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Takamizawa, Hisashi; Hata, Kuniki; Nishiyama, Yutaka; Toyama, Takeshi*; Nagai, Yasuyoshi*
Journal of Nuclear Materials, 556, p.153203_1 - 153203_10, 2021/12
Solute clusters (SCs) formed in pressurized water reactor surveillance test specimens neutron-irradiated to a fluence of 1 
10
n/cm
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 (
) and number density (
) decreased and increased, respectively, with increasing nominal Si content. The shift in the reference temperature for nil-ductility transition (
RT
) showed a good correlation with the square root of volume fraction (
) multiplied by 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 RT indicated that increasing of nominal Si content reduces the degree of embrittlement.
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：1 Percentile：39.17(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.
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(1), p.3429_1 - 3429_7, 2020/07
Times Cited Count：8 Percentile：81.73(Multidisciplinary Sciences)
-
-
Toyama, Takami; Mori, Michiyasu; Sota, Shigetoshi*
Physical Review B, 97(23), p.235137_1 - 235137_7, 2018/06
Times Cited Count：7 Percentile：50.91(Materials Science, Multidisciplinary)Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami; Maekawa, Sadamichi
Physical Review B, 97(14), p.144424_1 - 144424_10, 2018/04
Times Cited Count：2 Percentile：16.65(Materials Science, Multidisciplinary)
edgeIshii, 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：20 Percentile：75.96(Materials Science, Multidisciplinary)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)Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi
Physics Procedia, 75, p.861 - 867, 2015/12
Times Cited Count：0 Percentile：0.04Tsutsui, Kenji; Kaneshita, Eiji*; Toyama, Takami*
Physical Review B, 92(19), p.195103_1 - 195103_5, 2015/11
Times Cited Count：2 Percentile：12.06(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
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.
Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi
JPS Conference Proceedings (Internet), 8, p.034005_1 - 034005_6, 2015/09
Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi
Physical Review B, 92(12), p.125114_1 - 125114_6, 2015/09
Times Cited Count：7 Percentile：37.86(Materials Science, Multidisciplinary)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：12 Percentile：54.76(Materials Science, Multidisciplinary)Resonant inelastic X-ray scattering (RIXS) tuned for Cu 
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.
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
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.
Sugimoto, Takanori*; Mori, Michiyasu; Toyama, Takami*; Maekawa, Sadamichi
JPS Conference Proceedings (Internet), 3, p.014016_1 - 014016_5, 2014/06
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：75 Percentile：94.66(Multidisciplinary Sciences)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.
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：5 Percentile：28.48(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.
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：37.27(Materials Science, Multidisciplinary)no abstracts in English
Ishii, Kenji; Toyama, Takami*; Mizuki, Junichiro
Journal of the Physical Society of Japan, 82(2), p.021015_1 - 021015_24, 2013/02
Times Cited Count：21 Percentile：75.95(Physics, Multidisciplinary)Tsutsui, Kenji; Toyama, Takami*; Koshibae, Wataru*; Maekawa, Sadamichi
Journal of Physics; Conference Series, 400(3), p.032105_1 - 032105_4, 2012/12
Times Cited Count：0 Percentile：0.03The two-dimensional nickelate La
Sr
NiO
has received special attention as a reference system of high-
cuprates. The undoped nickelate (
=0) has the charge-transfer gap in the optical conductivity. Upon doping of holes, a broad spectrum appears in the gap. We have shown theoretically that the broad spectrum comes from excitations to the low spin states. In the Ni -edge resonant inelastic X-ray scattering (RIXS) measurement, the momentum-resolved charge excitations are obtained. In this study, we examine theoretically the RIXS spectra on nickelates by using numerically exact diagonalization techniques on the two-band Hubbard model. We also calculate other spectra such as the dynamical charge density function and discuss what excitations appear in the RIXS spectrum.
