効果が系の次元性に依存した振舞を示すことを見出した

  • 擬三角格子上の新奇な秩序状態と特異な外場応答の探索

    科学研究費助成事業(早稲田大学)  科学研究費助成事業(基盤研究(B))

    研究期間:

    2013年
    -
    2015年
     

     概要を見る

    単結晶合成:AV13O18系においてA=Ba1-xSrxの混晶系の純良単結晶の合成に成功した。特にx=0.1の試料について、電気抵抗・帯磁率ともに2つの転移温度(電荷整列と三量体相転移)で明確な異常を示すことを明らかにした。今後はこれらの試料を用いて、熱電測定、NMR測定を行う。また、Ba2Ti13O22のTiサイトにVをドーピング(Ti1-xVx)した単結晶試料を作製し、x=1程度で200K付近の電子相転移が消失することを見出した。
    新物質開発:hollandite AxB8O16はBイオンが三角格子から2列をとりだした梯子格子を形成する。BaxTi8O16の単結晶合成に成功し、作製時の還元条件によって室温でtetragonalとmonoclinicの試料が作り分けられること、またtetragonalの試料は200K付近に相転移があること、TEMの観察によりこの転移温度以下でincommensurateな超格子構造が現れることを明らかにした。
    特異な外場応答:AV10O15系に関してパルス電場を用いた非線形抵抗を測定した。その結果相転移のあるBaV10O15、相転移のないSrV10O15ともに非線形抵抗が現れること、温度や試料の種類によらず電場と電気抵抗の絶対値によってスケーリングすることを見出した。このことは、この非線形抵抗が試料内の自発的な不均一状態に起因したpercolationに由来することを示唆している。さらにhollandite BaxTi8O16の熱起電力、熱伝導度を測定し、200Kの相転移で異常を示すことを明らかにした。

  • ニッケル酸化物における電荷整列の研究

     概要を見る

    今年度はLa_<2-x>Sr_xNiO_4系のうちcommensurateな電荷整列を起こすx=1/3,1/2以外の組成の試料を作製し、その抵抗率、ホール係数、光学スペクトルの温度変化、ラマン散乱スペクトルなどを測定した。x=1/3から組成をずらすことによって電荷整列相転移にともなう抵抗率の異常が小さくなることがわかった。ホール係数の測定からはx=1/3前後の組成でキャリアの符号が反転し、x=1/3からのずれの分が余剰キャリアとして振る舞っていることを見いだした。x=1/3の組成で電荷整列転移温度以下で低エネルギー部分の光学伝導度が減少し、10Kにおいては0.26eVのギャップが開くことが我々の測定によって明らかにされていたが、x=1/3前後の組成、すなわちx=0.30,0.39の組成においても電荷整列転移温度以下でギャップ的な構造が成長することを見いだした。しかし最低温においてもそのギャップ構造はx=1/3に比してなまっており、この結果はホール係数の測定より得られた結論、すなわちx=1/3からのずれの分が余剰キャリアとして振る舞う、という描像と合致するものである。またラマン散乱スペクトルの測定から、電荷整列転移温度以下で電子ラマン散乱強度が急速に抑制され、2-マグノンによる散乱ピークが2本現れることを明らかにした。また、電荷整列にともない格子の対称性が低下フォノン散乱によるピークが活性化されるが、このフォノンピークは電荷整列転移温度以上から現れ始め、電荷整列転移の揺らぎをラマン散乱というダイナミカルなプローブによって観測した

  • 異常磁気伝導を示す量子物質の開発

     概要を見る

    電子の量子性が室温程度の高温領域で発現し,異常な物性を示す物質(異常量子物質)のうち,伝導電子が外部磁場や自分自身の磁化と相互作用する物質の開発を行った。対象とした物質群は,遷移金属酸化物を中心に窒化物,硫化物,有機物など多岐にわたった。特に,巨大非線形伝導を示す有機伝導体,希薄磁性半導体として振舞うチタン酸化物,強磁性と非磁性が縮退しているコバルト酸化物,磁気相に隣接して超伝導を示すアンチペロブスカイト窒化炭化物などを開発した

  • 酸化物におけるパルス電場誘起の不揮発かつ可逆的な物性変化

     概要を見る

    1.パルス電場印加によって電気抵抗が変化する物質の探索パルス電場印加によって電気抵抗が不揮発かつ可逆的に変化する物質がペロブスカイト型マンガン酸化物薄膜によって見出されている。様々な遷移金属酸化物(TiからCuまで)のエピタキシャルな薄膜を作製し、この現象がマンガン酸化物に特有のものかどうかを調べた。その結果、いずれの遷移金属酸化物においても、マンガン酸化物によく似た電気抵抗変化を起こすことを見出した。2.絶対光学反射率の空間分布を測定する装置の開発薄膜等の光学反射率の絶対値を、400nmから900nmまでの任意の波長で、1マイクロメートルの空間分解能でイメージングできる装置を開発した。これは、顕微分光用の顕微鏡にCCDカメラをとりつけ、絶対反射率が既知の参照試料と試料を正確に入れ替えることにより、1%程度の精度まで正確に試料の絶対反射率の空間依存性が測定できる装置である。3.顕微分光装置を用いたパルス電場誘起効果の研究2.で開発された顕微分光装置を用いて、電場印加によって電気抵抗変化が変化する薄膜の測定を行った。(LaSr)FeO3薄膜にITO電極をつけた試料に電場を印加したところ、不揮発に光学反射率の変化の空間分布が変化することが明らかになった。波長依存性の解析の結果、電極間に過剰酸素に由来する伝導度の高い部分が現れること、電場によって過剰酸素が集団的に動くことにより、伝導度の高い部分の位置も電極位置に対して動くこと、それが結果として電気抵抗の変化につながること、を明らかにした

  • 遷移金属酸化物の新奇な軌道状態とドメイン構造およびその外場制御

     概要を見る

    スピネル構造をとる様々な遷移金属酸化物において、新奇な軌道秩序を新たに見出した。また、その軌道秩序状態のドメインを磁場によって制御し、巨大な磁歪、磁気誘電性を得ることに成功した。さらに、強誘電性と強誘電性が共存したマルチフェロイク物質において第二高調波発生(SHG)を初めて観測した。さらに、SHG顕微鏡の開発を進めて、深さ方向も含めた3次元断層写真を得ることに成功した

  • Ti,V酸化物における軌道自由度付フラストレーション系の物質開発と物性

     概要を見る

    A2V13022におけるスピンシングレット三量体形成:A2V13022(A=Ba,Sr)は、Vが規則的に欠損した擬V三角格子が3層、fcc格子的に積層した構造をとる。Ti酸化物で同様の構造をとる物質は知られていたが、この物質は本研究で初めて合成されたものである。この物質の電気抵抗と帯磁率を測定した結果、Ba2V13022では290Kで、SrV13022では380Kで、それぞれ電気抵抗は上昇し、帯磁率は現象するという異常が観測されることが分かった。さらに、放射光x線粉末回折実験を行った結果、この温度以下でVが三量体を形成する構造相転移が起こることが分かった。この構造相転移において、13個のVが4つの三量体と1つの孤立Vイオンに分離する。さらに帯磁率の振る舞いとNMR測定の結果から、この三量体はスピンシングレット状態であり、孤立VイオンはS=1/2のスピンを持つことが明らかとなった。BaV10015の三量体構造相転移とSrV10015のスピンフラストレーション:AV10015(A=Ba,Sr)は、Vが規則的に欠損した擬V三角格子が2層積層した構造をとる。この物質の単結晶を初めて作製し、BaV10015は130Kの構造相転移で電気抵抗が3桁跳ぶこと、また光学伝導度スペクトルに0.2eV程度のギャップが開くこと、37K以下では、a軸方向にスピンの向く反強磁性を見出してきた。本年は、SrV10015の測定をした結果、電気抵抗は異常がなくvariable range hoppingの振る舞いを示すこと、光学スペクトルにはギャップは開かず、温度低下とともに連続的にウェイトが減少する振る舞いを示すこと、また低温には磁気転移に対応する比熱の異常は見られないが、NMRの1/T1に長距離秩序を示唆する異常が30K付近に見られることを明らかにした

  • 強相関電子系における多自由度の競合とマルチクリティカリティ

     概要を見る

    複数の秩序状態が競合して多重臨界点をつくる物質として、Ba^<1-x> Sr^xV^<13> O^<18>、La^5M^<o4> O^<16>を新たに見出し、その多重臨界点付近において、特徴的な物性を見いだした。さらに、MnV^2O^4の磁場下熱伝導度測定の結果から、転移点付近でスピン揺らぎと軌道揺らぎがともに臨界的に増大している振舞いを明らかにした。また、ぺロブスカイト型マンガン酸化物における電子相分離について、ローレンツ顕微鏡法を含めた電子線回折実験によって明らかにした

  • 軌道自由度付フラストレーション系における三量体の物理

     概要を見る

    A2V13022における三量体形成と不純物効果A2V13022(A=Ba,Sr)は、Vが規則的に欠損した擬V三角格子が3層、fcc格子的に積層した構造をとり、Baでは290K、Srでは380Kで三量体相転移を起こす。このV3+(3d^2)サイトに非磁性不純物Sc、磁性不純物Cr(3d^3)を導入した試料を作製した結果、不純物ドーピングによって、三量体相転移に伴う電気抵抗の増大、帯磁率の減少が消失することを昨年度に見出した。さらに、本年度は結晶構造の変化について調べ、歪測定における相転移での異常が不純物ドーピングによって消失すること、また電子線回折実験の結果から、三量体相転移によって起こるCmce=>Pbcaの構造相転移が不純物ドーピングによって消失することを見いだした。これらの結果から、不純物ドーピングによってスピンシングレット三量体が崩壊し、スピンが現れるという振舞いが明確になった。Ba2Ti13022単結晶の物性Ba2Ti13022は、上記のA2V13022と同じ結晶構造をとるが、V3+(3d^2)ではなくTi3+(3d'1)によって構成されている。この物質の単結晶をFZ法によって作製し、210Kで電気抵抗の増大と帯磁率の減少が起こることを昨年度見出した。今年度はこの210Kの相転移についてより詳しく実験を行い、電子線回折実験の結果からCmce=>C2/mの構造相転移が起こっていること、またこれはある3層と隣の3層が非等価になる相転移に対応していることを見いだした。さらに歪測定とx線粉末回折実験の結果から、この相転移でa軸が縮み、b軸が延びることを見いだした。また光学測定の結果、0.1eV以下でギャップが開く様子を明らかにした。以上のことから、この物質の210Kでの相転移は電荷密度波に近いことが明らかになった

▼全件表示

特定課題研究

  • 新奇トポロジカル相転移の探索

    2018年  

     概要を見る

    Topological phase transition, for example,Kosterlitz-Thouless transition, has long been a subject of theoretical physics,but experimental systems exhibiting such an exotic phase transition are quiterare. To find out materials that possibly exhibit a topological phasetransition, we performed the magnetic-force microscopy (MFM) measurement for alayered magnet, La5Mo4O16. It is known that this compound exhibits anantiferromagnetic ordering below 200 K, and there are additional phasetransitions accompanied by an anomaly in the lattice constants at 90 K and 70K. We found in the MFM that ferromagnetic domains with the size of ca. 100 nmappear below 70 K, but they disappear with the applied magnetic field of 0.3T. Together with the large positive magnetoresistance observed below 70 K, itis suggested that the boundaries of the magnetic domains are electricallyconducting, possibly due to the topologically protected edge state. Thisexperimental result may suggest that the anomaly at 70 K is caused by some kindof a topological phase transition.

  • 4d遷移金属酸化物における新奇磁気緩和現象とその応用

    2015年  

     概要を見る

    &nbsp;&nbsp;&nbsp; La5Mo4O16 has tetragonal lattices of Mo ions with a checkerboard arrangement of Mo4+ and Mo5+, and exhibits ferrimagnetic ordering of Mo4+ and Mo5+ moments within the tetragonal plane and the antiferromagnetic arrangement of the ferrimagnetic moment along the c axis (perpendicular to the plane) below 200 K. We grew the single crystals of this compound by the floating-zone technique and measured the magnetization and resistivity with applied magnetic field. We found new anomalies at 90 K and 70 K in the temperature dependence of both magnetization and resistivity, in addition to the ferrimagnetic ordering at 200 K.&nbsp;We also found a multiple step-like behavior in the field-dependence of magnetization (M(H) curve) and an anomalous relaxation behavior of the magnetization below 70 K. We also found positive magnetoresistance and its relaxation behavior, which is closely correlated with the magnetic relaxation.

  • 電子ネマティック状態における光誘起異方性の探索

    2013年  

     概要を見る

     V3+ (3d2)が軌道整列を起こすスピネル型 V 酸化物(AV2O4, A=Mn,Fe,Co etc.)に関して、A サイトを様々に変化させた試料、種々のドーピングを施した試料について単結晶を作製し、ポンププローブ分光法において、ポンプ光とプローブ光の偏光方向を変えて光誘起スペクトル変化を測定することにより、「電子ネマティック相」の探索をおこなった。 AlドープしたMnV2O4の光誘起相転移のポンプ光偏光依存性 57Kでフェリ磁性相転移と軌道整列に由来する構造相転移を起こすMnV2O4のVサイトにAlをドープすると、フェリ磁性相はそのままであるが、構造相転移が消失して再低温まで立方晶となる。この物質に関して、プローブ光の偏光を変えずに、ポンプ光の偏光方向のみを変えて、光誘起スペクトル変化に違いが生じるかどうかを調べたが、現在のところ有意な差は得られていない。CoV2O4の光誘起相転移のポンプ光偏光依存性 180K以下でフェリ磁性転移を起こすが、あらわな構造相転移を起こさないCoV2O4について、プローブ光の偏光を変えずに、ポンプ光の偏光方向のみを変えて、光誘起スペクトル変化に違いが生じるかどうかを調べたが、現在のところ有意な差は得られていない。CoV2O4に関する基礎研究 CoV2O4において、磁気相転移が存在するにもかかわらず、軌道整列に由来する構造相転移が観測されない理由について、単結晶と多結晶を用いて詳しい測定を行った。その結果、単結晶試料ではCoが一部Vを置換しており、その結果、ストイキオメトリックな多結晶に存在する60Kの異常が、単結晶では消失していることが分かった。さらに磁場下での歪測定により、フェリ磁性転移温度直下では磁化方向に結晶が伸びるが、さらに温度を下げると磁化方向に結晶が縮む、という変化を示すことが分かった。この結果は2つのタイプの軌道整列の競合として解釈できる。こうした結果は、CoV2O4の特定の温度で電子ネマティック相が実現している可能性を示唆するものである。

  • 軌道秩序状態におけるドメイン壁の構造とダイナミクス

    2012年  

     概要を見る

    BaV10O15のoffstoichiometryによる軌道整列相転移の変化BaV10O15は~120KでV三量体を形成する構造相転移を起こし、電気抵抗が3桁程度上昇すること、この三量体相転移がVのt2g軌道の整列に由来することが知られている。また120~200Kの中間温度相では局所的にV三量体が形成され、それが帯磁率や電気抵抗の温度依存性にも異常として現れる。我々は、この物質の作成条件を様々に変化させてoffstoichiometryを導入すると、三量体相転移温度が次第に減少していき、ついには相転移が消失することを見いだした。さらに、三量体相転移温度が減少すると、中間温度相での異常も消失していくことが分かった。また、offstoichiometryを導入して転移温度が消失した結晶の光学測定を行い、相転移のある試料の結果と比較した結果、低温でのギャップ構造がoffstoichiometryな試料では明確には観測されないこと、また(転移温度より上の)中間温度相の光学スペクトルの様子も、相転移のあるなしで異なる振舞いを示すことを明らかにした。BaV10O15の中間温度相における非線形伝導強電場下で電流が電場に比例しなくなる非線形伝導は様々な物質で報告されているが、「正しい」測定は容易ではない。これは強電場、大電流下では試料自体が発熱するため、温度変化に由来する電気抵抗の変化と区別がつかなくなるためである。我々は、パルス電場下での試料の発熱の効果を詳しく調べた結果、パルス幅の間に温度が上がる効果と、パルス印加を繰り返すことによって温度が上がる効果の両方を考慮する必要があることを見いだした。この知見をもとに、BaV10O15の非線形伝導を、パルス幅100μsの短いパルスを用いて、0.1%の小さなduty cycle(=パルス幅/パルス間隔)で測定することにより、試料自体の温度上昇を防ぎ、本質的な非線形伝導を測定することに成功した。その結果、120~200Kの中間温度相で電場印加とともに抵抗率が減少する非線形伝導を見いだした。

  • 三量体形成に基づく新規強誘電体の開発

    2010年  

     概要を見る

     イオンの三量体形成にともなって、三量体に垂直方向に(常に同方向に)他のイオンを「押し出す」ことによって、その方向へ自発分極が発生する物質群が知られている。こうした物質を研究し、さらに同様なメカニズムによって強誘電体となる新物質を開発することを目指した。三量体「強誘電体」SrV6O11の単結晶作製の試み 上記のメカニズムによりc軸方向に分極が発生する(しかし金属のため自発分極は発生しない)ことが知られているSrV6O11の単結晶作製を試みている。分解溶融のため単純なフローティングゾーン法では単結晶はできない。また適当なフラックスもないために、traveling solvent floating zone法もうまくいかない。しかし、フローティングゾーン法を繰り返すうちに、包晶のような形で結晶が成長する可能性を見いだした。現在のところ大きな結晶は出来ていないが、粉末x線で00lピークのみが強く出るなど、平板状の微結晶がたくさんできていることが示唆されている。今後条件をさらに詰めて、大きな単結晶を作製することを目指す。スピネル型V酸化物におけるBサイト1:3秩序型の物質開発 スピネル型V酸化物AV2O4において、V3+のうちの1/4を他の元素Bで置換した物質、AV1.5B0.5O4を作製し、そのVとBが1:3で秩序した系の作製を試みた。こうした物質においてVはカゴメ格子を形成し、このときV三量体が形成すると、カゴメ格子と垂直方向に自発分極が発生することが期待される。A=Li+, B=Nb5+など、様々な組み合わせで合成を試みているが、現在のところ単相試料の作製には成功していない。今後さらに条件を変えて、単相試料の作製を目指す。

  • 光学反射率を用いた動的比熱測定法の開発と固体中の二相共存状態への応用

    2007年  

     概要を見る

     物質中において2つの相が競合する場合、臨界点付近で2相共存状態が現れることがある。この2相共存状態のダイナミクスを特徴付ける物理量として、相境界が移動する緩和時間τが挙げられる。このτを測定するには、物質の温度を周期的に変調し、それに対するバルク量の変調を測定すればよい。すなわち、温度変調の周波数を大きくすると、相境界の移動が追随できずバルク量の変調が小さくなるため、バルク量の周波数依存性からτを見積もることができる。 従来、このような目的のために開発された実験手法として、3ω法と呼ばれる複素比熱測定法があった。しかしこの方法では、試料表面に直接金属を蒸着したものをヒーター兼温度計として用いるため、導電性の高い試料を測定できないという欠点がある。本研究において、導電性を示す物質の2相共存状態におけるダイナミクスを測定できる実験手法として、温度変調光学反射率測定系を開発した。時間変調したレーザー光により試料に周期的な温度変調を与え、それに対する光学反射率の変調を検出することにより、2相共存のダイナミクスを測定できる。 本研究では、Srをドープしたペロブスカイト型マンガン酸化物R0.6Sr0.4MnO3の2相共存状態のダイナミクスを研究対象とした。本物質系は、低温で短距離電荷整列相と強磁性金属相が競合しており、相転移温度近傍においてこの2相の共存が期待される。R=Nd (TC=270K), R=Nd0.5Sm0.5 (TC=210K), R=Sm (TC=128K) の温度変調反射率測定の結果、すべての試料において、転移温度近傍で温度変調反射率にピークが現れた。また、最も転移温度の低いR=Smにおいて、最も顕著に周波数分散が現れた。このことから、低温で相転移を起こす物質ほど、相転移近傍の2相共存の相境界の緩和時間τは長い、すなわち相境界は動きにくくなることがわかった。 

  • マルチフェロイクの物理とその応用

    2006年  

     概要を見る

    (1) 強誘電体-強磁性体ナノコンポジット薄膜の作製と物性強誘電体と強磁性体からなるナノコンポジット薄膜を作製し、その物性を調べた。BaTiO3 65%とCoFe2O4 35%からなるコンポジット薄膜をレーザーアブレーションで作製した結果、CoFe2O4が直径数nmの柱として、ほぼ三角格子状にBaTiO3の中に整列することが明らかになった。また、強誘電転移と自発磁化も観測され、強誘電体、強磁性体としての性質を兼ね備えることもわかった。さらに低温で0.6%程度の負のmagnetocapacitance(誘電率の磁場依存性)も観測された。以上の実験より、マルチフェロイクとしてナノコンポジットを作製することの有効性が確認された。(2) 誘電率がスイッチングを起こすスピネル型鉄バナジウム酸化物の発見スピネル型FeV2O4は150K以下で逐次構造相転移をおこし、105Kにフェリ磁性転移温度を持つ物質である。この物質の多結晶試料の誘電率を磁場下で測定し、磁場に対して複雑はヒステリシスを持つこと、ゼロ磁場の値が直前にかけた磁場方向に応じて2値をとること、すなわち磁場によって誘電率の値がスイッチングを起こすことを見出した。磁化の磁場依存性の測定結果から、磁化曲線が二重ヒステリシスループを描くことがわかり、このことがゼロ磁場における誘電率のスイッチングにつながっていることを見出した。これらの結果は新しいタイプのマルチフェロイクであり、今後も集中的に研究していきたいと考える。(3) 絶対光学反射率の空間分布を測定する装置の開発薄膜等の光学反射率の絶対値を、400nmから900nmまでの任意の波長で、1$mu;mの空間分解能でイメージングできる装置を開発した。これによって、電子構造の空間依存性が精密に議論できるようになった。これまでに、電場誘起で電気抵抗が変化する物質の測定を行い、電気抵抗変化と同期する光学反射率の変化の空間イメージングの測定に成功している。今後は偏光依存性測定も含めて、マルチフェロイクにこの測定手法を応用していく。

  • ペブロスカイト構造を持つ希薄磁性半導体の開発

    2004年  

     概要を見る

    SrTiO3は、立方晶ペロブスカイト構造をとり、Tiの3d軌道とOの2p軌道からなる伝導体を持った半導体である。2価のSrを3価のLaで置換することにより、Tiに電子がドープされて伝導体になることが知られている。このSrTiO3に、電子に加えて、スピンをドープすることにより、新しい希薄磁性半導体を作製することが可能である。本研究では、CrでTiを置換することによってスピンドーピングを行い、磁気抵抗を中心とした磁性と電気伝導の結合の様子を実験的に明らかにした。その結果,スピン濃度を固定して、電子濃度を増加させることによって、ワイス温度が負から正に変化することを見出した。これは、スピン間にキャリアーを通じた強磁性相互作用が働いていることを意味している。また、電気抵抗は低温に向かって増大し、これに磁場をかけると電気抵抗が大きく減少する(負の磁気抵抗)が明らかになった。これは、スピンの揺らぎによってキャリアーが散乱されることにより電気抵抗が増大し、それに磁場をかけるとスピンの向きが揃うことによってキャリアーの散乱が抑制され、電気抵抗が減少すると考えられる。電子の濃度とスピンの濃度を独立に制御することによって磁気抵抗の大きさを最大化することが可能であり、スピン濃度20%、電子濃度10%の組成で、最大70%の負の磁気抵抗(電気抵抗が磁場によって1/3以下に減少する)を見出した。

  • 巨大磁気誘電率物質の開発

    2002年  

     概要を見る

     六方晶RMnO3(R=Y, Lu etc.)は、誘電率が磁性と強く結合しており、反強磁性転移温度で誘電率が大きく変化することが知られている。しかし、反強磁性相互作用が強いために磁化率が小さく、その結果試料に磁場をかけても誘電率はほとんど変化しない。本研究では、このRMnO3に様々なタイプのドーピングを施し、誘電率の磁場依存性(磁気誘電率)を増大させることを試みた。 その結果、Rサイト(3価)をZr(4価)で置換することによって、磁化率が低温で増大し、それに伴って磁気誘電率 (magnetocapacitance)も増大することがわかった。例えば、Y0.8Zr0.2MnO3においては、誘電率は7 Tの磁場下で0.4%変化する。ところが、この試料を還元すると磁化率もmagnetocapacitanceも減少する。すなわち、還元によってドーピングの効果が打ち消されることがわかった。もし、3価のRを4価のZrへの置換がMnサイトへの電子ドーピング(Mn3+→Mn2+)と考えるならば、還元によってその効果はさらに増大するはずである。したがってこの実験結果は、Zr置換は単純なMnサイトへの電子ドーピング効果ではないことを意味する。 さらに、Mnサイト(3価)をTi(4価)で置換した場合でも、同様に磁化率が低温で増大し、magnetocapacitanceも増大することがわかった。RサイトのZr置換とMnサイトへのTi置換が同様の効果を示すことから、これらの効果は「フラストレートしたスピン系への乱れの導入による磁気モーメントの発生」と考えられることを示唆している。

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