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Journal Articles

Design and actual performance of J-PARC 3 GeV rapid cycling synchrotron for high-intensity operation

Yamamoto, Kazami; Kinsho, Michikazu; Hayashi, Naoki; Saha, P. K.; Tamura, Fumihiko; Yamamoto, Masanobu; Tani, Norio; Takayanagi, Tomohiro; Kamiya, Junichiro; Shobuda, Yoshihiro; et al.

Journal of Nuclear Science and Technology, 59(9), p.1174 - 1205, 2022/09

 Times Cited Count:3 Percentile:85.55(Nuclear Science & Technology)

In the Japan Proton Accelerator Research Complex, the purpose of the 3 GeV rapid cycling synchrotron (RCS) is to accelerate a 1 MW, high-intensity proton beam. To achieve beam operation at a repetition rate of 25 Hz at high intensities, the RCS was elaborately designed. After starting the RCS operation, we carefully verified the validity of its design and made certain improvements to establish a reliable operation at higher power as possible. Consequently, we demonstrated beam operation at a high power, namely, 1 MW. We then summarized the design, actual performance, and improvements of the RCS to achieve a 1 MW beam.

Journal Articles

The Role of silicon on solute clustering and embrittlement in highly neutron-irradiated pressurized water reactor surveillance test specimens

Takamizawa, Hisashi; Hata, Kuniki; Nishiyama, Yutaka; Toyama, Takeshi*; Nagai, Yasuyoshi*

Journal of Nuclear Materials, 556, p.153203_1 - 153203_10, 2021/12

 Times Cited Count:0 Percentile:0.01(Materials Science, Multidisciplinary)

Solute clusters (SCs) formed in pressurized water reactor surveillance test specimens neutron-irradiated to a fluence of 1 $$times$$ 10$$^{20}$$ n/cm$$^{2}$$ were analyzed via atom probe tomography to understand the effect of silicon on solute clustering and irradiation embrittlement of reactor pressure vessel steels. In high-Cu bearing materials, Cu atoms were aggregated at the center of cluster surrounded by the Ni, Mn, and Si atoms like a core-shell structure. In low-Cu bearing materials, Ni, Mn, and Si atoms formed cluster and these solutes were not comprised core-shell structure in SCs. While the number of Cu atoms in clusters was decreased with decreasing nominal Cu content, the number of Si atoms had clearly increased. The cluster radius ($$r$$) and number density ($$N_{d}$$) decreased and increased, respectively, with increasing nominal Si content. The shift in the reference temperature for nil-ductility transition ($$Delta$$RT$$_{NDT}$$) showed a good correlation with the square root of volume fraction ($$V_{f}$$) multiplied by r ($$sqrt{V_{f}times {r}}$$). This suggested that the dislocation cutting through the particles mechanism dominates the precipitation hardening responsible for irradiation embrittlement. The negative relation between the nominal Si content and $$Delta$$RT$$_{NDT}$$ indicated that increasing of nominal Si content reduces the degree of embrittlement.

Journal Articles

Mean-field study of the Bose-Hubbard model in the Penrose lattice

Ghadimi, R.*; Sugimoto, Takanori*; Toyama, Takami

Physical Review B, 102(22), p.224201_1 - 224201_9, 2020/12

Journal Articles

Atomistic modeling of hardening in spinodally-decomposed Fe-Cr binary alloys

Suzudo, Tomoaki; Takamizawa, Hisashi; Nishiyama, Yutaka; Caro, A.*; Toyama, Takeshi*; Nagai, Yasuyoshi*

Journal of Nuclear Materials, 540, p.152306_1 - 152306_10, 2020/11

 Times Cited Count:8 Percentile:74.98(Materials Science, Multidisciplinary)

Spinodal decomposition in thermally aged Fe-Cr alloys leads to significant hardening, which is the direct cause of the so-called 475C-embrittlement. To illustrate how spinodal decomposition induces hardening by atomistic interactions, we conducted a series of numerical simulations as well as reference experiments. The numerical results indicated that the hardness scales linearly with the short-range order (SRO) parameter, while the experimental result reproduced this relationship within statistical error. Both seemingly suggest that neighboring Cr-Cr atomic pairs essentially cause hardening, because SRO is by definition uniquely dependent on the appearance probability of such pairs. A further numerical investigation supported this notion, as it suggests that the dominant cause of hardening is the pinning effect of dislocations passing over such Cr-Cr pairs.

Journal Articles

Gapless spin liquid in a square-kagome lattice antiferromagnet

Fujihara, Masayoshi*; Morita, Katsuhiro*; Mole, R.*; Mitsuda, Setsuo*; Toyama, Takami*; Yano, Shinichiro*; Yu, D.*; Sota, Shigetoshi*; Kuwai, Tomohiko*; Koda, Akihiro*; et al.

Nature Communications (Internet), 11, p.3429_1 - 3429_7, 2020/07

 Times Cited Count:25 Percentile:88.08(Multidisciplinary Sciences)

Journal Articles

Dynamical density matrix renormalization group study of spin and charge excitations in the four-leg $$t$$-$$t'$$-$$J$$

Toyama, Takami; Mori, Michiyasu; Sota, Shigetoshi*

Physical Review B, 97(23), p.235137_1 - 235137_7, 2018/06


 Times Cited Count:9 Percentile:44.06(Materials Science, Multidisciplinary)

Journal Articles

Magnetic phase diagram of a frustrated spin ladder

Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami; Maekawa, Sadamichi

Physical Review B, 97(14), p.144424_1 - 144424_10, 2018/04


 Times Cited Count:5 Percentile:27.01(Materials Science, Multidisciplinary)

Journal Articles

Observation of momentum-dependent charge excitations in hole-doped cuprates using resonant inelastic X-ray scattering at the oxygen $$K$$ edge

Ishii, Kenji*; Toyama, Takami*; Asano, Shun*; Sato, Kentaro*; Fujita, Masaki*; Wakimoto, Shuichi; Tsutsui, Kenji*; Sota, Shigetoshi*; Miyawaki, Jun*; Niwa, Hideharu*; et al.

Physical Review B, 96(11), p.115148_1 - 115148_8, 2017/09


 Times Cited Count:28 Percentile:78.89(Materials Science, Multidisciplinary)

Journal Articles

Predoping effects of boron and phosphorous on arsenic diffusion along grain boundaries in polycrystalline silicon investigated by atom probe tomography

Takamizawa, Hisashi; Shimizu, Yasuo*; Inoue, Koji*; Nozawa, Yasuko*; Toyama, Takeshi*; Yano, Fumiko*; Inoue, Masao*; Nishida, Akio*; Nagai, Yasuyoshi*

Applied Physics Express, 9(10), p.106601_1 - 106601_4, 2016/10

 Times Cited Count:0 Percentile:0(Physics, Applied)

Journal Articles

Quasi-spin correlations in a frustrated quantum spin ladder

Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi

Physics Procedia, 75, p.861 - 867, 2015/12

 Times Cited Count:0 Percentile:0.04

Journal Articles

Orbital-selective electronic excitations in iron arsenides revealed by simulated nonresonant inelastic X-ray scattering

Tsutsui, Kenji; Kaneshita, Eiji*; Toyama, Takami*

Physical Review B, 92(19), p.195103_1 - 195103_5, 2015/11

 Times Cited Count:2 Percentile:10.11(Materials Science, Multidisciplinary)

Nonresonant inelastic X-ray scattering (NIXS) is a possible tool to detect charge excitations in electron systems. In addition, multipole transitions at high-momentum-transfer region open a new possibility to determine orbital-selective electronic excitations in multi-orbital itinerant 3$$d$$ electron systems. As a theoretical example, we chose the antiferromagnetic state of iron arsenides and demonstrate that the orbital-selective excitations are detectable by choosing appropriate momentum transfer in NIXS. We propose that both NIXS and resonant inelastic X-ray scattering are complementary to each other for fully understanding the nature of orbital excitations in multi-orbital itinerant electron systems.

Journal Articles

Lifshitz transition induced by magnetic field in frustrated two-leg spin-ladder systems

Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi

JPS Conference Proceedings (Internet), 8, p.034005_1 - 034005_6, 2015/09

Journal Articles

Magnetization plateaus by reconstructed quasispinons in a frustrated two-leg spin ladder under a magnetic field

Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi

Physical Review B, 92(12), p.125114_1 - 125114_6, 2015/09


 Times Cited Count:9 Percentile:39.68(Materials Science, Multidisciplinary)

Journal Articles

Enhanced charge excitations in electron-doped cuprates by resonant inelastic X-ray scattering

Toyama, Takami*; Tsutsui, Kenji; Mori, Michiyasu; Sota, Shigetoshi*; Yunoki, Seiji*

Physical Review B, 92(1), p.014515_1 - 014515_8, 2015/07

 Times Cited Count:13 Percentile:50.85(Materials Science, Multidisciplinary)

Resonant inelastic X-ray scattering (RIXS) tuned for Cu $$L$$ edge is a possible tool to detect charge excitations in cuprate superconductors. We theoretically investigate the possibility for observing a collective charge excitation by the RIXS. The RIXS process via the intermediate state inevitably makes the spectral weight of charge excitation stronger in electron doping than in hole doping. Electron-hole asymmetry also appears in dynamical charge structure factor, showing a new enhanced small-momentum low-energy mode in electron doping. These facts indicate a possibility of detecting the new charge mode by RIXS in electron-doped systems.

Journal Articles

Theoretical study of electronic states and excitation spectra in Ni-substituted cuprates

Tsutsui, Kenji; Toyama, Takami*; Maekawa, Sadamichi

JPS Conference Proceedings (Internet), 3, p.017008_1 - 017008_6, 2014/06

Impurity substitution for copper in high-Tc cuprate superconductors has attracted much attention. For Ni substitution, it has been argued from several experiments that hole carriers are strongly localized at around Ni sites. We have demonstrated theoretically that a hole can be bound in the NiO$$_{4}$$ plaquette forming the Zhang-Rice doublet. In this study, we examine the interaction between the Zhang-Rice doublet and hole carriers. We carry out the numerically exact diagonalization on small clusters of the d-p model with a Ni impurity site and calculate the resonant inelastic X-ray scattering spectra for Ni K-edge.

Journal Articles

Spin-spin correlation enhanced by impurities in a frustrated two-leg spin ladder

Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi

JPS Conference Proceedings (Internet), 3, p.014016_1 - 014016_5, 2014/06

Journal Articles

High-energy spin and charge excitations in electron-doped copper oxide superconductors

Ishii, Kenji; Fujita, Masaki*; Sasaki, Takanori*; Minola, M.*; Dellea, G.*; Mazzoli, C.*; Kummer, K.*; Ghiringhelli, G.*; Braicovich, L.*; Toyama, Takami*; et al.

Nature Communications (Internet), 5, p.3714_1 - 3714_8, 2014/04

 Times Cited Count:84 Percentile:94.41(Multidisciplinary Sciences)

Journal Articles

Optical conductivity of antiferromagnetic metallic chromium; Mean-field calculation for the multi-orbital Hubbard model

Sugimoto, Kodai*; Toyama, Takami*; Kaneshita, Eiji*; Tsutsui, Kenji

Journal of the Korean Physical Society, 63(3), p.632 - 635, 2013/08

 Times Cited Count:0 Percentile:0(Physics, Multidisciplinary)

Cr is the representative material of multi-orbital antiferromagnetism. We numerically calculate the band structure and density of states in commensurate antiferromagnetic Cr by multi-orbital Hubbard model with mean-field approximation. The result is adopted to the calculation of optical conductivity. The spectra of the optical conductivity is consistent with experimental data, and this promises the validity of our model.

Journal Articles

Spin dynamics and resonant inelastic X-ray scattering in chromium with commensurate spin-density wave order

Sugimoto, Kodai*; Li, Z.*; Kaneshita, Eiji*; Tsutsui, Kenji; Toyama, Takami*

Physical Review B, 87(13), p.134418_1 - 134418_8, 2013/04

 Times Cited Count:7 Percentile:33.86(Materials Science, Multidisciplinary)

We theoretically investigate spin dynamics and L-edge resonant inelastic X-ray scattering (RIXS) of chromiumwith commensurate spin-density wave (SDW) order, based on a multiband Hubbard model. Obtaining the ground state with the SDW mean-field approximation, we calculate the dynamical spin susceptibility by using random-phase approximation. We find that a collectivespin-wave excitation hardly damps up to 0.6 eV. Above the energy, the excitation overlaps individual particle-hole excitations, leading to broad spectral weight.

Journal Articles

Effects of frustration on magnetic excitations in a two-leg spin-ladder system

Sugimoto, Takanori; Mori, Michiyasu; Toyama, Takami; Maekawa, Sadamichi

Physical Review B, 87(15), p.155143_1 - 155143_7, 2013/04

 Times Cited Count:7 Percentile:33.86(Materials Science, Multidisciplinary)

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95 (Records 1-20 displayed on this page)