Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.35848/1347-4065/acc6da

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acs.nanolett.3c00972

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Publishing type：Research paper (scientific journal)   Publisher：American Chemical Society (ACS)  

DOI： 10.1021/acsnano.3c01608

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Publishing type：Research paper (international conference proceedings)   Publisher：Optica Publishing Group  

Janus transition metal dichloride (TMDC) is a TMDC with two different chalcogenide elements on the upper and lower surfaces. This special structure is reported to break the out-of-plane symmetry of the monolayer TMDC and produce out-of-plane polarization. This is expected to lead to the development of new electronic properties, such as Rashba spin splitting and piezoelectricity. There have been several reports on the synthesis of Janus TMDCs, and in recent years, the use of chalcogen spices containing H₂ plasma for atomic substitution on the surface of TMDC has attracted attention since Janus TMDCs can be generated at room temperature [1,2]. However, the generation process of Janus TMDCs with this method has not been well under-stood. In addition, the physical properties in the intermediate state between TMDC and Janus TMDC have also not been clarified, although the control of the intermediate state is important for tuning the physical properties of Janus TMDCs. Thus, elucidating the intermediate partially sub-stituted Janus (PSJ) state between MoSe₂ and Janus MoSeS state is essential for understanding the synthesis dynamics and tuning the electrical properties of TMDCs.

DOI： 10.1364/jsapo.2023.20p_a602_5

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

Based on the concept of band topology in phonon dispersion, we designed a topological phononic crystal in a thin plate for developing an efficient elastic waveguide. Despite that various topological phononic structures have been actively proposed, a quantitative design strategy of the phononic band and its robustness assessment in an elastic regime are still missing, hampering the realization of topological acoustic devices. We adopted a snowflake-like structure for the crystal unit cell and determined the optimal structure that exhibited the topological phase transition of the planar phononic crystal by changing the unit cell structure. The bandgap width could be adjusted by varying the length of the snow-side branch, and a topological phase transition occurred in the unit cell structure with threefold rotational symmetry. Elastic waveguides based on edge modes appearing at interfaces between crystals with different band topologies were designed, and their transmission efficiencies were evaluated numerically and experimentally. The results demonstrate the robustness of the elastic wave propagation in thin plates. Moreover, we experimentally estimated the backscattering length, which measures the robustness of the topologically protected propagating states against structural inhomogeneities. The results quantitatively indicated that degradation of the immunization against the backscattering occurs predominantly at the corners in the waveguides, indicating that the edge mode observed is a relatively weak topological state.

DOI： 10.3390/sym14102133

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Authorship：Last author   Language：English   Publishing type：Research paper (international conference proceedings)  

DOI： 10.1380/ejssnt.2022-041

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Language：English   Publishing type：Research paper (scientific journal)  

Transition metal dichalcogenides (TMDCs), including MoS2 and WS2, are potential candidates for next-generation semiconducting materials owing to their atomically thin structure and strong optoelectrical responses, which allow for flexible optoelectronic applications. Monolayer TMDCs have been grown utilizing chemical vapor deposition (CVD) techniques. Enhancing the domain size with high crystallinity and forming heterostructures are important topics for practical applications. In this study, the size of monolayer WS2 increased via the vapor-liquid-solid growth-based CVD technique utilizing the confined space of the substrate-stacked microreactor. The use of spin-coated metal salts (Na2WO4 and Na2MoO4) and organosulfur vapor allowed us to precisely control the source supply and investigate the growth in a systematic manner. We obtained a relatively low activation energy for growth (1.02 eV), which is consistent with the surface diffusion-limited growth regime observed in the confined space. Through systematic photoluminescence (PL) analysis, we determined that a growth temperature of ∼820 °C is optimal for producing high-quality WS2 with a narrow PL peak width (∼35 meV). By controlling the source balance of W and S, we obtained large-sized fully monolayered WS2 (∼560 μm) and monolayer WS2 with bilayer spots (∼1100 μm). Combining two distinct sources of transition metals, WS2/MoS2 lateral heterostructures were successfully created. In electrical transport measurements, the monolayer WS2 grown under optimal conditions has a high on-current (∼70 μA/μm), on/off ratio (∼5 × 108), and a field-effect mobility of ∼7 cm2/(V s). The field-effect transistor displayed an intrinsic photoresponse with wavelength selectivity that originated from the photoexcited carriers.

DOI： 10.1021/acsnano.2c05076

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Publishing type：Research paper (scientific journal)  

Particle damping is an effective method for increasing structural damping and is utilized in many fields. Using laser powder bed fusion (LPBF) additive manufacturing (AM), a new integrated particle damper can be produced by deliberately leaving unfused powder inside the structure. This study focuses on experimentally and numerically investigating the damping mechanism and performance of additively manufactured particle dampers in the low-frequency range (< 100 Hz). A numerical simulation approach based on the discrete element method was developed to predict the damping performance of a particle damper. To reduce the computational cost, a multi-unit particle damper (MUPD) was introduced. A series of particle dampers of 316 L stainless steel with different numbers and sizes of unit cells were built using LPBF. The damping mechanism and performance of additively manufactured MUPDs (AM-MUPD) were studied, the results cross-verified via experiments and simulations, and the effects of unit cell size and number investigated.

DOI： 10.1016/j.powtec.2021.11.029

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.apmt.2021.101167

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/abe2e5

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Authorship：Corresponding author   Publishing type：Research paper (international conference proceedings)  

We demonstrate an automatic materials design method using continuous representation of molecule and its atomic arrangement via a neural network algorithm. This method is applied to optimizing and predicting the HOMO-LUMO gap within the molecules composed of carbon, oxygen, nitrogen, fluorine, and hydrogen. Adopting the Quantum Machine 9 (QM9) dataset as a training dataset for the molecules, we first established a continuous representation of molecules in a latent space, then predicted molecules that have target values of the HOMO-LUMO gap. In the gap maximization calculation, CF4 with the largest gap value in the QM9 dataset was automatically found despite there is no a priori data for the gap. In the case of a target gap value of 0.10 hartree, we found a new molecule whose gap value is closer to 0.10 hartree than any other molecules in the QM9 dataset.

DOI： 10.4028/www.scientific.net/MSF.1016.1492

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0021338

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Language：Japanese   Publishing type：Research paper (scientific journal)  

Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/5.0012331

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/ab7c0e

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IOP} Publishing  

DOI： 10.35848/1347-4065/ab7e3e

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.14723/tmrsj.45.67

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Publishing type：Research paper (scientific journal)   Publisher：AIP Publishing  

DOI： 10.1063/1.5127993

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41598-020-63591-7

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41467-019-12116-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACS Publications  

A two-dimensional nanocarbon, graphene, has attracted substantial interest due to its excellent properties. The reduction of graphene oxide (GO) has been investigated for the mass production of graphene used in practical applications. Different reduction processes produce different properties in graphene, affecting the performance of the final materials or devices. Therefore, an understanding of the mechanisms of GO reduction is important for controlling the properties of functional two-dimensional systems. Here, we determined the average structure of reduced GO prepared via heating and photoexcitation and clearly distinguished their reduction mechanisms using ultrafast time-resolved electron diffraction, time-resolved infrared vibrational spectroscopy, and time-dependent density functional theory calculations. The oxygen atoms of epoxy groups are selectively removed from the basal plane of GO by photoexcitation (photon mode), in stark contrast to the behavior observed for the thermal reduction of hydroxyl and epoxy groups (thermal mode). The difference originates from the selective excitation of epoxy bonds via an electronic transition due to their antibonding character. This work will enable the preparation of the optimum GO for the intended applications and expands the application scope of two-dimensional systems.

DOI： 10.1021/acsnano.9b03060

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Authorship：Corresponding author   Language：English   Publishing type：Research paper (international conference proceedings)  

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41598-019-38540-8

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)  

DOI： 10.1557/adv.2019.329

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Authorship：Corresponding author   Publishing type：Research paper (international conference proceedings)  

Using an optical visualization technique, we experimentally measure ultrasonic propagation in phononic crystals consisting of a two-dimensional square array of copper wires in the water. Two different phononic crystals are designed and fabricated to exhibit waveguiding property and negative refraction of an ultrasonic wave at 200 kHz. The Fresnel diffraction method is employed to directly visualize intensity profile of the wavefront propagating in the phononic crystals, proving waveguiding and negative refraction of the ultrasonic wave. Non-invasive time-resolved imaging of an ultrasonic wave in artificial acoustic structures is thereby demonstrated, paving the way toward an efficient device design for an advanced ultrasonic wave control.

DOI： 10.1109/ULTSYM.2018.8580129

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Authorship：Lead author,　Corresponding author   Publishing type：Research paper (international conference proceedings)  

We designed a new acoustic diode model, based on a one-dimensional structure consisting of piezoelectric ceramic films placed periodically in an epoxy plate. Specifically, the model is designed to exhibit non-reciprocal wave propagation in the audible and ultrasonic range. Our model includes no artificial forces to generate the asymmetry in the system but utilizes a mode-conversion mechanisms of elastic-wave propagation at interfaces. We demonstrate numerically that the model can achieve non-reciprocity at a particular frequency range for both S-mode and A-mode incidence of the Lamb wave. We further attempt to develop an additional model that can suppress unwanted backward propagation due to a large mismatch at the heterojunction interface.

DOI： 10.1109/ULTSYM.2018.8579922

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IEEE}  

DOI： 10.1109/ultsym.2018.8579923

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Authorship：Corresponding author   Publishing type：Research paper (scientific journal)   Publisher：{IEEE}  

DOI： 10.1109/ultsym.2018.8579977

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Institute of Physics Inc.  

Daytime radiative cooling potentially offers efficient passive cooling, but the performance is naturally limited by the environment, such as the ambient temperature and humidity. Here, we investigate the performance limit of daytime radiative cooling under warm and humid conditions in Okayama, Japan. A cooling device, consisting of alternating layers of SiO2 and poly(methyl methacrylate) on an Al mirror, is fabricated and characterized to demonstrate a high reflectance for sunlight and a selective thermal radiation in the mid-infrared region. In the temperature measurement under the sunlight irradiation, the device shows 3.4 °C cooler than a bare Al mirror, but 2.8 °C warmer than the ambient of 35 °C. The corresponding numerical analyses reveal that the atmospheric window in λ = 16 ∼ 25 μm is closed due to a high humidity, thereby limiting the net emission power of the device. Our study on the humidity influence on the cooling performance provides a general guide line of how one can achieve practical passive cooling in a warm humid environment.

DOI： 10.1063/1.5030156

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

We have developed a combined approach of metaheuristic optimization algorithms (MOA), such as the genetic algorithm, with an ab-initio materials simulation engine. Concurrent run of the ab-initio calculations with each different parameter set selected by the MOA searches the optimum condition within a given input-parameter space. Using this methodology, the optimum dopant and its position/structure at a graphene edge are found to be a multiple N-atoms doping at graphitic sites, which predicts to lead to better charging/discharging performance when it is used as an anode material of Li-ion battery.

DOI： 10.4028/www.scientific.net/MSF.941.2356

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE  

Engineered metallodielectric nanostructures offer a new platform for controlling thermal emission in a desired manner, thus promise potential applications in passive cooling devices. Here, we present optimization design of metallodielectric multilayer structures for high-efficiency daytime radiative cooling and experimentally characterize their cooling performance. The device structure consists of alternating layers of SiO2 and PMMA on an Ag mirror, which works as a selective thermal emitter at 8-13 μm with a high reflectance for sunlight. Automated design scheme based on simulated annealing method combined with photonic-thermal analysis is developed and applied to search the optimized number and thickness of the layers. The evaluation function in the optimization process is predefined such that a net emission power would be maximized at the ambient temperature of 27 °C (300 K) under the sunlight irradiation of AM1.5G. The numerical results prove efficient radiative cooling to 3.0 °C lower than the ambient temperature, corresponding to 6.6 °C below the bare Ag mirror temperature. Based on the optimized design, the device is fabricated on an Al mirror by using reactive evaporation and spin-coating process over an area of 25 × 25 mm2. The reflectance and absorption properties of the fabricated device are characterized to demonstrate the selective thermal radiation through an atmospheric window. The equilibrium temperature of the device is also investigated to demonstrate the cooling performance under the direct sunlight irradiation.

DOI： 10.1117/12.2273640

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

A technique of selective electroless plating onto PLA-ABS (Polylactic Acid-Acrylonitrile Butadiene Styrene) composite structures fabricated by three-dimensional (3D) printing is demonstrated to construct 3D microwave metamaterials. The reducing activity of the PLA surface is selectively enhanced by the chemical modification involving Sn2+ in a simple wet process, thereby forming a highly conductive Ag-plated membrane only onto the PLA surface. The fabricated metamaterial composed of Ag-plated PLA and non-plated ABS parts is characterized experimentally and numerically to demonstrate the important bi-anisotropic microwave responses arising from the 3D nature of metallodielectric structures. Our approach based on a simple wet chemical process allows for the creation of highly complex 3D metal-insulator structures, thus paving the way toward the sophisticated microwave applications of the 3D printing technology. Published by AIP Publishing.

DOI： 10.1063/1.4986203

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

We demonstrate automatic design of infrared (IR) metamaterials using a genetic algorithm (GA) and experimentally characterize their IR properties. To implement the automated design scheme of the metamaterial structures, we adopt a digital metamaterial consisting of 7 x 7 Au nano-pixels with an area of 200 nm x 200 nm, and their placements are coded as binary genes in the GA optimization process. The GA combined with three-dimensional (3D) finite element method (FEM) simulation is developed and applied to automatically construct a digital metamaterial to exhibit pronounced plasmonic resonances at the target IR frequencies. Based on the numerical results, the metamaterials are fabricated on a Si substrate over an area of 1 mm x 1 mm by using an EB lithography, Cr/Au (2/20 nm) depositions, and liftoff process. In the FT-IR measurement, pronounced plasmonic responses of each metamaterial are clearly observed near the targeted frequencies, although the synthesized pixel arrangements of the metamaterials are seemingly random. The corresponding numerical simulations reveal the important resonant behavior of each pixel and their hybridized systems. Our approach is fully computer-aided without artificial manipulation, thus paving the way toward the novel device design for next-generation plasmonic device applications.

DOI： 10.1117/12.2273607

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We numerically demonstrate a non-reciprocal acoustic waveguide by utilizing the indirect interband transition between two guided modes. The waveguide, consisting of a water core sandwiched between parallel steel plates, is designed to support symmetric and asymmetric guided modes with different frequencies and wavevectors. Dynamic mode conversion is then achieved by applying spatio-temporal density modulation in the core of the waveguide to induce both frequency and wavevector shifts of the incident guided wave. Numerical simulations prove that the phase matching condition is satisfied only for the forward propagation, not for the backward one, thus realizing the non-reciprocal acoustic waveguide. Our approach based on a linear dynamic system may achieve wide-band tunable operation with a low-energy consumption, paving the way toward the sophisticated acoustic diode applications. (C) 2017 The Japan Society of Applied Physics.

DOI： 10.7567/JJAP.56.07JB01

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

Hafnium disulfide (HfS2) is one of the transition metal dichalcogenides which is expected to have the high electron mobility and the finite bandgap. However, the fabrication process for HfS2 = based electron devices is not established, and it is required to bring out the superior transport properties of HfS2. In this report, we have investigated the effects of the atomic layer deposited HfO2 passivation on the current properties of HfS2 transistors. HfO2 passivation of the HfS2 surface enhanced the drain current and significantly reduced the hysteresis. Moreover, HfO2 passivation allows the use of a higher annealing temperature and further improvement of the drain current.

DOI： 10.1109/TNANO.2017.2661403

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Plasmonic metamaterials have overcome fundamental limitations in conventional optics by their capability to engineer material resonances and dispersions at will, holding great promise for sensing applications. Recent demonstrations of metamaterial sensors, however, have mainly relied on their resonant nature for strong optical interactions with molecules, but few examples fully exploit their functionality to manipulate the polarization of light. Here, we present cross-polarized surface-enhanced infrared absorption (SEIRA) by the Fano-resonant asymmetric metamaterial allowing for strong background suppression as well as significant field enhancement. The metamaterial is designed to exhibit the controlled Fano resonance with the cross-polarization conversion property at 1730 cm(-1), which spectrally overlaps with the C=O vibrational mode. In the cross-polarized SEIRA measurement, the C=O mode of poly(methyl methacrylate) molecules is clearly observed as a distinct dip within a Fano-resonant transmission peak of the metamaterial. The vibrational signal contrast is then improved based on the cross-polarized detection scheme where only the light interacting with the metamaterial-molecular coupled system is detected by totally eliminating the unwanted background light. Our metamaterial approach achieves the zeptomole sensitivity with a large signal-to-noise ratio in the far-field measurement, paving the way toward the realization of ultrasensitive IR inspection technologies.

DOI： 10.1038/s41598-017-03545-8

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

High-density and long-lived plasmonic hot-spots are an ideal system for high-sensitive surface-enhanced infrared absorption (SEIRA), but these conditions arc usually incompatible due to unwanted near-field coupling between the adjacent unit structures. Here, by fully controlling plasmonic interference in a metamaterial lattice, we experimentally demonstrate densely packed long-lived quadrupole plasmons for high-sensitive SEIRA. The metamaterial consists of a strongly coupled array of super-and sub-radiant plasmonic elements to exhibit an electromagnetic transparency mode at 1730 cm(-1), which spectrally overlaps with the C=O vibrational mode. In the SEIRA measurement, the C=O mode of poly(methyl methacrylate) molecules is clearly observed as a distinct dip within a transmission peak of the metamaterial. The corresponding numerical simulations reveal that constructive interference uniformly forms coherent quadrupole plasmons over the metamaterial lattice, leading to a stronger molecular signal from the system. Our metamaterial approach provides a robust way to construct ideal hot-spots over the sample, paving the way toward a reliable sensing platform of advanced infrared inspection technologies. Published by AIP Publishing.

DOI： 10.1063/1.5004703

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

Based on a heterostructured plate consisting of piezoelectric-ceramic/epoxy-resin composites with different periodicities, we design a novel acoustic diode for the symmetrical/asymmetrical (S/A) mode of Lamb wave at audible ranges. The acoustic diode is constructed with two parts, i.e., the mode conversion part and the mode selection part, and the mode conversion mechanism at the interface is applied to the mode hybridization from S to S+A and for the mode conversion from A to S. The phonon band structures for each part are calculated and optimized so that the mode selection is realized for a specific mode at the junction. Finite-element simulations prove that the proposed acoustic diode achieves efficient rectification at audio frequency ranges for both S and A mode incidences of the Lamb wave. (C) 2016 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.55.07KB02

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：SPIE-INT SOC OPTICAL ENGINEERING  

A high-sensitive polarized surface-enhanced infrared absorption (polarized SEIRA) is proposed and demonstrated by utilizing the resonant coupling of Fano-resonant mode of the asymmetric metamaterials and IR vibrational mode of a polymer nano-film. The asymmetric metamaterials consisting of an Au nano-rod pair with a coupling nano-antenna were fabricated and characterized to demonstrate the controlled Fano resonances at 1730 cm(-1), which spectrally overlapped with the C=O stretching vibrational mode. In the co-polarized SEIRA measurement, the C=O mode of the PMMA nano-film was clearly observed as a conventional anti-resonant peak within the Fano line-shape of the metamaterial. For the cross-polarized case, on the other hand, a distinctive dip appeared within a cross-polarized transmission peak of the metamaterial. Since the unwanted background is strongly suppressed in the cross-polarized detection scheme, the signal contrast was dramatically improved, allowing for the attomole detection of the C=O bond in the far-field measurement. Our metamaterial approach achieves the significant improvement of signal-to-background ratio in the far-field measurement, thus paving the way toward the high-sensitive analysis of functional group in direct IR spectroscopy.

DOI： 10.1117/12.2237559

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Using ab-initio theoretical methods, we demonstrate possible enhancement of photo-conversion efficiency of an organic solar cell via intentional doping in molecular graphene-fullerene heterojunction [the hexabenzocoronene (HBC)-triethylene glycol (TEG)-C-60 molecule]. Photoabsorption analysis indicates oxygen substitution into HBC leads to an extension of the spectra up to an infrared regime. A quantum-mechanical molecular dynamics simulation incorporating nonadiabatic electronic transitions reveals that a dissociated charge state (D+ and A(-)) in the O-doped system is more stable than the pristine case due to the presence of an effective barrier by the TEG HOMO/LUMO level. We also find that oxygen doping in HBC enhances the intermolecular carrier mobility after charge separation. On the other hand, the pristine molecule undergoes rapid recombination between donor and acceptor charges at the interface. These analyses suggest that the graphene oxidation opens a new window in the application of organic super-molecules to solar cells. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

DOI： 10.1063/1.4939848

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

HfS2 is the novel transition metal dichalcogenide, which has not been experimentally investigated as the material for electron devices. As per the theoretical calculations, HfS2 has the potential for well-balanced mobility (1,800 cm(2)/V.s) and bandgap (1.2 eV) and hence it can be a good candidate for realizing low-power devices. In this paper, the fundamental properties of few-layer HfS2 flakes were experimentally evaluated. Micromechanical exfoliation using scotch tape extracted atomically thin HfS2 flakes with varying colour contrasts associated with the number of layers and resonant Raman peaks. We demonstrated the I-V characteristics of the back-gated few-layer (3.8 nm) HfS2 transistor with the robust current saturation. The on/off ratio was more than 10(4) and the maximum drain current of 0.2 mu A/mu m was observed. Moreover, using the electric double-layer gate structure with LiClO4: PEO electrolyte, the drain current of the HfS2 transistor significantly increased to 0.75 mA/mu m and the mobility was estimated to be 45 cm(2)/V.s at least. This improved current seemed to indicate superior intrinsic properties of HfS2. These results provides the basic information for the experimental researches of electron devices based on HfS2.

DOI： 10.1038/srep22277

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Hafnium Disulfide (HfS2) is one of the transition metal dichalcogenides which is expected to have high electron mobility and finite bandgap. However, the fabrication process for HfS2 based electron devices has not been established, and it is required to bring out the superior transport properties of HfS2. In this report, we have investigated the effects of ALD HfO2 passivation on the current properties of HfS2 Transistors. HfO2 passivation of HfS2 surface achieved the improvement in drain current and significant reduction of hysteresis. The charge trapping at the outermost surface seems to be the dominant factor for degradation of the current stability.

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.2.0_1504

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

We propose a tunable metasurface consisting of an array of graphene ribbons on a silver mirror with a SiO2 gap layer to control reflected wavefront at terahertz frequencies. The graphene ribbons exhibit localized plasmon resonances depending on their Fermi levels to introduce abrupt phase shifts along the metasurface. With interference of the Fabry-Perot resonances in the SiO2 layer, phase shift through the system is largely accumulated, covering the 0-to-2 pi range for full control of the wavefront. Numerical simulations prove that wide-angle beam steering up to 53 degrees with a high reflection efficiency of 60% is achieved at 5 THz within a switching time shorter than 0.6 ps. (C) 2015 AIP Publishing LLC.

DOI： 10.1063/1.4927824

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Other Link： http://orcid.org/0000-0002-1447-2530

Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

By atomistic simulation, we investigate an acoustic wave at THz frequencies in nanoscale thin films of aluminum-nitride piezoelectric material. A mode analysis reveals that the thickness longitudinal mode along the [0001] direction exists stably at the atomic level. To control the acoustic wave, we introduce a phononic crystal (PC) structure in the films. We determine the band-gap frequency in the phonon dispersion of the PC structure and confirm via molecular dynamics simulation that the acoustic wave within the band-gap frequency can be confined by a waveguide structure with a PC. The possibility of designing and controlling a THz acoustic wave in a nanoscale thin film with a PC is thereby demonstrated. (C) 2015 The Japan Society of Applied Physics

DOI： 10.7567/JJAP.54.07HB07

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

DOI： 10.1109/DRC.2015.7175643

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Other Link： http://orcid.org/0000-0002-1447-2530

Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Materials Research Society  

We propose and numerically investigate a tunable metasurface made of an array of graphene ribbons to dynamically control terahertz (THz) wavefront. The metasurface consists of graphene micro ribbons on a silver mirror with a SiO2 gap layer. The graphene ribbons are designed to exhibit localized plasmon resonances depending on their Fermi levels to introduce abrupt phase shifts along the metasurface. With interference of the Fabry-Perot resonances in the SiO2 layer, phase shift through the system is largely accumulated, covering up to 2jt range for full control of the THz wavefront. Numerical simulations prove that wide-angle reflected THz beam steering from -53° to +53° with a high reflection efficiency as high as 60% is achieved at 5 THz while the propagation direction of THz beam could be switched within 0.6 ps.

DOI： 10.1557/opl.2015.730

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Other Link： http://orcid.org/0000-0002-1447-2530

Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Recent oxygen permeation experiments with cc-alumina have shown that the two major diffusing species at the grain boundaries, i.e. oxygen and aluminum vacancies, switch over in terms of their dominance depending on the oxygen partial pressure at high temperatures (the so-called p-n transition). In order to investigate this transition behavior the formation energies of charged oxygen and aluminum vacancies at Al2O3 grain boundaries were evaluated using density functional theory. The electronic structures affecting the carrier concentrations are assessed for three types of grain boundaries. The ratios of the grain boundary band gaps relative to the bulk value lie in the range of 0.6-0.85 depending on the degree of grain boundary misorientation. From the formation energies we find that, at band gaps around 60% compared to the bulk value, the dominant vacancy alters depending on the pressure, consistent with the oxygen permeation experiments. Our results suggest that the band bap narrowing occurring at grain boundaries enhances the creation of oxygen vacancies and conduction electrons, facilitating the observed transition. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2014.01.059

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Using large-scale molecular dynamics (MD) simulations, we investigate the energetics and local structures/stresses of partial dislocations, 1/3 &lt; 01 (1) over bar0 &gt; and 1/3 &lt; 10 (1) over bar0 &gt;, dissociated from the 1/3 &lt; 11 (2) over bar0 &gt; perfect basal edge dislocation in alpha-Al2O3. The validity of the model adopted in the simulation is confirmed by comparing with theoretical stress/strain distributions and with those experimentally obtained from a high-resolution transmission electron microscopy (HRTEM) observation. Partial dislocation pairs have a stable inter-core distance (similar to 2 nm), which is also a phenomenon that is observed in the HRTEM experiments. The distance between the partials can be explained quantitatively by the balance between an elastic core core repulsion and an effective attractive force against the extension of stacking faults (SFs). A comparison is made for two types of core structures of partial dislocations: a pair of partials with Al-terminated/O-terminated extra-half planes and that with Al-/Al-terminated ones. The overall tendency of the inter-core interaction and the equilibrium distances are the same in both cases, whereas the Al-O-terminated pair is slightly favourable in energy at the equilibrium distance. A residual shear stress on the SF plane is observed in the MD results, which can be attributed to local atomic structure in the SF. (C) 2013 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.actamat.2013.11.035

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

The lens effect of acoustic waves in a two-dimensional (2D) phononic crystal is studied by numerical simulation based on the finite-difference time-domain (FDTD) method. We calculate the phonon band structure of 2D phononic crystals, consisting of metal/ceramic cylinders placed periodically in water. The lens effect is observed for the far-field wave in phononic crystals with two-layered and graded-index structure. We compare the focal intensity and the energy transmission efficiency in these numerical experiments, and show that a novel energy-transmission device can be designed using phononic crystals. (c) 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 97(1): 22-27, 2014; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11491

DOI： 10.1002/ecj.11491

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

DOI： 10.1299/jsmecmd.2014.27.325

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SUMMARYAn optically powered device using an InGaAs photodiode has been developed. This study aims to harvest light energy (2.8 to 500 mu W) from the FTTH (fiber-to-the-Home) network and to utilize it for operating remote sensors without an external energy source. We designed the booster circuit and evaluated its characteristics, confirming that it could boost an input voltage of 0.3 V to 3.0 V. We also evaluated the characteristics of an InGaAs photodiode and confirmed that it could output a voltage over 0.3 V using 10-mu W input light. We demonstrate that a ready-made sensor can be operated with an input optical power as low as 10 mu W. (c) 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(10): 37-42, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11472

DOI： 10.1002/ecj.11472

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL  

In this paper, a transducer that is suitable for the ultrasonic wireless power transmission system is discussed. The transducer is designed based on Mason's equivalent circuit theory, basically. The transducer was developed using design parameters optimized by multiphysics analysis. As a result, it is confirmed that the transmission eciency is drastically improved to more than 50%. This result suggests that wireless power transmission by acoustic wave is feasible. (c) 2013 Wiley Periodicals, Inc. Electr Eng Jpn, 184(3): 2735, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.22406

DOI： 10.1002/eej.22406

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Language：English   Publisher：The Materials Research Society of Japan  

Vacancy interaction in crystalline Si wafer is studied by numerical simulation based on the Tight-Binding Molecular Dynamics (TBMD) method. We evaluate an effective interaction between vacancy pair in bulk and in a thin slab as model systems of a Si wafer. In the case of vacancy pair in bulk crystal, results reveal that there is an effective "capture radius" of the vacancies to form a divacancy at around 8Å at 300K and 13Å at 600K. Surface structure involving vacancies in Si slab undergoes a reconstruction accompanying dimer-rows in [110] direction initiated by the vacancies. The capture radius is also found to exist in the slab and binding energy of the divacancy has depth dependence due to the surface effect.

DOI： 10.14723/tmrsj.37.143

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We investigated a fiber-based multimode interference phenomenon in the wavelength domain by using a white light source and an optical spectrum analyzer. This phenomenon was produced by a larger-core optical fiber joined at both ends with smaller-core optical fibers. We examined the variation of interference wavelength with changes in the length of the larger-core fiber. The interference wavelengths were blue-shifted and the interference signals were sharpened with an increase in the length of the larger-core fiber. The calculated results agreed well with the measured results. Next, we investigated how the input and output fibers with a small core influence the interference signal characteristics. By comparing the amplitude differences of the interference signal we find the conditions of input and output (I/O) fibers for higher sensitivity. In addition, an interference-signal shift was observed by changing the medium surrounding an multimode interference (MMI) structure. The amount of shift increased at a longer wavelength. This leads to the sensitive detection of the refractive index. Finally, a demonstration of the optical fiber refractometer with a multimode interference structure was given by refractive-index measurements of ethanol/water solutions. (C) 2012 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.51.04DG14

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Institute of Electrical Engineers of Japan  

In this paper, the transducer that is suitable for the ultrasonic wireless power transmission system is discussed. The transducer is designed based on the Mason's equivalent circuit theory, basically. Design parameters of the transducer are optimized for by using the multi physics analysis and developed. As results, it is confirmed that the transmission efficiency is drastically improved to more than 50%. This result suggests that the WPT by acoustic wave is practicable. © 2012 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejeiss.132.337

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DOI： 10.1016/j.ssc.2011.07.020

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN SOC APPLIED PHYSICS  

The negative refraction of acoustic waves in a two-dimensional phononic-crystal slab is studied by numerical simulation based on the finite-difference time-domain (FDTD) method and by ultrasonic measurement. The incident-angle dependences of energy-transmission efficiency in the simulation and experiment are in good agreement in the frequency range of approximately 1.2 MHz. Using the FDTD method, we optimize the efficiency by varying structural parameters such as filling fraction and slab thickness. The effect of deviation from the ideal crystallinity is also evaluated quantitatively via the simulation. These results indicate that an energetically efficient acoustic lens can be fabricated by carefully optimizing the structure of the phononic crystal. (C) 2011 The Japan Society of Applied Physics

DOI： 10.1143/JJAP.50.067301

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS  

The atomic- and electronic-level structures of a dislocation loop and a stacking fault in 4H-SiC crystal are investigated by using large-scale tight-binding (TB) molecular-dynamics simulation. We employ a linear-scaling TB method implemented on a parallel computer in order to accelerate the 9,600-atoms calculation which is required for such a nanoscale simulation. We find that the initial configuration that involves unstable C-C networks around the dislocation loop is relaxed to a structure having six-membered rings, and that the distribution of electron populations is inhomogeneous on the loop. The local electronic density of states shows two peaks in the bulk band gap, where one of these peaks may correspond to the defect state observed in EBIC and CL experiments. [doi:10.2320/matertrans.MBW201024]

DOI： 10.2320/matertrans.MBW201024

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

We developed a fundamental testbed and investigated the suitable transducer for the wireless power transmission (WPT). We also had reported experimental results of the transmission efficiency in water by using the commercial transducer (COM) which only designed to match to the acoustic impedance of water by its structure and the electric impedance of the transmitter and receiver by matching networks. In this paper, we investigate the transducer (DEV) for improving the transmission efficiency based on the Mason's equivalent circuit. For deciding a structure and materials of the transducer, we used ANSYS well-known as the multi-physics simulator.
In this result, it is confirmed that the transmission efficiency is drastically improved from less than 1% to 60% compared to our previous report. In addition, we manufactured designed transducer, and confirmed the system's efficiency is about 50 % experimentally. This result also suggests that WPT by the acoustic wave be practicable.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

RF-DC conversion circuit is one of the most important components in the wireless power transmission technologies. For realizing highly efficient wireless power transmission, it is necessary to design the RF-DC conversion circuit with high conversion efficiency, and the designing method of the circuit is actively studied. In this paper, the influences of diode parameters, such as its resistance, junction capacitance, parasitic reactance, and so on, on the conversion efficiency of the RF-DC conversion circuit are investigated to clarify the characteristic of the diode for obtaining highly efficient RF-DC conversion circuit.

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Other Link： http://orcid.org/0000-0002-1447-2530

Language：English   Publishing type：Research paper (international conference proceedings)  

We developed a fundamental testbed and investigated the suitable transducer for the wireless power transmission (WPT). We also reported experimental results of the transmission efficiency in water by using the commercial transducer (COM) which only designed to match to the acoustic impedance of water and the electric impedance of the transmitter and receiver. In this paper, we investigate the transducer for improving the efficiency based on the Mason's equivalent circuit. For deciding a structure and materials of the transducer, we used ANSYS well-known as the multi-physics simulator. In this result, it is confirmed that the transmission efficiency is drastically improved from less than 1% to 60%. In addition, we made designed transducer, and confirmed the systems' efficiency is about 50 %. This result suggests that the WPT by acoustic wave is practicable. © 2011 IEEE.

DOI： 10.1109/IMWS.2011.5877115

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：HINDAWI PUBLISHING CORPORATION  

Tight-binding molecular-dynamics (TBMDs) simulations are performed to study atomic and electronic structures during high-temperature consolidation processes of nanocrystalline silicon carbide under external pressure. We employ a linear-scaling method (the Fermi-operator expansion method) with a scalable parallel algorithm for efficient calculations of the long time-scale phenomena. The results show that microscopic processes of the consolidation depend strongly on initial orientations of the nanocrystals. It is observed that an orientational rearrangement of the nanocrystals initially misaligned is induced by an instantaneous shearing force between nanocrystals, whereas the aligned system undergoes densification without shearing. Analysis on an effective-charge distribution and an average bond-order distribution reveals electronic-structure evolutions during these processes.

DOI： 10.1155/2011/308495

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

Extra-long molecular-dynamics simulations are performed to study the interplay between dielectric properties and microstructures of silicon oxynitride. We quantitatively obtained the ionic permittivity and its linear dependence on nitrogen concentration. Analyses on microstructure of the oxynitride show that, as the N concentration increases, N atoms enter Si-O networks creating smaller-sized rings. While overall tendency of dielectric constant is monotonic increase on the N concentration, microscopic characteristics, such as local displacement of atoms, are strongly depend on local bonding environments.

DOI： 10.4028/www.scientific.net/KEM.485.287

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We design and characterize a two-dimensional sonic crystal (SC) to manipulate the propagation of acoustic waves by numerical simulation based on the finite-difference time-domain (FDTD) method and by ultrasonic measurement. Two types of SC for an acoustic lens effect are studied. One is a homogeneous periodic structure exhibiting negative refraction, and the other has a hetero-structure. Negative refraction behavior in the simulation is good agreement with the experiment in the frequency range around 1.2MHz. We also observe a lens effect for far-field wave in the hetero-structured SC. These results indicate that a novel energy-transmission/harvesting device can be designed using SCs. © 2011 IEEE.

DOI： 10.1109/IMWS.2011.5877131

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOC  

Recent experiments on ultrasonic measurements of non-doped and boron-doped silicon indicate that vacancies in crystalline silicon can be detected through the elastic softening at low temperature. This is attributed to enhanced response of electronic quadrupole localized at the vacancies to the elastic strain. In the present work, the electronic quadrupole moment of the vacancy orbital in silicon and their strain susceptibility are evaluated quantitatively by using the density-functional method. We show the orbital of gap state is localized around vacancy but extended over several neighbors. The effect of applied magnetic field on the vacancy orbital and its multipole structures are also investigated. We find that the results obtained from these calculations are consistent with the ultrasonic experiments.

DOI： 10.1557/PROC-1195-B08-11

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：JAPAN INST METALS  

We perform molecular-dynamics simulation,, to investigate the atomic and electronic structures of a basal edge dislocation it) alpha-Al2O3. The core structure consisting of two non-stoichiometric partial dislocations, which has been recently proposed by an experiment, is examined by an empirical interatomic-potential model and by a hybrid quantum/classical approach. The atomic rearrangements in the full and in the partial dislocation cores are analyzed. The local electronic structure in the full dislocation core is evaluated by the density-functional method applied for a quantum-cluster region in the hybrid simulations. Interaction potentials between partial dislocations are investigated by the classical model. Results preliminarily obtained show that the partials aligned normal to a basal plane ({10001}) has a short-ranged repulsive nature approximately within 8 angstrom. [doi: 10.2320/matertrans.MC200833]

DOI： 10.2320/matertrans.MC200833

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Metamaterial with negative permittivity and permeability is studied by means of computer simulations. We analyze the electromagnetic response of nanostructured metamaterials to evanescent waves at optical frequency via the finite-difference time-domain simulations on parallel computer. Effects of the nanostructure on dielectric and magnetic properties are taken into account by introducing the Drude-Lorentz model in the materials dispersion. Size effect on the dispersion is examined by comparing the model with that of a noble metal particle. A re-focusing and an amplification of the evanescent waves propagating through a metamaterial, consisting of metal slabs/vacuum stacking, are demonstrated for the frequency range at 614-744 THz. By properly treating the materials dispersion, we show that the nanostructured metamaterial may behave as a left-handed material in the optical range. [doi:10.2320/matertrans.MC200822]

DOI： 10.2320/matertrans.MC200822

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Negative refraction of acoustic waves in a two-dimensional (2D) phononic crystal is studied by numerical simulation based on the finite-difference time-domain (FDTD) method. We calculate the phonon band structure of 2D phononic crystals, consisting of metal cylinders placed periodically in a liquid. By comparing several combinations of materials for metal cylinder and liquid, we analyze the dependence of the band structures on sound speed and density of liquid media. The negative refraction of the acoustic waves is observed at the interfaces between the phononic crystal slab and the liquid. We find that an acoustic "lens effect" with the slab appears due to the negative refractions. The relationship between the focal intensity in the lens effect and the band structure of the phononic crystal is quantitatively discussed. [doi: 10.2320/matertrans.MC200823]

DOI： 10.2320/matertrans.MC200823

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A method for numerically simulating quantum systems is proposed and applied to the two-dimensional electron fluid at T = 0. This method maps quantum systems onto classical ones in the spirit of the classical-map hypernetted-chain theory and performs simulations on the latter. The results of the simulations are free from the assumption of the hypernetted-chain approximation and the neglect of the bridge diagrams. A merit of this method is the applicability to systems with geometrical complexity and finite sizes including the cases at finite temperatures. Monte Carlo and molecular dynamics simulations are performed corresponding to two previous proposals for the &apos;quantum&apos; temperature and an improvement in the description of the diffraction effect. It is shown that one of these two proposals with the improved diffraction effect gives significantly better agreement with quantum Monte Carlo results reported previously for the range of 5 &lt;= r(s) &lt;= 40. These results may serve as the basis for the application of this method to finite non-periodic systems like quantum dots and systems at finite temperatures.

DOI： 10.1088/0953-8984/21/4/045502

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We analyze the ground state of the two-dimensional quantum system of electrons confined in a parabolic potential with the system size around 100 at 0 K. We map the system onto a classical system on the basis of the classical-map hypernetted-chain (CHNC) method which has been proven to work in the integral-equation-based analyses of uniform systems and apply classical Monte Carlo and molecular dynamics simulations. We find that, when we decrease the strength of confinement keeping the number of confined electrons fixed, the energy of the spin-polarized state with somewhat lower average density becomes smaller than that of the spin-unpolarized state with somewhat higher average density. This system thus undergoes the transition from the spin-unpolarized state to the spin polarized state and the corresponding critical value of r(s) estimated from the average density is as low as r(s) similar to 0.4 which is much smaller than the r(s) value for the Wigner lattice formation. When we compare the energies of spin-unpolarized and spin-polarized states for given average density, our data give the critical r(s) value for the transition between unpolarized and polarized states around 10 which is close to but still smaller than the known possibility of polarization at r(s) similar to 27. The advantage of our method is a direct applicability to geometrically complex systems which are difficult to analyze by integral equations and this is an example. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physleta.2008.08.036

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Language：English   Publishing type：Research paper (bulletin of university, research institution)   Publisher：Faculty of Engineering, Okayama University  

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Structural, electronic, and magnetic properties of isolated Ni impurities at point defects and a dislocation core in diamond are investigated using tight-binding molecular-dynamics simulations. The results show the structural stabilization associated by lowering local symmetry in the cases of point defects. The segregation energies of Ni impurity for a substitutional site and for an interstitial site in the dislocation core are estimated to be in the same order within 0.2 eV. The local electronic density-of-states reveals that the gap states appeared by the insertion of Ni impurity are strongly localized around the Ni sites. Magnetic moments on neighboring C atoms are induced so as to screen the moment on the Ni atom except for the case of interstitial Ni impurity in which the total magnetic moment remains non-zero. Analyses indicate that localized atomic d states on the Ni atom and the p-d coupling between Ni and neighboring C atoms are responsible for the residual magnetic moment in the system with an interstitial Ni defect. In the other systems investigated, on the other hand, the bonding states between Ni impurity and its neighboring C atoms are dominated by the s-p coupling. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.commatsci.2006.06.003

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

By extensive Monte Carlo simulations, two separate peaks are found in the specific heat of relatively small spherical clusters of particles with 100 &lt; N &lt; 400, interacting via the Yukawa and Coulomb potentials and confined in the external field. The analyses of the diffusion and positional fluctuations show that these peaks at lower and higher temperatures reflect the loss of intrashell two-dimensional ordering and of the radial ordering, respectively. The magnitudes of the fluctuations in the outermost shell are consistent with the conventional Lindemann criterion. The results can be verified by experiments in ion traps and in dusty plasmas under microgravity or equivalent isotropic environments.

DOI： 10.1143/JPSJ.75.123501

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DOI： 10.1088/0305-4470/39/17/S36

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Spherical Yukawa and Coulomb clusters are composed of charged particles interacting via the Yukawa and Coulomb potential and confined by the external potential. Melting transitions of these clusters are analyzed by Monte Carlo (MC) and molecular dynamics (MD) simulations. By MC simulations, it is found that the specific heat of relatively small clusters (100 &lt;= N &lt;= 400) has two peaks. The diffusion analyses by MC and MD simulations and the fluctuation analyses related to the Lindemann criteria indicate that two peaks at lower and higher temperatures reflect the losses in ordering in azimuthal and radial directions, respectively.

DOI： 10.1063/1.2387929

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Structures in the ground state and thermodynamics related to melting of spherical Yukawa and Coulomb clusters confined in external fields of less than 103 particles are discussed on the basis of molecular dynamics and Monte Carlo simulations and theoretical methods. At low temperatures, particles form concentric shell structures which are expressed by simple formulas and insensitive to the strength of screening. Structures are reproduced by the shell model when the intrashell cohesive energy is properly taken into account. The specific heat of clusters of relatively small sizes 100 &lt;= N-d &lt;= 400 has two peaks as a function of temperature. By analyses of diffusions, it is shown that the peaks at lower and higher temparatures corresponds to the loss of ordering in the azimuthal and radial (and therefore total) ordering, respectively. Magnitudes of fluctuations in the outermost shell are consistent with the conventional Lindemann criterion.

DOI： 10.1063/1.2387928

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The structure of confined two-dimensional systems of dust particles in dusty plasmas is analyzed at finite temperatures by numerical simulations and theoretical approaches. The finite Yukawa system is adopted as a model for these systems and the variational method which has been successful in reproducing structures at low temperatures is extended to finite temperatures through replacement of the cohesive energy by the Helmholtz free energy recently given in the form of simple interpolation formulas. The results are compared with those of numerical simulations and their satisfactory agreement has been obtained. Finite-temperature structures presented here will be useful to evaluate the physical parameters of the two-dimensional systems of dust particles often formed near the boundary between the plasma bulk and the sheath.

DOI： 10.1063/1.2116827

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DOI： 10.1103/PhysRevE.72.036406

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DOI： 10.1103/PhysRevE.71.045401

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Thermodynamic quantities of a two-dimensional Yukawa system, a model for various systems including single-layered dust particles observed in dusty plasmas, are obtained and expressed by simple interpolation formulas. In the domain of weak coupling, the analytical method based on the cluster expansion is applied and, in the domain of intermediate and strong coupling, numerical simulations are performed. Due to reduced dimensionality, the treatment based on the mean field fails at the short range and exact behavior of the binary correlation is to be taken into account even in the case of weak coupling.

DOI： 10.1103/PhysRevE.70.016405

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Thermodynamic quantities of a two-dimensional Yukawa system, a model for various systems including single-layered dust particles observed in dusty plasmas, are obtained and expressed by simple interpolation formulas. In the domain of weak coupling, the analytical method based on the cluster expansion is applied and, in the domain of intermediate and strong coupling, numerical simulations are performed. Due to reduced dimensionality, the treatment based on the mean field fails at the short range and exact behavior of the binary correlation is to be taken into account even in the case of weak coupling. © 2004 The American Physical Society.

DOI： 10.1103/PhysRevE.70.016405

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DOI： 10.1103/PhysRevE.68.049901

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Single-layered two-dimensional crystals of dust particles are often observed in dusty plasma experiments and the data on their structure can be obtained by analyzing the images usually taken by charge-cupled device cameras. We give here some formulas for practical purposes of estimating the screening length and the electric charge on a dust particle from the surface density and the radius of such finite two-dimensional dust crystals. The formulas are derived on the basis of our theoretical approach which has been successful in reproducing results of molecular-dynamics simulations on dusty plasmas. An example of application is also shown.

DOI： 10.1103/PhysRevE.68.017401

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Language：English   Publishing type：Research paper (scientific journal)  

Single-layered two-dimensional crystals of dust particles are often observed in dusty plasma experiments and the data on their structure can be obtained by analyzing the images usually taken by charge-cupled device cameras. We give here some formulas for practical purposes of estimating the screening length and the electric charge on a dust particle from the surface density and the radius of such finite two-dimensional dust crystals. The formulas are derived on the basis of our theoretical approach which has been successful in reproducing results of molecular-dynamics simulations on dusty plasmas. An example of application is also shown. © 2003 The American Physical Society.

DOI： 10.1103/PhysRevE.68.017401

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The lowest-energy state of spherical clusters made up of single-species charged particles in a three-dimensional confining potential is investigated by molecular dynamics simulations for a system size of 5 x 10(3) to 1.2 x 10(5). The energy per particle is compared between shell-structured clusters and spherical finite-bcc lattices with relaxed surfaces. The shell structure in the interior is the lowest-energy configuration for ion numbers lower than about 10(4), while for higher ion numbers, an interior with bcc ordering surrounded by a few shells on the outside has lower energy. The formation of a small bcc lattice (nucleation) in the shell-structured cluster of 2 x 10(4) ions is observed.

DOI： 10.1103/PhysRevLett.88.125002

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The structures of Yukawa systems spherically confined under the condition of micro-gravity are analyzed by molecular dynamics simulations. It is shown that the system is separated into the outer and the inner parts and well-developed shells appear in the outer part at low temperatures. In the case of Yukawa mixtures, there occurs the separation of species according to the magnitude of charges.

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The ground state of the system of charged particles of one species confined by the three-dimensional, isotropic, and parabolic potential is investigated by molecular dynamics simulations. It is shown that, with the increase of the system size or the number of particles in the system N, the ground state changes from the shell-structured system to the finite bee lattice with reconstructed surface. The critical value of the transition is estimated to be between N = 10(4) and N = 2 x 10(4). The nucleation of the bee lattice in the she I I- structured cluster of 2 x 10(4) ions is observed.

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Dust particles in plasmas are often confined near the boundary between the plasma bulk and the sheath where the gravitation is balanced by electrostatic force. To keep dust particles from running away horizontally, an electrostatic potential is usually applied to the electrode surrounding these dusty plasmas and, under appropriate conditions. we have finite two-dimensional lattices of dust particles. Modeling the interaction between dust particles as the isotropic Yukawa interaction, structures of finite two-dimensional Yukawa systems at low temperatures have been analyzed both by numerical simulations and variational methods. The effect of the correlation energy between dust particles is shown to play an important role in the formation of the one-body distribution in these systems.

DOI： 10.1103/PhysRevE.64.066402

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Tight-binding molecular dynamics (TBMD) simulations are performed to investigate atomic and electronic structures during neck formation processes of nanocrystalline silicon carbide at high temperature. For calculating the electronic energy and forces we use a linear-scaling method (the Fermi-operator expansion method) with a scalable parallel algorithm. The TBMD simulations of collision of SiC nanospheres show that processes of neck formation depend strongly on contact angles between the two grains. Electronic populations at grain boundaries are rather uniform, even in a disordered structure of the grain boundary between misaligned nanospheres, Atomic diffusions at elevated temperature are, on the other hand, quite different in the necks formed with different orientations of particles.

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Atomistic and electronic structures in the process of intergranular neck formation of nanocrystalline silicon carbide (SiC) have been investigated by a tight-binding molecular dynamics method. An order-N parallel algorithm is employed for efficient calculations of electronic energy and forces. We find that a defect-free neck is formed between SIC nanocrystals aligned along [11 (2) over bar] axis at T = 1000 K. In the case of neck formation between tilted nanocrystals (with an orientation equivalent to a {122}, Sigma = 9 grain boundary), surface reconstruction before sintering, that is Si-Si bond formation, significantly affects the grain-boundary formation. The spatial distribution of the electronic population and the electronic density of states show that structural defects, accompanied by gap states, are formed around the reconstructed regions after the sintering.

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We perform large-scale molecular dynamics (MD) simulations of Coulomb clusters in a spherical trapping field. The fast multipole method have been employed to simulate clusters of up to 5 x 10(4) particles while previous investigations are limited to clusters of less than 5000 particles, It is found that the Madelung energy of bce clusters with reconstructed surfaces is substantially smaller than the extrapolated values based on the previous results for smaller multilayered icosahedral clusters. (C) 2001 Elsevier Science B.V. All rights reserved.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SWEDISH ACAD SCIENCES  

The structures of systems of particles with repulsive Yukawa interactions are analyzed by molecular dynamics simulations with emphasis on the behavior of Yukawa mixtures. To keep the density of particles with mutual repulsion at finite values, we need some confining mechanism and the latter has strong effects on the structure formation. Under the gravity, the confinement is one-dimensional and we have layered structures. In this case, Yukawa mixtures are separated by the difference in the charge to mass ratios. In the case without gravity, we may have three-dimensional confinement and the behavior of the system may be analyzed by assuming spherical symmetry. We show that there also occurs a separation of species according to the magnitude of charges under the condition of no gravity and we have well-organized structures when the temperature is sufficiently low.

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Language：English   Publishing type：Research paper (scientific journal)  

Dust particles in plasmas are often confined near the boundary between the plasma bulk and the sheath where the gravitation is balanced by electrostatic force. To keep dust particles from running away horizontally, an electrostatic potential is usually applied to the electrode surrounding these dusty plasmas and, under appropriate conditions, we have finite two-dimensional lattices of dust particles. Modeling the interaction between dust particles as the isotropic Yukawa interaction, structures of finite two-dimensional Yukawa systems at low temperatures have been analyzed both by numerical simulations and variational methods. The effect of the correlation energy between dust particles is shown to play an important role in the formation of the one-body distribution in these systems. © 2001 The American Physical Society.

DOI： 10.1103/PhysRevE.64.066402

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Publishing type：Research paper (scientific journal)   Publisher：IOP Publishing  

DOI： 10.1238/physica.topical.089a00117

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

A suite of scalable molecular-dynamics (MD) algorithms has been developed for materials simulations. The linear scaling MD algorithms encompass a wide spectrum of physical reality: (i) classical MD based on a many-body interatomic potential model; (ii) environment-dependent, variable-charge MD: (iii) quantum mechanical MD based on the tight-binding method; and (iv) self-consistent quantum MD based on the density functional theory. Benchmark tests on 1024 Gray T3E processors including 1.02-billion-atom many-body and 22 500-atom density functional MD simulations demonstrate that these algorithms are highly scalable. A design-space diagram spanning seven decades of system size and computational time is constructed for materials scientists to design an optimal MD simulation incorporating maximal physical realism within a given computational budget. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0167-739X(00)00087-X

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

The stability of surface structure and space-charge distribution in free TiO2 nanoparticles are investigated for both rutile and anatase phases using a variable-charge interaction potential in which atomic charges vary dynamically depending on their environment. We find the dynamic charge transfer: (i) enhances atomic diffusion at surfaces of the spherical nanoparticles at high temperatures; and (ii) creates additional repulsive force between the two nanospheres through formation of a double-charge surface layer in each nanosphere. The surface diffusion due to the charge transfer clearly distinguishes the two nanospheres with different underlying crystalline structures; the surface diffusion constant of the anatase sphere is almost three times as great as that of the rutile sphere. Variable-charge molecular dynamics simulations are then applied to sintering of two TiO2 nanospheres. It turns out that the enhanced surface diffusion in the anatase nanosphere at high temperatures significantly promotes neck formation between the two spheres. (C) 2000 American Institute of Physics. [S0021-8979(00)08323-7].

DOI： 10.1063/1.1321785

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Structure, mechanical properties, and sintering of nanostructured SiC (n-SiC) are investigated with neutron scattering and molecular-dynamics (MD) techniques. Both MD and the experiment indicate the onset of sintering around 1500 K. During sintering, the pores shrink while maintaining their morphology: the fractal dimension is similar to 2 and the surface roughness exponent is similar to 0.45. Structural analyses reveal that interfacial regions in n-SiC are disordered with nearly the same number of three- and fourfold coordinated Si atoms. The elastic moduli scale with the density as similar to rho(mu), where mu=3.4 +/- 0.1. (C) 2000 American Institute of Physics. [S0003-6951(00)04334-5].

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：E D P SCIENCES  

The relaxation processes and the equilibrium distribution of two-component nonneutral plasmas in the Penning-Malmberg trap are analyzed by molecular dynamics simulations and theoretical approaches. As an example, mixtures of antiprotons and electrons for antiproton cooling experiments are considered. The relaxation of antiproton energy is calculated based on the rate equations and it is shown that the slow energy transfer between parallel and perpendicular components of strongly magnetized electrons does not affect the whole relaxation process seriously. The equilibrium distribution of each species are obtained by extensive molecular dynamics simulations for various values of parameters and the results are reproduced by theoretical calculations to a good accuracy. The condition for the centrifugal separation is obtained.

DOI： 10.1051/jp4:2000549

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：E D P SCIENCES  

As a method of quantum simulation for large systems with surfaces and inhomogenieties, the density functional molecular dynamics is applied to the hydrogen plasma in the domain of liquid metallic hydrogen and the performance of a PC cluster is measured. Usefulness of relatively small scale PC cluter is shown even without much efforts in tuning the programs for parallel computation.

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DOI： 10.1051/jp4:2000582

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

Regarded as Yukawa systems in external confining potentials, structures and transitions in dusty plasmas have been analyzed theoretically and by numerical simulations. The one-dimensional confinement in three dimensions and the radial confinement in two dimensions are considered. The results of simulations have been reproduced to a good accuracy in both cases. It is shown that the inclusion of the correlation energy or the effect of strong coupling is of essential importance in the theory. A crossover from the surface freezing of Coulomb system to surface melting of systems of short-ranged interactions is observed.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOCIETY  

A new interatomic potential has been developed for molecular-dynamics simulations of TiO2 based on the formalism of Streitz and Mintmire [J. Adhesion Sci. Technol. 8, 853 (1994)], in which atomic charges vary dynamically according to the generalized electronegativity-equalization principle. The present potential reproduces various quantities of rutile crystal including vibrational density of states, static dielectric constants, melting temperature, elastic moduli, and surface relaxation. Calculated cohesive-energy and dielectric constants for anatase crystal agree well with experimental data The potential is applied to TiO2 nanoclusters (size 60-80 Angstrom) for both anatase and rutile phases to analyze their equilibrium configuration and space-charge distribution. Stable double-charge layer is found in the surface region of a spherical nanocluster for both rutile and anatase, resulting in enhanced Coulomb-repulsion between the nanoclusters at close proximity.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

An interatomic potential model has been developed for molecular-dynamics simulations of TiO2 (rutile) based on the formalism of Streitz and Mintmire [J. Adhes. Sci. Technol. 8, 853 (1994)], in which atomic charges vary dynamically according to the generalized electronegativity equalization principle. The present model potential reproduces the vibrational density of states, the pressure-dependent static dielectric constants, the melting temperature, and the surface relaxation of the rutile crystal, as well as the cohesive energy, the lattice constants, and the elastic moduli. We find the physical properties of rutile are significantly affected by dynamic charge transfer between Ti and O atoms. The potential allows us to perform atomistic simulations on nanostructured TiO2 with various kinds of interfaces (surfaces, grain boundaries, dislocations, etc.). (C) 1999 American Institute of Physics. [S0021-8979(99)07618-5].

DOI： 10.1063/1.371165

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCI LTD  

'Grand Challenge' atomistic simulations of high-temperature structural materials ave performed on multiple, parallel platforms. The simulations focus on sintering of ceramic nanoclusters, structure and mechanical properties of nanophase ceramics, and hypervelocity impact damage in diamond coatings. (C) 1999 Elsevier Science Ltd. All rights reserved.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

Dusty plasmas can be regarded as assemblies of Yukawa particles in a one-dimensional potential well. We extend the analyses on dusty plasmas in two directions: the two-dimensional Yukawa system and Yukawa mixtures. (1) Under appropriate conditions at low temperatures, dust particles sit in a plane which is perpendicular to the gravitational field. When they are also confined laterally by an electrode, we have a finite two-dimensional system of Yukawa particles. The low temperature structures are obtained by molecular dynamics simulations and the results are reproduced by theoretical analyses. Through these analyses we show that the correlation energy of the Yukawa system plays an essential role in structure formations. As for dynamics of this system, a crossover from the surface freezing of Coulomb system to surface melting of systems of short-ranged interactions is observed. (2) In the case of mixtures, we have an extra parameter characterizing the difference in the gravity on each species. We obtain the low temperature structures of this system and compare them with theoretical predictions based on the results for the case of one component.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AMER INST PHYSICS  

The behavior of the two-component nonneutral plasma in the Penning Malmberg trap is analyzed by simulations and theoretical approaches. The parameters expected in experiments of antiproton cooling by electrons are assumed. The relaxation of antiproton energy is followed by the rate equation with proper account taken into account for the slow transfer of energy between parallel and perpendicular components of strongly magnetized electrons. The equilibrium distribution of each species are obtained by molecular dynamics simulations for various Values of parameters and the results are reproduced by theoretical calculations to a good accuracy. The condition for the centrifugal separation is obtained.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

DOI： 10.1103/PhysRevLett.82.4018

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：OXFORD UNIV PRESS  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE COMPUTER SOC  

Materials and devices with microstructures on the nanometer scale are revolutionizing technology, but until recently simulation at this scale has bean problematic. Developments in parallel computing are now allowing atomistic simulation using multiresolution algorithms, such as fast multipole methods. With these algorithms, researchers may soon be able to simulate applications up to one billion atoms.

DOI： 10.1109/99.735897

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER CERAMIC SOC  

Consolidation and fracture dynamics in nanophase amorphous Si3N4 are investigated using 10(6)-atom molecular-dynamics simulations. At a pressure of 15 GPa and 2000 K, the nanophase system is almost fully consolidated within a fraction of a nanosecond. The consolidation process is well-described by the classical theory of sintering. Under an applied strain the consolidated system develops several cracks which propagate parallel to each other, causing failure at multiple sites. The critical strain at which the nanophase system fractures is much larger than that for crystalline Si3N4.

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Oxidation of an aluminum nanocluster (252,158 atoms) of radius 100 Angstrom placed in gaseous oxygen (530,727 atoms) is investigated by performing molecular-dynamics simulations on parallel computers. The simulation takes into account the effect of charge transfer between Al and O based on the electronegativity equalization principles. We find that the oxidation starts at the surface of the cluster and the oxide layer grows to a thickness of similar to 28 Angstrom. Evolutions of local temperature and densities of Al and O are investigated. The surface oxide melts because of the high temperature resulting from the release of energy associated with Al-O bondings. Amorphous surface-oxides are obtained by quenching the cluster. Vibrational density-of-states for the surface oxide is analyzed through comparisons with those for crystalline Al, Al nanocluster, and alpha-Al2O3.

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Using 10(6)-atom molecular-dynamics simulations, we investigate dynamic fracture in nanophase Si3N4. The simulations reveal that intercluster regions are amorphous, and they deflect cracks and give rise to local crack branching. As a result, the nanophase system is able to sustain an order-of-magnitude larger external strain than crystalline Si3N4. We also determine the morphology of fracture surfaces: For in-plane fracture surface profiles the roughness exponent zeta = 0.57 and for out-of-plane profiles the exponents zeta(perpendicular to) = 0.84 and zeta(parallel to) = 0.75 are in excellent agreement with experiments.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

Million atom molecular-dynamics simulations are performed to investigate the structure, dynamics, and mechanical behavior of cluster-assembled Si3N4. These solids contain highly disordered interfacial regions with 50% undercoordinated atoms. Systems sintered at low pressures have percolating pores whose surface morphologies are well characterized by two values of the roughness exponent, 0.46 and 0.86; these are close to the experimental values found by Bouchaud et al. for fracture surfaces. Results for elastic moduli are consistent with a three-phase model for heterogeneous materials.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOCIETY  

Million-atom molecular-dynamics (MD) simulations are performed to study the structure, mechanical properties, and dynamic fracture in nanophase Si3N4. We find that intercluster regions are highly disordered: 50% of Si atoms in intercluster regions are three-fold coordinated. Elastic moduli of nanophase Si3N4 as a function of grain size and porosity are well described by a multiphase model for heterogeneous materials. The study of fracture in the nanophase Si3N4 reveals that the system can sustain an order-of-magnitude larger external load than crystalline Si3N4. This is due to branching and pinning of the crack front by nanoscale microstructures.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MINERALS, METALS & MATERIALS SOC  

Structural correlations and dynamic fracture in nanocluster-assembled silicon nitride are investigated with molecular-dynamics (MD) simulations involving 1.08 million particles. These simulations reveal that the consolidated nanophase Si3N4 has highly disordered intercluster regions that contain 50% under-coordinated atoms. Amorphous interfacial regions deflect cracks and give rise to local crack branching. These dissipative mechanisms enable the nanophase system to sustain an order-of-magnitude larger external strain than crystalline Si3N4. The morphology of fracture surfaces in nanophase Si3N4 is also determined. The MD results for roughness exponents are very close to experimental values even though the materials and length scales are very different.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOCIETY  

New multiscale algorithms and a load-balancing scheme are combined for molecular-dynamics simulations of nanocluster-assembled ceramics on parallel computers. Million-atom simulations of the dynamic fracture in nanophase silicon nitride reveal anisotropic self-affine structures and crossover phenomena associated with fracture surfaces.

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Language：English   Publisher：CURRENT SCIENCE LTD  

Recent advances in computing technology - parallel computer architectures, portable software and development of robust O(N) algorithms - have revolutionized the field of computer simulation. Using the space-time multiresolution molecular dynamics algorithms it is possible to carry out multimillion atom simulations of materials in different ranges of density, temperature and uniaxial strain.

DOI： 10.1016/S1359-0286(96)80113-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EDITIONS PHYSIQUE  

Sintering of Si3N4 nanoclusters is investigated With the molecular-dynamics approach. At 2000 K thermally rough nanocrystals develop an asymmetric neck in 100 picoseconds. The neck contains more fourfold than threefold coordinated Si atoms, Amorphous nanoclusters develop a symmetric neck which has nearly equal number of threefold and fourfold coordinated Si atoms. In both cases, sintering is driven by surface diffusion of Si and N atoms. The diffusion is much more rapid in the neck joining amorphous nanoclusters than in the neck region of nanocrystals.

DOI： 10.1209/epl/i1996-00359-2

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOC  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：AIP PRESS  

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DOI： 10.1103/PhysRevE.51.3788

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

Screening potential, the balance between the bare Coulomb repulsion and the potential of mean force in a charged liquid, plays an essential role in the theoretical estimation of enhancement for the nuclear reaction rates in dense stellar matter. For the accurate assessment of the screening potential at short interparticle separations, we revisit the first-principles calculations for the coefficients of the Widom expansion and undertake extra-long Monte Carlo samplings in specifically designed binary-ionic systems. The results are compared with those obtained from an approximate estimation based on extrapolation from intermediate distances. We also present model calculations of the screening potential with two-ion clusters, which shed light on the relation between the short- and intermediate-range behaviors.

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By computer-simulation and analytic studies for binding energies and microscopic shell structures, it is shown that Coulomb clusters assume particularly stable configurations at certain ''magic numbers'' of particles. Through analyses with the spherical Voronoi numbers, a special role that the icosahedral geometry plays in shaping the particle configurations on the spherical surfaces is elucidated. Gross features in the cluster structures, such as the shell radii and intershell partition of particles, are well accounted for by an appropriate shell model.

DOI： 10.1103/PhysRevA.48.1339

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The collisional equipartition rate between the parallel and perpendicular velocity components is calculated for a weakly correlated electron plasma that is immersed in a uniform magnetic field. Here, parallel and perpendicular refer to the direction of the magnetic field. The rate depends on the parameter κ̄ = (b̄/rc)/√2, where rc = √T/m/Ωc is the cyclotron radius and b̄ = 2e 2/T is twice the distance of closest approach. For a strongly magnetized plasma (i.e., κ̄ ≫ 1), the equipartition rate is exponentially small (v∼exp[ -5(3πκ̄)2/5/6]). For a weakly magnetized plasma (i.e., κ̄ ≪ 1), the rate is the same as for an unmagnetized plasma except that rc/b̄ replaces λD/b̄ in the Coulomb logarithm. (It is assumed here that rc &lt
λD
for rc &gt
λD, the plasma is effectively unmagnetized.) This paper contains a numerical treatment that spans the intermediate regime κ̄∼1, and connects onto asymptotic results in the two limits κ̄ ≪ 1 and κ̄ ≫ 1. Also, an improved asymptotic expression for the rate in the high-field limit is derived. The present theoretical results are in good agreement with recent measurements of the equipartition rate over eight decades in κ̄ and four decades in the scaled rate v/nv̄b̄2, where n is the electron density and v̄ = √2T/m. © 1992 American Institute of Physics.

DOI： 10.1063/1.860124

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Language：English   Publisher：Steering committee of symposium on ultrasonic electronics  

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

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Language：English   Publisher：Steering committee of symposium on ultrasonic electronics  

In this paper, we discussed the design of the ultrasonic WPT system, consisting of impedance-matched transducers, phononic-crystal structure, and the Schlieren optical measurement system. The transducers we designed show 60% energy-transmission efficiency at 1.2MHz. The phononic structure is designed via phonon-band analysis and fabricated by using 3D printer for controlling ultrasonic wave. The Schlieren optical measurement system is also designed for analyzing actual wave propagation in water at MHz regime without disturbing the wave propagation.

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Using large-scale molecular dynamics simulation, we investigate the stability of screw dislocation in α-alumina. Energies of perfect and partial screw dislocations in α-alumina crystal are compared at several temperatures. In the case of screw dislocation results reveal that perfect dislocation in alumina is more stable than partial dislocations. This result is consistent with the experimental observations in that the screw dislocation in alumina is stable against dissociation into partials. We also evaluate the temperature dependence of rigidity modulus of alumina from the stress-distribution calculations.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

We demonstrate the effectiveness of a hybrid Monte Carlo (MC)/molecular dynamics (MD) method in analysis of grain-boundary segregation, a long-time phenomenon from the atomistic-simulation point of view. The Σ9 coincidence grain boundary of Si crystal is adopted as a testbed for the method. We find an efficient sampling of hopping diffusion of a vacancy in the system can be achieved, while the internal energy evaluated from the MD runs at the same temperature as the MC simulation includes the structural relaxation of the system at the temperature.

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The device called Rectenna as an antenna for Receive Antenna of radio electric power transmission is used. The RF-DC conversion circuit of high conversion efficiency which is a component of Rectenna cause wireless power transimission of high transmission efficiency. In this paper, The conversion efficiency analysis of RF-DC conversion circuit in change of component oh the diode are investigated to clarify the characteristic of the diode for obtaining highly efficiency conversion circuit.

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DOI： 10.1541/ieejiess.132.337

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DOI： 10.1541/ieejiess.132.337

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Language：English   Publisher：Institute of Electrical Engineers of Japan  

An optically powered device with using InGaAs-Photodiode has been developed. This study aims to harvest light energy (2.8∼500 μW) from the FTTH (Fiber To The Home) network and to utilize it for operating remote sensors without external energy sources. First, we designed and evaluated the characteristics of the booster circuit and confirmed that it could boost an input voltage of 0.3 V to 3.0 V. Next, we also evaluated the characteristics of InGaAs photodiode and confirmed that it can output a voltage over 0.3 V at 10-μW input light. We demonstrate that a ready-made sensor can be operated with an input optical power as low as 10 μW. © 2012 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejeiss.132.495

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Language：English   Publisher：Institute of Electrical Engineers of Japan  

An optically powered device with using InGaAs-Photodiode has been developed. This study aims to harvest light energy (2.8∼500 μW) from the FTTH (Fiber To The Home) network and to utilize it for operating remote sensors without external energy sources. First, we designed and evaluated the characteristics of the booster circuit and confirmed that it could boost an input voltage of 0.3 V to 3.0 V. Next, we also evaluated the characteristics of InGaAs photodiode and confirmed that it can output a voltage over 0.3 V at 10-μW input light. We demonstrate that a ready-made sensor can be operated with an input optical power as low as 10 μW. © 2012 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejeiss.132.495

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Lens effect of acoustic wave in two-dimensional (2D) phononic crystal is studied by numerical simulation based on the finite-difference time-domain (FDTD) method. We calculate the phonon band structure of 2D phononic crystals, consisting of metal/ceramic cylinders placed periodically in water. Lens effect is observed for far-field wave in the phononic crystals with two-layered and graded-index structure. We compare the focal intensity and the energy transmission efficiency in these numerical experiments, and show that a novel energy-transmission device can be designed using phononic crystals. © 2012 The Institute of Electrical Engineers of Japan.

DOI： 10.1541/ieejeiss.132.686

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

A graphite nanotube is composed of Gemini type hexabenzocoronene (Gemini-HBC) as the hole-transport layer and the fullerene as the electron-transport layer. We study the doping effect on the electronic properties of Gemini-HBC to make the absorption peak broader, expecting to cover an unharvested energy resource. Here we assume that structure of doped Gemini-HBC is well represented by substituting dopant atom to one of carbon sites. Calculations of the energy and absorption spectrum are performed by the ab-initio computational method. As a result, the absorption peak of the Gemini-HBC is about 0.5μm, whereas the first and second peaks for doped Gemini-HBC are also approximately 0.5μm. In addition, the absorption spectrum of F-doped Gemini-HBC has an additional peak around 0.95μm, implying the sunlight with the longer wavelength may be absorbed.

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Lithium ion battery (LiB) is widely used in portable electronic devices, such as cellular phones and laptop computers. Recently, needs for higher efficiency of the battery has been increasing rapidly due to the demand for electric and hybrid electric vehicle. At present, anode of LiB is mainly made of graphite. Recently, graphene has been attracting more attention as an alternative anode material. The effect of impurity doping to graphene has also been investigated recently. To optimize the materials combination for the anode applications microscopic mechanisms of such enhancement of electric capacity in LiB have to be identified. Using an ab-initio computational method, based on the density-functional theory, and its hybridized method with the classical molecular dynamics, we investigate dynamical behavior, such as diffusion processes, of Li atom in the carbon based materials with pristine, defected, and doped graphene.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Vacancy interaction in Si crystal is studied by molecular-dynamics simulations. Using tight-binding molecular dynamics and classical molecular dynamics methods, we calculate an effective interaction potential of Si vacancy pair in bulk and in a thin slab at several temperatures. In the case of vacancy pair in bulk crystal, results reveal that there is an effective "capture radius" of the vacancies at around 8 Åat 300K and 13 Å at 600K and 1000K. Stable structure and electronic states around the vacancies in Si slab near the surface are qualitatively different from their bulk properties, reflecting a proximity surface electronic state. Results of diffusion behavior and interaction potential of the vacancy pair in the slab at finite temperatures are also reported.

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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We perform molecular-dynamics simulations to investigate the atomic and electronic structures of a basal edge dislocation in α-Al_2O_3. Inter-core interactions between a dissociated partial pair are evaluated by a million-atom classical molecular dynamics. The interactions consists of a short-ranged repulsion due to the elastic interaction between two singular points and a long-ranged attraction due to the stacking-fault formed between the two partials. The atomic rearrangements in the full and in the partial dislocation cores are analyzed by a hybrid quantum/classical molecular dynamics method. The local electronic structure in the dislocation core is evaluated by the density-functional method applied for a quantum-cluster region in the hybrid simulation. Effects of a doped Si atom segregated to the dislocation core are also analyzed.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Using the two dimensional Finite Difference Time Domain method, we demonstrate a light-confinement phenomenon by a structure composed of a negative-refractive-indexed (NRI) material. The structure, consisting of triangular lattice of mediums with a NRI and a normal material, is designed to confine an incident light by peculiar refractions at the materials boundaries. To model realistically the NRI material, we introduce the Drude-Lorentz model for the dielectric response at optical frequencies. The effects of dielectric loss in the NRI medium on the confinement efficiency of the proposed structure is reported.

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Language：Japanese   Publisher：The Japan Society of Mechanical Engineers  

Lens effect of acoustic waves in a two-dimensional (2D) phononic crystal is studied by numerical simulation based on the finite-difference time-domain (FDTD) method. We calculate the phonon band structure of 2D phononic crystals, consisting of metal cylinders placed periodically in water. Lens effect is observed by the negative refraction of acoustic waves, which results in acoustic waves refocused at the point outside the crystal. To increase the focal intensity, we introduce a 2D phononic crystal shield with a different composition of material, which reflects incident acoustic waves from the source. We show how the focal intensity is improved by using the 2D phononic crystal shield.

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publisher：The Japan Institute of Metals and Materials  

DOI： 10.2320/materia.48.303

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Other Link： https://jlc.jst.go.jp/DN/JALC/00333014217?from=CiNii

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Language：English   Publisher：TRANS TECH PUBLICATIONS LTD  

We present some attempts to simulate nanoscale phenomena, which involve different length-scales and time-scales, using multiscale molecular-dynamics approaches. To simulate realistically an impurity-segregated nanostructure, we have developed the hybrid quantum/classical approach, The method can describe seamlessly both dynamical changes of local chemical bonding and nanoscale atomic relaxations. We apply the method to hydrogen diffusion in Si grain boundary. We find that the hydrogen is strongly trapped in (001)Sigma 5 twist boundary below 1000K, whereas it starts diffusing along the grain boundary above 1000K. For long-time processes in nanostructure formation, we apply the stochastic-difference-equation method to accelerate the simulations for microstructure evolution. The method bridges the states separated by high-energy barriers in a configuration space by optimizing an action, defined as an error accumulation along a reaction pathway. As an example, a SDE simulation is performed for Cu thin-film formation via nanocluster deposition. We show that the method can be applied effectively to search for the long-time process which involves a rare event due to a large potential barrier between two atomic configurations.

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Language：English   Publisher：Faculty of Engineering, Okayama University  

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The dynamical form factor and the wave dispersion relations of the two-dimensional Yukawa fluids are obtained by extensive molecular dynamics simulations for a wide range of coupling and the screening parameters. It is observed that, for weak screening, the transverse wave dispersion has a cut-off at a long wavelength. The simulation results are compared with the earlier theoretical predictions of the Yukawa dispersion relations and good agreement is found in the regimes of interest.

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