Spin-orbit fluctuations in frustrated heavy-fermion metal LiVO

Tomiyasu, Keisuke*; Iwasa, Kazuaki*; Ueda, Hiroaki*; Niitaka, Seiji*; Takagi, Hidenori*; Kawamura, Seiko; Kikuchi, Tatsuya; Inamura, Yasuhiro; Nakajima, Kenji; Yamada, Kazuyoshi*

Physical Review Letters, 113(23), p.236402_1 - 236402_5, 2014/12

https://doi.org/10.1103/PhysRevLett.113.236402

Spin fluctuations were studied over a wide momentum () and energy () space in the frustrated -electron heavy-fermion metal LiVO by time-of-flight inelastic neutron scattering. We observed the overall evolutions near the characteristic = 0.6 peak and found another weak broad magnetic peak around 2.4 . The data are described by a simple response function, a partially delocalized magnetic form factor, and antiferromagnetic short-range spatial correlations, indicating that heavy-fermion formation is attributable to spin-orbit fluctuations with orbital hybridization.

Anomalous metallic state in the vicinity of metal to valence-bond solid insulator transition in LiVS

Katayama, Naoyuki*; Uchida, Masaya*; Hashizume, Daisuke*; Niitaka, Seiji*; Matsuno, Jobu*; Matsumura, Daiju; Nishihata, Yasuo; Mizuki, Junichiro; Takeshita, Nao*; Gauzzi, A.*; et al.

Physical Review Letters, 103(14), p.146405_1 - 146405_4, 2009/10

https://doi.org/10.1103/PhysRevLett.103.146405

We investigate LiVS and LiVSe with a triangular lattice as itinerant analogues of LiVO known for the formation of a valence-bond solid (VBS) state out of an S = 1 frustrated magnet. LiVS, which is located at the border between a metal and a correlated insulator, shows a first order transition from a paramagnetic metal to a VBS insulator at T 305 K upon cooling. The presence of a VBS state in the close vicinity of insulator-metal transition may suggest the importance of itinerancy in the formation of a VBS state. We argue that the high temperature metallic phase of LiVS has a pseudogap, likely originating from the VBS fluctuation. LiVSe was found to be a paramagnetic metal down to 2 K.

Evaluation of containment failure probability due to ex-vessel steam explosions in LWR severe accidents

Moriyama, Kiyofumi; Takagi, Seiji*; Muramatsu, Ken; Nakamura, Hideo; Maruyama, Yu

JAEA-Research 2007-072, 54 Pages, 2007/11

JAEA-Research-2007-072.pdf:25.89MB

The containment failure probability due to ex-vessel steam explosions was evaluated for BWR and PWR model plants. A stratified Monte Carlo technique (Latin Hypercube Sampling (LHS)) was applied for the evaluation of steam explosion loads, in which a steam explosion simulation code JASMINE was used as a physics model. The evaluation was made for three scenarios: a steam explosion in the pedestal area or in the suppression pool of a BWR model plant with Mark-II containment, and in the reactor cavity of a PWR model plant. The scenario connecting the generation of steam explosion loads and the containment failure was assumed to be displacement of the reactor vessel and pipings, and failure at the penetration in the containment boundary. We evaluated the conditional containment failure probability (CCFP) based on the preconditions of failure of molten core retention within the reactor vessel, relocation of the core melt into the water pool without significant interference, and a strong triggering at the time of maximum premixed mass. The obtained mean and median values of the CCPF were 6.4E-2 (mean) and 3.9E-2 (median) for the BWR suppression pool case, 2.2E-3 (mean) and 2.8E-10 (median) for the BWR pedestal case, and 6.8E-2 (mean) and 1.4E-2 (median) for the PWR cavity case. The evaluation of CCFPs on the basis of core damage needs consideration of probabilities for the above-mentioned preconditions. Thus, the CCFPs per core damage should be lower than the values given above. The specific values of the probability were most dependent on the assumed range of melt flow rate and fragility curve that involved conservatism and uncertainty due to simplified scenarios and limited information. Additionally, the source term significance of the fine particles generated by steam explosions was discussed.

Evaluation of ex-vessel steam explosion induced containment failure probability for Japanese BWR

Moriyama, Kiyofumi; Takagi, Seiji; Muramatsu, Ken; Nakamura, Hideo; Maruyama, Yu

Proceedings of 2005 International Congress on Advances in Nuclear Power Plants (ICAPP '05) (CD-ROM), 9 Pages, 2005/05

The containment failure probability due to ex-vessel steam explosions were evaluated for a BWR Mk-II model plant. The evaluation was made for two scenarios: a steam explosion in the pedestal area, or in the suppression pool. A probabilistic approach, Latin Hypercube Sampling (LHS), was applied for the evaluation of steam explosion loads, in which a steam explosion simulation code JASMINE was used as a physics model. The fragility curves connecting the steam explosion loads and containment failure were developed based on simplified assumptions on the containment failure scenarios. The mean conditional probabilities of containment failure per occurrence of a steam explosion were for suppression pool and for pedestal area. Note that the results depend on the assumed range of input parameters and fragility curves that involve conservatism and simplification.

Source term analysis for severe accident conditions of a nuclear power plant

Ishikawa, Jun; Shintani, Kiyonori; Takagi, Seiji; Muramatsu, Ken

Nihon Kikai Gakkai Dai-8-Kai Doryoku, Enerugi Gijutsu Shimpojiumu Koen Rombunshu, p.203 - 208, 2002/00

no abstracts in English

Giant TMR effect and spin states of C-Co thin films

Sakai, Seiji; Matsumoto, Yoshihiro; Sugai, Isamu*; Mitani, Seiji*; Takanashi, Koki; Takagi, Yasumasa*; Nakagawa, Takeshi*; Yokoyama, Toshihiko*; Shimada, Toshihiro*; Maeda, Yoshihito

no journal, , 

no abstracts in English

Soft X-ray circular dichroism study of electronic and magnetic states in C-Co mixture film

Matsumoto, Yoshihiro; Sakai, Seiji; Naramoto, Hiroshi*; Nakagawa, Takeshi*; Takagi, Yasumasa*; Yokoyama, Toshihiko*; Takanashi, Koki; Mitani, Seiji*; Shimada, Toshihiro*; Maeda, Yoshihito

no journal, , 

no abstracts in English

X-ray magnetic circular dichroism of codeposited C60-Co films with giant tunnel magnetoresistance

Matsumoto, Yoshihiro; Sakai, Seiji; Naramoto, Hiroshi*; Nakagawa, Takeshi*; Takagi, Yasumasa*; Yokoyama, Toshihiko*; Hirao, Norie; Baba, Yuji; Shimada, Toshihiro*; Maeda, Yoshihito

no journal, , 

Recently, we have found that the codeposited C60-Co films where Co nanoparticles are dispersed into the matrix of the C60-Co compound show large tunnel magnetoresistanene (TMR) effects at low temperatures. The observed MR ratios cannot be explained by a spin-polarization of crystalline Co, so it is expected that the C60-Co compound mediates an enhancing effects to the spin-dependent transport in the films. In the present work, we have investigated the spin states of the C60-Co films by X-ray magnetic circular dichroism spectroscopy. As the results, MCD spectra of the C60-Co compound show the sensitivity to the sample temperature, which is consistent to the temperature dependence of MR ratios. This indicates clearly that spin-polarized states of the C60-Co compound influences the degree of spin-dependent transport in the films.

Electronic and spin states of C-Co thin films studied by XPS and X-ray magnetic circular dichroism

Matsumoto, Yoshihiro; Sakai, Seiji; Takagi, Yasumasa*; Nakagawa, Takeshi*; Yokoyama, Toshihiko*; Mitani, Seiji*; Takanashi, Koki; Shimada, Toshihiro*; Naramoto, Hiroshi*; Maeda, Yoshihito

no journal, , 

no abstracts in English

Giant TMR effect and spin states in fullerene-cobalt hybirid films

Sakai, Seiji; Matsumoto, Yoshihiro; Mitani, Seiji*; Takanashi, Koki; Takagi, Yasumasa*; Nakagawa, Takeshi*; Yokoyama, Toshihiko*; Shimada, Toshihiro*; Naramoto, Hiroshi*; Maeda, Yoshihito

no journal, , 

Anomalously large tunnel magnetoresistance (TMR) effects were found for the granular structured C60-Co films consisting of a C60-Co compound matrix and Co nanoparticles dispersed therein. The observed MR ratios of 90% at the maximum are significantly higher than the MR ratios of the inorganic granular films. Our recent X-ray magnetic circular dichroism study revealed that the C60-Co compound in the C60-Co films shows the paramagnetic behaviors due to the spin-polarized states (Co d-spin) derived from the Co 3d orbitals. We found that the T-dependence of the zero-bias MR ratio is represented by considering the similar T-dependence of the spin-polarization(P) of tunneling electrons to the observed T-dependence of the magnetization of the Co d-spin in the C60-Co compound, and by assuming P=100% at low temperature. This result suggests the incorporation of the spin-polarized Co 3d-derived states of the C60-Co compound into the spin-transport process in the granular C60-Co films.