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Nakamura Nobutomo, Matsuura Koji, Ishii Akio and Ogi Hirotsugu. Restructuring in bimetallic core-shell nanoparticles: Real-time observation. Physical Review B, 105, 125401, 2022. https://doi.org/10.1103/PhysRevB.105.125401., The formation process of core-shell bimetallic nanoparticles synthesized by sputtering onto a substrate is observed in real time using an originally developed acoustic technique. The technique enables us to evaluate the structural change of nanoparticles at room temperature without contacting the nanoparticles or substrate. In the experiments, the sputtering of metal A followed by metal B tended to form B-shell/A-core nanoparticles. However, in Pd-Au alloy system, notable restructuring occurred during synthesis, resulting in the formation of A-shell/B-core nanoparticles. The formation process is analyzed using the molecular dynamics simulation, revealing that this restructuring occurs on a short timescale, and high diffusivity of Au plays an important role.

This is the Accepted Manuscript version of an article accepted for publication in Japanese Journal of Applied Physics. IOP Publishing Ltd are not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.35848/1347-4065/abea50., For visualizing propagation of an acoustic wave and evaluating the local sound velocity in micro/nano-meter structures, confocal picosecond ultrasonics is developed. In conventional picosecond ultrasonics, the measurement of local sound velocity was difficult, and macroscopic sound velocity and elastic properties were measured. However, in confocal picosecond ultrasonics, a confocal laser-scanning microscope is embedded in the optics of the picosecond ultrasonics, and the depth resolution is improved. Using the developed optics, we successfully observed propagation of an acoustic wave in thin film. In this paper, the optics we developed and the experimental results on silica film are reported.

This is the Accepted Manuscript version of an article accepted for publication in Japanese Journal of Applied Physics. IOP Publishing Ltd are not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.35848/1347-4065/ab78e2., Sensitivity of a hydrogen-gas sensor based on lattice expansion of palladium highly depends on the surface morphology of palladium. We find that the sensitivity can be significantly improved by exposing the as-deposited palladium film to low-power plasma. The hydrogen-gas detection was performed by a wireless-electrodeless AT-cut quartz-crystal resonator with 125 MHz fundamental resonance frequency. It detects hydrogen gas through bending deformation of the resonator caused by the volume expansion of palladium. The surface morphology of palladium film before and after the plasma treatment were analyzed using atomic force microscopy. The plasma treated palladium film exhibits a rougher surface, finer grains, voids, and grain boundary extension. Such morphology and structure changes along with defects induced by the ion bombardment during the plasma treatment strongly contribute to increase in hydrogen absorption rate and then the sensor sensitivity. We further investigate the thickness dependence of the sensitivity, revealing an optimum palladium film thickness of 300 nm.

This is the Accepted Manuscript version of an article accepted for publication in Japanese Journal of Applied Physics. IOP Publishing Ltd are not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The Version of Record is available online at https://doi.org/10.35848/1347-4065/abea50., In metallic multilayers, elastic stiffness is often reported to decrease with an increasing number of interfaces. This phenomenon is generally attributed to softening at interfaces, and a decrease of 50% or more in interface stiffness is expected. This interpretation of the elastic stiffness of metallic multilayers is commonly accepted; however, we propose that the decrease of interface stiffness is doubtful because it is difficult to explain the large decrease using existing models. In the present study, to confirm the validity of the decrease of interface stiffness, the longitudinal-wave elastic stiffness in the stacking direction is measured using picosecond ultrasonics. The elastic stiffness of Cr/Fe, Cu/Co, and Nb/Cu multilayers with several thickness ratios is measured, and the decrease in multilayer stiffness is experimentally demonstrated to arise from the decrease in stiffness of each layer, not from a decrease in interface stiffness.

Ogi H., Ishihara T., Ishida H., et al. Physical Review Letters, 117(19), 195901, 2016. Copyright 2016 by the American Physical Society., Heat conduction possesses (thermal) modes in analogy with acoustics even without oscillation. Here, we establish thermal mode spectroscopy to measure the thermal diffusivity of small specimens. Local heating with a light pulse excites such modes that show antinodes at the heating point, and photothermal detection at another antinode spot allows measuring relaxation behavior of the desired mode selectively: The relaxation time yields thermal diffusivity. The Ritz method is proposed for arbitrary geometry specimens. This method is applicable even to a diamond crystal with ∼1 mm dimensions.

Ogi H., Masuda S., Nagakubo A., et al. Physical Review B, 93(2), 024112, 2016. Copyright 2016 by the American Physical Society., Surface waves are often excited by interdigitated transducers consisting of many nanostrips attached on a substrate, and it has been recognized that the mass and stiffness of the attached nanostrips affect surface-wave resonances to some extent. Here, we reveal the more noticeable influence of the interfacial stiffness between strips and substrate at high frequencies. This influence is confirmed by exciting and detecting surface-wave resonances up to ∼6 GHz by picosecond ultrasound spectroscopy. The resonance frequency significantly decreases and attenuation increases as the interfacial stiffness decreases for silicon substrate. However, low-attenuation branches appear along the Rayleigh-wave-resonance dispersion curve for silica substrate, and the resonance frequencies remain nearly identical to those of the Rayleigh waves. Previous models fail to reproduce these surface-wave resonance behaviors. The proposed theoretical model, involving the interfacial stiffness, consistently explained them, indicating the importance of the interface bond strength in designing surface-wave resonators.

Point-focusing electromagnetic-acoustic transducers (PF-EMATs) for shear-vertical (SV) waves were developed for crack inspection of stainless-steel pipes. The transducer has improved defect detectability by accumulating SV waves generated by concentric line sources at a focal point in phase. An optimum frequency for defect detection was found to be 2MHz, with which a crack of 0.5mm depth near a weld was clearly detected. The EMAT exhibited defect detectability comparable to that of a conventional phased-array piezoelectric transducer, indicating that this new EMAT is highly practical for the non-contacting evaluation of stress-corrosion cracking in stainless steels.

Takashi Takishita, Kazuhiro Ashida, Nobutomo Nakamura, Hirotsugu Ogi and Masahiko Hirao. Development of shear-vertical-wave point-focusing electromagnetic acoustic transducer. Japanese Journal of Applied Physics, 2015, 54,(7S1), 07HC04. https://doi.org/10.7567/JJAP.54.07HC04., A shear-vertical-wave point-focusing electromagnetic acoustic transducer is developed for the purpose of detecting stress corrosion cracking in stainless steel. The transducer is composed of a permanent magnet and two identical concentric meander-line coils, one for the transmitter and the other for the receiver, and the shear-vertical waves are excited and detected by the Lorentz force mechanism. The meander-line coils are designed so that the phases of all the excited shear-vertical waves are focused in phase at a focal point. The focal area is evaluated experimentally, and it is confirmed that the developed transducer is capable of detecting slit defects deeper than 0.05mm at the bottom surface of a stainless-steel plate 20mm thick.