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Journal Articles

Self-learning hybrid Monte Carlo; A First-principles approach

Nagai, Yuki; Okumura, Masahiko; Kobayashi, Keita*; Shiga, Motoyuki

Physical Review B, 102(4), p.041124_1 - 041124_6, 2020/07

https://doi.org/10.1103/PhysRevB.102.041124

Times Cited Count：13 Percentile：66.36(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Self-learning Monte Carlo method with Behler-Parrinello neural networks

Nagai, Yuki; Okumura, Masahiko; Tanaka, Akinori*

Physical Review B, 101(11), p.115111_1 - 115111_12, 2020/03

https://doi.org/10.1103/PhysRevB.101.115111

Times Cited Count：17 Percentile：74.78(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

Dry cleaning process test for fuel assembly of fast reactor plant system, 1; Pilot scale test for fuel pin bundle

Kudo, Hideyuki*; Otani, Yuichi*; Hara, Masahide*; Kato, Atsushi; Ishikawa, Nobuyuki; Otaka, Masahiko; Nagai, Keiichi; Saito, Junichi; Ara, Kuniaki; Ide, Akihiro*

Proceedings of 2019 International Congress on Advances in Nuclear Power Plants (ICAPP 2019) (Internet), 10 Pages, 2019/05

A next generation SFR in Japan has adopted an advanced dry cleaning system which consists of the argon gas blowing process to reduce the amount of metallic residual sodium remaining on spent fuel assemblies. This paper describes experimental and analytical work focusing on the amount of residual sodium remaining on a fuel pin bundle before and after the argon gas blowing process. The experiments were conducted using a sodium test loop and a short specimen consisting of a 7 pin bundle. The effects of the blowing gas velocity and the blowing time were quantitatively analyzed in the experiments. On the basis of these experimental results, evaluation models predicting the amount of the residual sodium were constructed.

Journal Articles

Dry cleaning process test for fuel assembly of fast reactor plant system, 2; Laboratory scale test for fuel assembly and evaluation of the amount of residual sodium

Ide, Akihiro*; Kudo, Hideyuki*; Inuzuka, Taisuke*; Hara, Masahide*; Kato, Atsushi; Ishikawa, Nobuyuki; Otaka, Masahiko; Nagai, Keiichi; Saito, Junichi; Ara, Kuniaki

Proceedings of 2019 International Congress on Advances in Nuclear Power Plants (ICAPP 2019) (Internet), 10 Pages, 2019/05

A next generation SFR in Japan has adopted an advanced dry cleaning system which consists of the following process of argon gas blowing to reduce the amount of metallic sodium, moist argon gas blowing to deactivate the residual sodium, and direct storage in the SFP without using storage containers. This three-step process increases economic competitiveness and reduces waste products. In this Research and Development work, the amount of residual sodium and performance of the dry cleaning process were investigated. This paper describes experimental and analytical work for all parts of a fuel assembly except for a fuel pin bundle.

Journal Articles

Dry cleaning process test for fuel assembly of fast reactor plant system

Kato, Atsushi; Nagai, Keiichi; Ara, Kuniaki; Otaka, Masahiko; Oka, Nobuki*; Tanaka, Masako*; Otani, Yuichi*; Ide, Akihiro*

Proceedings of 2017 International Congress on Advances in Nuclear Power Plants (ICAPP 2017) (CD-ROM), 8 Pages, 2017/04

In a fuel handling system (FHS) of a sodium-cooled fast reactor, it is necessary to reduce residual sodium on a spent fuel subassembly before storing at a spent fuel water-pool (SFP) in order to minimize design loads. Although the wet cleaning process adopted on MONJU could eliminate almost all of residual sodium, a large amount of radioactive liquid waste occurs and it needs long duration of cleaning treatment and large plant commodities. On the other hand, Japan sodium-cooled fast reactor adopted an advanced dry cleaning system which consists of roughly blowing massive sodium on the fuel subassembly out by 300 $^{circ}$ C argon gas, inactivation of residual sodium to NaOH by moist argon gas and directly immersion into the SFP to achieve economic competitiveness and waste reduction. This paper reports current status of recent R&D activities to demonstrate a performance of the dry cleaning process in Japan which are for improvement of the cleaning performance and optimizing the FHS design.

Journal Articles

Theory of low-energy behaviors in topological s-wave pairing superconductors

Ota, Yukihiro; Nagai, Yuki; Machida, Masahiko

Physica C, 518, p.5 - 9, 2015/11

https://doi.org/10.1016/j.physc.2015.08.002

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

Topological superconductors are notable materials, owing to the mathematical interest as topological matter and the application potential of quantum engineering. To clarify their characters relying on gap-function types, we build up a low-energy effective theory, focusing on a model of superconductor Cu $_{x}$ Bi $_{x}$ Se $_{3}$ . The non-magnetic impurity effects in our previous numerical study indicates that the system has both p-wave and s-wave aspects. A perturbation study is insightful for explaining this peculiar behavior. Since the normal electrons are dominated by the Dirac-type dispersion relation, we assign a low-energy case as a large-Dirac-mass one. When the system has an odd-parity fully-gapped pair potential, our approach indicates that the effective gap function in the low-energy domain has not only a chiral p-wave-like component, but also an s-wave-like one. This peculiar mixture of p-wave and s-wave leads to intriguing responses in the present model.

Journal Articles

Inverse coherence effects in nuclear magnetic relaxation rates as a sign of topological superconductivity

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Physical Review B, 92(18), p.180502_1 - 180502_5, 2015/11

https://doi.org/10.1103/PhysRevB.92.180502

Times Cited Count：3 Percentile：14.69(Materials Science, Multidisciplinary)

no abstracts in English

Journal Articles

First-principles study of antimony doping effects on the iron-based superconductor CaFe(Sb $_{x}$ As $_{1-x}$ ) $_{2}$

Nagai, Yuki; Nakamura, Hiroki; Machida, Masahiko; Kuroki, Kazuhiko*

Journal of the Physical Society of Japan, 84(9), p.093702_1 - 093702_4, 2015/09

https://doi.org/10.7566/JPSJ.84.093702

Times Cited Count：6 Percentile：45.25(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Surface states around a vortex in topological superconductors; Intersection of a surface and a vortex

Nagai, Yuki; Nakamura, Hiroki; Machida, Masahiko

Journal of the Physical Society of Japan, 84(3), p.033703_1 - 033703_4, 2015/03

https://doi.org/10.7566/JPSJ.84.033703

Times Cited Count：1 Percentile：11.26(Physics, Multidisciplinary)

We numerically show that zero-energy Majorana surface states are suppressed around a vortex in three-dimensional topological superconductors such as Cu $_{x}$ Bi $_{2}$ Se $_{3}$ and Sn $_{1-x}$ In $_{x}$ Te. On the other hand, zero-energy Majorana bound states along the vortex line are robust against a cut on the surface. The suppression of the surface-bound states is similar to that with a magnetic impurity on the surface of a topological insulator. The suppression of the surface-bound states around a vortex can be observed as an unconventional energy dependence of imaging by scanning tunneling microscopy/spectroscopy.

Journal Articles

Topological -wave pairing superconductivity with spatial inhomogeneity; Midgap-state appearance and robustness of superconductivity

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of the Physical Society of Japan, 84(3), p.034711_1 - 034711_8, 2015/03

https://doi.org/10.7566/JPSJ.84.034711

Times Cited Count：6 Percentile：45.25(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Robustness against non-magnetic impurities in topological superconductors

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of Physics; Conference Series, 568(2), p.022030_1 - 022030_5, 2014/12

https://doi.org/10.1088/1742-6596/568/2/022030

Times Cited Count：0 Percentile：0.05(Physics, Applied)

We study the robustness against non-magnetic impurities in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Bogoliubov-de Gennes (BdG) Hamiltonian with -wave on-site pairing) of Cu $_{x}$ Bi $_{2}$ Se $_{3}$ with the parameter set determined by the first-principles calculation. With the use of the self-consistent -matrix approximation for impurity scattering, we discuss the impurity-concentration dependence of the zero-energy density of states. We show that a single material variable, measuring relativistic effects in the Dirac-BdG Hamiltonian, well characterizes the numerical results. In the nonrelativistic limit, the odd-parity fully-gapped topological superconductivity is fragile against non-magnetic impurities, since this superconductivity can be mapped onto the -wave superconductivity. On the other hand, in the ultrarelativistic limit, the superconductivity is robust against the non-magnetic impurities, since the effective model has the -wave superconductivity. We derive the effective Hamiltonian in the both limit.

Journal Articles

Impurity effects in a two-dimensional topological superconductor; A Link of $T_{rm c}$ -robustness with a topological number

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Journal of the Physical Society of Japan, 83(9), p.094722_1 - 094722_5, 2014/09

https://doi.org/10.7566/JPSJ.83.094722

Times Cited Count：9 Percentile：55.36(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Inhomogeneity effects in topological superconductors

Nagai, Yuki; Nakamura, Hiroki; Machida, Masahiko

JPS Conference Proceedings (Internet), 3, p.015013_1 - 015013_6, 2014/06

https://doi.org/10.7566/JPSCP.3.015013

no abstracts in English

Journal Articles

Spin-polarized majorana bound states inside a vortex core in topological superconductors

Nagai, Yuki; Nakamura, Hiroki; Machida, Masahiko

Journal of the Physical Society of Japan, 83(6), p.064703_1 - 064703_7, 2014/06

https://doi.org/10.7566/JPSJ.83.064703

Times Cited Count：24 Percentile：76.91(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Nonmagnetic impurity effects in a three-dimensional topological superconductor; From - to -wave behaviors

Nagai, Yuki; Ota, Yukihiro; Machida, Masahiko

Physical Review B, 89(21), p.214506_1 - 214506_6, 2014/06

https://doi.org/10.1103/PhysRevB.89.214506

Times Cited Count：23 Percentile：67.92(Materials Science, Multidisciplinary)

Unconventional features in superconductivity are revealed by responses to impurity scattering. We study nonmagnetic impurity effects in a three-dimensional topological superconductor, focusing on an effective model (massive Dirac Hamiltonian with s-wave on-site pairing) of copper-doped bismuth-selenium compounds. Using a self-consistent T-matrix approach for impurity scattering, we examine in-gap states in density of states. We find that the results are well characterized by a single material variable, which measures relativistic effects in the Dirac Hamiltonian. In the nonrelativistic regime, an odd-parity superconducting state is fragile against nonmagnetic impurities. We show that this behavior is caused by a p-wave character involved in the topological superconducting state. In contrast, we show that in the relativistic regime the superconductivity is robust against nonmagnetic impurities, owing to an s-wave character. To summarize, the system has two aspects, p- and s-wave features, depending on the weight of relativistic effects.

Journal Articles