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Journal Articles

Inelastic neutron scattering of hydrogen in palladium studied by semiclassical dynamics

Shiga, Motoyuki; Thomsen, B.; Kimizuka, Hajime*

Physical Review B, 109(5), p.054303_1 - 054303_12, 2024/02

Inelastic neutron scattering spectra of hydrogen in palladium were calculated considering nuclear quantum effects at finite temperatures. A computational method combining semiclassical molecular dynamics based on path integrals and machine learning potentials was used. The calculated spectra agree well with the experimental spectra with respect to the positions and intensities of the peaks corresponding to the fundamental and first harmonic of the vibrational excitation of hydrogen atoms. Comparison with classical molecular dynamics shows that nuclear quantum effects play an essential role in the inelastic neutron scattering spectra.

Journal Articles

Accurate description of hydrogen diffusivity in bcc metals using machine-learning moment tensor potentials and path-integral methods

Kwon, H.*; Shiga, Motoyuki; Kimizuka, Hajime*; Oda, Takuji*

Acta Materialia, 247, p.118739_1 - 118739_11, 2023/04

We estimate the diffusivity of dilute hydrogen in body-centered-cubic metals, Nb, Fe, and W, from path integral simulations using machine-learning moment tensor potentials with an accuracy level of density functional theory. Our computational results show great agreement with some experimental results that appear to be accurate. The isotope effects are also reproduced consistently with the experimental data.

Journal Articles

Artificial neural network-based path integral simulations of hydrogen isotope diffusion in palladium

Kimizuka, Hajime*; Thomsen, B.; Shiga, Motoyuki

Journal of Physics; Energy (Internet), 4(3), p.034004_1 - 034004_13, 2022/07

 Times Cited Count:3 Percentile:73.76(Energy & Fuels)

Artificial neural network-based interatomic potential for a system of palladium and hydrogen was developed, and path integral molecular dynamics simulations were performed to study the quantum diffusion of hydrogen isotopes in palladium crystals. Diffusion coefficients of light and heavy hydrogen were calculated over a wide temperature range of 50-1500 K to clarify the difference in diffusion mechanisms at low and high temperatures.

Journal Articles

Two distinct non-Arrhenius behaviors of hydrogen diffusivities in fcc aluminum, silver, and copper determined by ${it ab initio}$ path integral simulations

Kimizuka, Hajime*; Shiga, Motoyuki

Physical Review Materials (Internet), 5(6), p.065406_1 - 065406_9, 2021/06

 Times Cited Count:7 Percentile:62.98(Materials Science, Multidisciplinary)

Nuclear quantum effects are a non-negligible factor in the dynamic behavior of hydrogen in metals. In this study, we investigated the hydrogen diffusion in the face-centered cubic metals Al, Ag, and Cu using a first-principles integral molecular dynamics simulation that takes into account the nuclear quantum effects. It was found that the temperature dependence of hydrogen diffusion in Ag and Cu is inverted S-shaped, while the temperature dependence of hydrogen diffusion in Al is C-shaped. This difference is due to the fact that the most stable position of hydrogen is the octahedral site in Ag and Cu, while it is the tetrahedral site in Al. Therefore, it is found that the nuclear quantum effects of hydrogen diffusion (zero-point oscillation and tunneling) differ qualitatively depending on metals with different stable sites.

Journal Articles

Diffusion mechanisms of hydrogen in metals explored via path-integral methods

Kimizuka, Hajime*; Ogata, Shigenobu*; Shiga, Motoyuki

Nihon Butsuri Gakkai-Shi, 75(8), p.484 - 490, 2020/08

Hydrogen is an element having quantum nature, and exhibits a unique diffusion behavior not found in other impurity atoms. In this study, the diffusion behavior of hydrogen isotopes in palladium crystals was predicted at a wide range of temperatures using a first-principles approach that combines the path integral simulation and electronic structure calculation. It was shown that in the high temperature region, the influence of quantum fluctuations becomes apparent as the temperature decreases, causing the suppression of diffusion where the Arrhenius plot bends upward. On the other hand, in the low temperature region, the diffusion barrier decreases due to the manifestation of the quantum tunneling effect, and the Arrhenius plot bents downward. The competition of quantum effects in different temperature regions clearly explains the anomalous isotopic effect of hydrogen diffusion.

Journal Articles

Unraveling anomalous isotope effect on hydrogen diffusivities in fcc metals from first principles including nuclear quantum effects

Kimizuka, Hajime*; Ogata, Shigenobu*; Shiga, Motoyuki

Physical Review B, 100(2), p.024104_1 - 024104_9, 2019/07

 Times Cited Count:13 Percentile:64.56(Materials Science, Multidisciplinary)

The behavior of H isotopes in crystals is a fundamental and recurrent theme in materials physics. Especially, the information on H diffusion over a wide temperature range provides a critical insight into the quantum mechanical nature of the subject; however, this is not yet fully explored. From state-of-the-art ab initio calculations to treat both electrons and nuclei quantum mechanically, we found that the temperature dependence of H isotope diffusivities in face-centered-cubic (fcc) Pd has an unconventional "reversed S" shape on Arrhenius plots. Such irregular behavior is ascribed to the competition between different nuclear quantum effects with different temperature and mass dependencies, which leads to a peculiar situation, where the heavier tritium ($$^3$$H) diffuses faster than the lighter protium ($$^1$$H) in the limited temperature range of 80 - 400 K. This unveils the mechanism of anomalous crossovers between the normal and reversed isotope effects observed in the experiments at high and low temperatures.

Journal Articles

Mechanism of fast lattice diffusion of hydrogen in palladium; Interplay of quantum fluctuations and lattice strain

Kimizuka, Hajime*; Ogata, Shigenobu*; Shiga, Motoyuki

Physical Review B, 97(1), p.014102_1 - 014102_11, 2018/01

AA2017-0587.pdf:0.9MB

 Times Cited Count:24 Percentile:76.45(Materials Science, Multidisciplinary)

To understand the mechanism of high diffusivity of hydrogen in palladium, we have studied the process of fast diffusion of interstitial H in face-centered cubic Pd, based on a first principles simulation taking account of quantum mechanical nature of both electrons and nuclei under finite strains. The simulated results revealed that the activation barrier for hydrogen migration was drastically increased with decreasing temperature owing to nuclear quantum effects on the octahedral sites. However, under the lattice expansion, nuclear quantum effects became less important since the tetrahedral site is stabilized. This implies that the diffusion mechanism gradually changes from quantum-like to classical-like as the strain is increased.

Journal Articles

Quantum-thermal crossover of hydrogen and tritium diffusion in $$alpha$$-iron

Yoshikawa, Takehiro*; Takayanagi, Toshiyuki*; Kimizuka, Hajime*; Shiga, Motoyuki

Journal of Physical Chemistry C, 116(43), p.23113 - 23119, 2012/11

 Times Cited Count:15 Percentile:45.72(Chemistry, Physical)

The diffusion coefficients of hydrogen (H) and tritium (T) in $$alpha$$-Fe have been computed using two approximate quantum dynamical techniques, i.e. centroid molecular dynamics (CMD) and ring polymer molecular dynamics (RPMD), in the temperature range of T = 100-1000 K using the embedded-atom-method (EAM) potential. It has been found that the RPMD and CMD methods give very similar results. From a further analysis based on quantum transition state theory (centroid density QTST) combined with path integral molecular dynamics (PIMD), it has been clear that there is a crossover between thermal and quantum mechanisms at about T = 500 K and 300 K for H and T diffusions, respectively. The importance of nuclear quantum effects at low temperatures has been illustrated in terms of the effective free energy surface map.

Journal Articles

Dynamical thermal conductivity of argon crystal

Kaburaki, Hideo; Li, J.*; Yip, S.*; Kimizuka, Hajime*

Journal of Applied Physics, 102(4), p.043514_1 - 043514_6, 2007/08

 Times Cited Count:46 Percentile:82.03(Physics, Applied)

The thermal conductivity of a rare-gas crystal (Ar) is computed using equilibrium molecular dynamics in conjunction with the Green-Kubo linear response formalism, and the Lennard-Jones potential with an appropriately long cutoff. Besides predicting absolute values of the conductivity from low temperature up to the liquid, the approach allows heat conduction to be understood as a dynamical process through the temporal behavior of the heat current correlation function. At low temperatures the correlation function shows a characteristic two-stage decay, a short-time relaxation which we attribute to single-particle motions in a local environment, and a more extended component corresponding to collective atomic motions (phonons). As temperature increases the second correlation component diminishes much faster than the first component, indicating a transition from mainly phase-coherent phonon transport to mainly phase-incoherent interatomic energy transfer in solids.

Journal Articles

First-principles calculation on screw dislocation core properties in BCC molybdenum

Shimizu, Futoshi; Ogata, Shigenobu*; Kimizuka, Hajime*; Kano, Takuma; Li, J.*; Kaburaki, Hideo

Journal of the Earth Simulator, 7, p.17 - 21, 2007/06

Journal Articles

Molecular dynamics study on the formation of stacking fault tetrahedra and unfaulting of Frank loops in FCC metals

Kadoyoshi, Tomoko; Kaburaki, Hideo; Shimizu, Futoshi; Kimizuka, Hajime*; Jitsukawa, Shiro; Li, J.*

Acta Materialia, 55(9), p.3073 - 3080, 2007/05

 Times Cited Count:57 Percentile:90.91(Materials Science, Multidisciplinary)

Critical conditions have been determined for intrinsic transformation of a vacancy Frank loop into a stacking fault tetrahedron in a face centered cubic metal by the molecular dynamics method. We found that a stacking fault tetrahedron can be formed from the scalene hexagonal vacancy Frank loops of wide range of sizes due to the dissociation of dislocations. We have also found atomistically the dynamical process in which vacancy and interstitial faulted Frank loops transform into perfect loops by the application of the external shear stress or by raising the temperature. We have determined numerically the critical shear stress and temperature for the initiation of unfaulting. The simulation results clearly unveiled the important role of temperature in the unfaulting mechanism of an interstitial Frank loop.

Journal Articles

Development of parallel molecular-dynamics simulation stencil

Shimizu, Futoshi; Kimizuka, Hajime*; Kaburaki, Hideo; Arakawa, Chuichi*

Keisan Kogaku Koenkai Rombunshu, 7(1), p.163 - 166, 2002/05

no abstracts in English

Journal Articles

Development of parallel molecular dynamics stencil

Shimizu, Futoshi; Kimizuka, Hajime*; Kaburaki, Hideo; Arakawa, Chuichi*

Nihon Keisan Kogakkai Rombunshu, 4, p.225 - 230, 2002/04

no abstracts in English

Journal Articles

High speed calculation for solid molecular dynamics on vector processors

Itakura, Kenichi; Yokokawa, Mitsuo; Shimizu, Futoshi; Kimizuka, Hajime*; Kaburaki, Hideo

Joho Shori Gakkai Kenkyu Hokoku 2001-HPC-88, p.67 - 72, 2001/10

he Earth Simulator which is under development has 640 processor nodes and its peak performance is 40 Tflop/s. In this study, we have evaluated performance of solid molecular dynamics simulation on an SMP node of the Earth Simulator. In molecular dynamics simulation, each particle is influenced by all particles within a cut-off region and the representation of these pairs of particles is made by a matrix. Two matrix representations, compressed row form and jagged diagonal form, are considered for vectorization. The jagged diagonal form is better than the compressed row form in performance on a vector processor for the force calculation of every pairs, because the vector length of the former is longer than that of the latter. However, computational cost for converting the normal matrix form to the jagged diagonal form is quite expensive and the total performance in using the jagged diagonal form is low. Speedup by parallelization with the compressed row form is 2.4 to 2.7 with 8 vector processors.

Journal Articles

PARCEL; Performance evaluation by structural analysis calculation

Yamada, Susumu; Shimizu, Futoshi; Imai, Ryuta*; Kimizuka, Hajime*; Kaji, Yoshiyuki; Kaburaki, Hideo

Keisan Kogaku Koenkai Rombunshu, 6(1), p.233 - 236, 2001/05

no abstracts in English

Journal Articles

Parrallel molecular dynamics simulation on elastic properties of solid argon

Shimizu, Futoshi; Kimizuka, Hajime*; Kaburaki, Hideo; Li, J.*; Yip, S.*

Proceedings of 4th International Conference on Supercomputing in Nuclear Applications (SNA 2000) (CD-ROM), 10 Pages, 2000/09

no abstracts in English

Journal Articles

Mechanism for negative poisson ratios over the $$alpha$$-$$beta$$ transition of cristobalite, SiO$$_{2}$$; A Molecular-dynamics study

Kimizuka, Hajime*; Kaburaki, Hideo; Kogure, Yoshiaki*

Physical Review Letters, 84(24), p.5548 - 5551, 2000/06

 Times Cited Count:134 Percentile:94.43(Physics, Multidisciplinary)

no abstracts in English

Journal Articles

Parallel molecular-dynamics simulation stencil; A Study on separation of simulation and parallel algorithms

Shimizu, Futoshi; Kimizuka, Hajime*; Kaburaki, Hideo

Keisan Kogaku Koenkai Rombunshu, p.361 - 364, 2000/05

no abstracts in English

Journal Articles

RARCEL; Prallelization of application programs for science and engineering

Yamada, Susumu; Shimizu, Futoshi; Kobayashi, Kenichi*; Kimizuka, Hajime*; Kishida, Norio*; Kaburaki, Hideo

Keisan Kogaku Koenkai Rombunshu, p.369 - 372, 2000/05

no abstracts in English

Journal Articles

Molecular dynamics analysis of negative poisson ratios over the $$alpha$$-$$beta$$ transition in cristobalite, SiO$$_{2}$$

Kimizuka, Hajime*; Kaburaki, Hideo; Shimizu, Futoshi; Kogure, Yoshiaki*

Progress of Theoretical Physics Supplement, (138), p.229 - 233, 2000/04

no abstracts in English

24 (Records 1-20 displayed on this page)