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The dynamics of magnetization is important in spintronics, where the coupling between phonon and magnon attracts much attention. In this work, we study the angular dependence of the coupling between longitudinal-wave phonon and magnon. We investigated the magnetization dynamics using the time-resolved magneto-optical Kerr effect, which allows measuring spin-wave resonances and the magnetic echo signal. The frequency, mode number, and amplitude of the spin-wave resonance change with the out-of-plane angle of the external magnetic field. The amplitude of the magnetic echo signal caused by the strain pulse also changes with the angle. We calculate these angular dependences based on the Landau–Lifshitz–Gilbert equation and find that the angles of the external field and magnetic moment are important factors for the phonon–magnon coupling when phonon propagates in the thickness direction under the out-of-plane magnetic field.

We acoustically measured the energy level for thermally activated conduction (TAC) in high-resistivity Fe-doped GaN using the non-contacting antenna-transmission acoustic-resonance method. The acoustic attenuation takes a maximum at a specific temperature, where the TAC is accelerated with the help of phonon energy. The Debye-type relaxation is thus observed for acoustic attenuation, and its activation energy (0.54 ± 0.04 eV) was determined with attenuation measurements at various frequencies and temperatures. This value agrees with the E3 level in GaN, indicating that thermally associated conduction originates from the E3 trap level. We also measured the five independent elastic constants at high temperatures.

Recent nano-scale fabrication has realized the progress of integrated circuits, where current density increases with the downscaling of each electronic component, leading to failure due to electromigration. Electromigration is the transport of materials via momentum exchange from high-density conducting electrons to metal atoms. Defects of atoms result in the disconnection of the circuits and the accumulation of atoms causes the short circuit. Therefore, electromigration due to highly dense current flow has been studied for long years. However, it is difficult to non-destructively evaluate the structural and physical property changes due to the electromigration in an in-situ and microscopic measurement. In this study, we develop a real-time monitoring method of structural and mechanical property changes of a single nanowire using picosecond ultrasonics. Picosecond ultrasonics is ultrafast pump-probe laser ultrasonics using femtosecond pulse laser to excite and detect sub-THz ultrasound. We can focus the laser light within 1-micrometer diameter, enabling us to evaluate the mechanical property changes in a single nanowire from the ultrasound propagation. By applying direct current and observing ultrasound, we discuss the relationship among mechanical, electrical, and structural changes due to electromigration.

Living cells sense various mechanical stimuli and respond to them by adjusting their tissue morphogenesis, self-renewal, and differentiation. Because stem cells seem to be more sensitive to the stimuli, many studies have been conducted to control their behavior. However, many of these mechanisms remain unexplored, because stimulus extends throughout the cells or causes cell-damaging. In this study, we have developed a focused ultrasound spectroscopic technique for noninvasively applying local mechanical stimulation in a single cell and combined it with an optical microscopy to systematically investigate its effects on the function and morphology of the cell. Our original system allows ultrasonically monitoring living cells for longer than 24 h. Appling this technique for human iPS cells, we find out an ultrasound absorption band at ∼160 MHz, which we attribute to the resonance of the nucleus. This indicates that we can stimulate cell nucleus selectively and effectively. Furthermore, we have established a method to evaluate the height distribution of cell colonies and revealed that human iPS cell colonies show very characteristic changes in height distribution during their growth.

We propose a high sensitivity biosensor based on a GHz phononic crystal (PnC) waveguide, and demonstrate its operation by numerical simulations. The geometry consists of a micron-scale freestanding PnC silica waveguide plate with embedded Au nanopillars for bioparticle attachment, the PnC plate lying between two groups of periodic metal strips for GHz Lamb-wave acoustic generation and detection with ultrashort light pulses. By precise choice of the waveguide defect width, this biosensor is designed to work using a single, isolated waveguide mode. We study the influence of the waveguide defect width on the acoustic dispersion and transmission of this mode. Bioparticle attachment is simulated by investigation of the Au nanopillar mass loading, and is shown to shift the waveguide transmission peak to lower frequencies. We thereby demonstrate femtogram detection, showing that our approach provides a new methodology for label-free ultra-sensitive biosensing.

Weng H.K., Nagakubo A., Ogi H., et al. "Phonon propagation in isotopic diamond superlattices", Physical Review B, 104(5), 054112, 2021. https://doi.org/10.1103/physrevb.104.054112., The out-of-plane thermal conductivity and elastic constant of epitaxial [100] C12/C13 superlattice diamonds with layer thicknesses of 1, 30, and 100 nm are evaluated by picosecond ultrasound spectroscopy. The measured elastic constants of the superlattices are equivalent to those of single-layer diamond thin films. This result confirms our success in synthesizing superlattice specimens with few defects at the interfaces. Therefore, the phonon transport behavior is governed by the mass difference, not the interfacial defects. The measured thermal conductivity of the superlattices is lower than that of a pure C12 isotope diamond thin film. We estimated the lattice thermal conductivity using the lattice dynamics calculation, attributing the lowered thermal conductivity to the decrease in the phonon group velocity in superlattices. We further consider the effect of mini-umklapp scattering in the superlattice, which explains the dependence of the thermal conductivity on the layer thickness. We reveal that the mini-umklapp scattering effect becomes significant only for an isotope diamond superlattice because of the high Debye temperature and large relative mass difference.

A novel label-free biosensor array is proposed, which uses the ultrafast heat transport from a metallic nano-plate into surrounding solution across proteins captured on the nano-plate. Using electron-beam lithography, 2500 gold nano-plates of 30 nm thick with 5 × 5 μm2 area were fabricated on a fused silica substrate, on which receptor proteins were immobilized. Thermal phonons were induced on the back surface of a single nano-plate by focusing ultrafast light pulses from the substrate side, which propagated toward surrounding solution through surface proteins. Because the ultrafast heat-transport behavior, which is completed within ∼1 ns, is affected by existence of target proteins on the nano-plate, each nano-plate works as a single label-free biosensor. A theoretical calculation was made for predicting the cooling-down behavior of the nano-plate after an impulsive heating, and a pump–probe optics was developed for monitoring the ultrafast temperature change after excitation with a femtosecond fiber laser. We then confirm that each nano-plate acts as a single label-free biosensor by performing antigen–antibody binding reactions, indicating that a large scale number of multichannel measurement is made possible with this phenomenon.

Since surface acoustic waves (SAW) are often generated on substrates to which nanostrips are periodically attached, it is very important to consider the effect of interface between the deposited strip and the substrate surface, which is an unavoidable issue in manufacturing. In this paper, we propose a theoretical model that takes into account the interface damping and calculate the dispersion relationships both for frequency and attenuation of SAW resonance. This results show that the interface damping has an insignificant effect on resonance frequency, but, interestingly, attenuation of the SAW can decrease significantly in the high frequency region as the interface damping increases. Using picosecond ultrasound spectroscopy, we confirm the validity of our theory; the experimental results show similar trends both for resonant frequency and attenuation in the SAW resonance. Furthermore, the resonant behavior of the SAW is simulated using the finite element method, and the intrinsic cause of interface damping on the vibrating system is discussed. These findings strongly indicate the necessity of considering interfacial damping in the design of SAW devices.

We study the lattice thermal conductivity of isotope diamond superlattices consisting of 12C and 13C diamond layers at various superlattice periods. It is found that the thermal conductivity of a superlattice is significantly deduced from that of pure diamond because of the reduction of the phonon group velocity near the folded Brillouin zone. The results show that asymmetric superlattices with a different number of layers of 12C and 13C diamonds exhibit higher thermal conductivity than symmetric superlattices even with the same superlattice period, and we find that this can be explained by the trade-off between the effects of phonon specific heat and phonon group velocity. Furthermore, impurities and imperfect superlattice structures are also found to significantly reduce the thermal conductivity, suggesting that these effects can be exploited to control the thermal conductivity over a wide range.

The interplanar bond strength in graphite has been identified to be very low owing to the contribution of the van der Waals interaction. However, in this study, we use microscopic picosecond ultrasound to demonstrate that the elastic constant, $C_{33}$, along the $c$ axis of defect-free monocrystalline graphite exceeds 45 GPa, which is higher than reported values by 20\%. Existing theories fail to reproduce this strongly correlated interplanar system, and our results, thus, indicate the necessity for improvement. Since the LDA+U+RPA method, including both random phase approximation correlation and short-range correlation in $p$ Wannier orbitals, shows better agreement with the observation than LDA or even than ACFDT-RPA, the experimental results indicate non-negligible electron correlation effects with respect to both the short-range and long-range interactions., (Main text) 5 pages, 3 figures, (Supplementary material A) 2 pages, 2 figures, (Supplementary material B) 12 pages, 1 figure

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/abec5a., Elastic constants and sound velocities of nm order thin films are essential for designing acoustic filters. However, it is difficult to measure them for dielectric thin films. In this study, we use a three-layer structure where a dielectric nano-thin film is sandwiched between thicker metallic films to measure the longitudinal elastic constant of the dielectric film. We propose an efficiency function to estimate the optimal thicknesses of the components. We use Pt/NiO/Pt three-layer films for confirming our proposed method. The determined elastic constant of NiO deposited at room temperature is smaller than the bulk value by ∼40%. However, it approaches the bulk value as the deposition temperature increases. We also reveal that the uncertainty of the elastic constant of the Pt film insignificantly affects the accuracy of the determined elastic constant of NiO in this structure.

A highly sensitive hydrogen-gas sensor fabricated using MEMS technology is presented. The sensor chip consists of glass substrates, silicon substrate, and an AT-cut quartz crystal resonator, which is embedded in the microchannel constructed on the substrates. The quartz resonator has a fundamental resonant frequency of 165 MHz and a 200 nm palladium film deposited on its single surface as the hydrogen-gas sensing material. The MEMS hydrogen-gas sensor operates in a wireless manner by exciting and detecting the resonator vibration using the non-contacting antennas. The curvature induced resonant frequency change of the resonator plate caused by the expansion of the palladium film is used for the detection of the hydrogen gas. We succeeded in improving the hydrogen absorption rate and then the sensitivity for the hydrogen-gas detection by applying the air-plasma treatment method, and clarified the role of palladium oxide in lowering the energy barrier for the hydrogen-atom migration from surface to subsurface with the X-ray photoelectron spectroscopy. Thus sensitivity enhanced MEMS hydrogen-gas sensor exhibits a detection limit of 10 ppm or less at room temperature both in nitrogen and air.

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.

Nagakubo A., Lee H.T., Ogi H., et al. Elastic constants of beta tungsten thin films studied by picosecond ultrasonics and density functional theory. Applied Physics Letters, 116(2), 021901 (2020) https://doi.org/10.1063/1.5131768., Tungsten thin films are used for various applications and sometimes exhibit an A15 structure (β-W). They have some superior properties in comparison to the bcc structure (α-W), such as a higher superconducting transition temperature and larger spin Hall angle. However, elastic constants of β-W are unclear, which restricts mechanical applications and reliable density-functional-theory calculations. In this study, we synthesized α-W, β-W, and mixed-phase W films and determined their elastic constants by picosecond ultrasonics. We also calculated the elastic constants based on density functional theory and reveal that β-W has a larger elastic anisotropy and smaller shear modulus. Our calculation further indicates a stable stacking faulted β-W, which leads to a monoclinic structure.

Amorphous thin films are used as components of acoustic resonators because they could control temperature coefficient of resonator. Because elastic constants of thin films are often different from those of bulk materials, the direct measurement of sound velocity and temperature coefficient of velocity (TCV) for individual thin films becomes important in designing resonators. Because The TCV of amorphous silicon oxide is opposite in sign to that of silicon nitride, we expect that a specific-composition SiON thin film show the zero TCV. We here measure the longitudinal-wave velocity of SiON thin films with various nitride concentrations using picosecond ultrasound method.

This is the Accepted Manuscript version of an article accepted for publication in Applied Physics Express. 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/10.7567/1882-0786/ab600d., Progress in the wireless communication requires higher frequency bandpass filters, and the bulk acoustic-wave filter is especially the key device for the GHz-range communication. Its resonance frequencies depend on the elastic constants of individual layers, but it is never straightforward to measure them. In this study, we apply resonant ultrasound spectroscopy to a GHz-range free-standing AlN/Ru/Cr resonator to simultaneously determine the elastic constant of each layer. Using picosecond ultrasonics, we measure free-vibration resonance frequencies up to ∼70 GHz and inversely determine it. The obtained values are reasonable as thin films elasticity, confirming the applicability of the proposed method.

Zhou L., Nakamura N., Nagakubo A., et al. Highly sensitive hydrogen detection using curvature change of wireless-electrodeless quartz resonators. Applied Physics Letters, 115(17), 171901 (2019) https://doi.org/10.1063/1.5126135., We propose a hydrogen detection method using a thin quartz resonator with a palladium thin film on its single side, which absorbs hydrogen, resulting in the film expansion and geometry change of the resonator, shifting the resonant frequency. Because the resonator is driven in a wireless manner, the other resonator side can remain electrodeless, enhancing the geometry change and then the detection sensitivity. The detection limit is 1 ppm or less. We also investigate the reaction kinetics between palladium and hydrogen through the temperature dependence of the reaction velocity constant, yielding the activation energy of 0.372 ± 0.003 eV. This is close to the activation energy for the hydrogen-atom transition from the surface to subsurface states, indicating that the adsorption reaction is dominated by the transition rather than the bulk diffusion in palladium.

Nagakubo A., Tsuboi S., Kabe Y., et al. Zero temperature coefficient of sound velocity in vitreous silicon oxynitride thin films. Applied Physics Letters, 114(25), 251905 (2019) https://doi.org/10.1063/1.5098354., Vitreous silicon oxide (v-SiO2) shows anomalous phonon properties such as the positive temperature coefficient of velocity (TCV). Variation of the Si-O-Si bond angle between SiO4 tetrahedrons has been recognized to be the key, but the origin of TCV still remains unclear. In this study, we controlled the bond angle by doping nitrogen and measured TCV of vitreous silicon oxynitride thin films with various nitrogen concentrations using picosecond ultrasonics. TCV significantly decreases by adding a small amount of nitrogen, and it shows positive to negative values as the nitrogen concentration increases. We evaluated the bond-angle change by Fourier-transform infrared spectroscopy, which decreases with the increase in the nitrogen content. We also find that the temperature rise in nondoped v-SiO2 decreases the bond angle, leading to an increase in the sound velocity. We then reveal theoretically that the bond-angle change dominates the origin of the positive TCV. This study indicates the existence of a zero-TCV single material, and we discover that the specific content of v-SiO1.71N0.19 achieves this.

The elastic properties of single-crystalline ω (hexagonal) phase of titanium are studied. Understanding the elastic properties is important for the development of biomedical titanium alloys with a low Young’s modulus. However, the elastic properties of the ω phase have remained unclear because of the difficulty in preparing a large single crystal consisting of a single phase of the ω phase, even though the ω phase has been believed to exhibit a higher elastic modulus than the β (body-centered cubic) phase. In this work, pure titanium was severely deformed by high-pressure torsion processing, to obtain polycrystalline specimens consisting exclusively of the ω phase, which is metastable at room temperature. For the ω-phase polycrystal, the complete set of elastic stiffness components was measured by RUS combined with laser Doppler interferometery. By analyzing the elastic stiffness of the ω-phase polycrystal on the basis of an inverse Voigt–Reuss–Hill approximation, the elastic stiffness components of the single-crystalline ω phase were determined. The Young’s modulus of the ω phase along 〈0001〉 was found to be clearly higher than that along 〈 1 1 2 ¯ 0 〉 , and the shear modulus also exhibited anisotropy. Importantly, the Young’s modulus and shear modulus of the metastable ω phase were higher than those of the β phase and also higher than those of the α (hexagonal close-packed) phase, which is stable at room temperature. Furthermore, analysis by a micromechanics model using the determined elastic stiffness deduced the effect of ω phase formation on the elastic properties of β-phase titanium alloys.

We measured light scattering in a single crystal of ZrW2O8 (ZWO) in the frequency range between 0.1 and 1100 cm−1 and in the temperature range between 3.5 and 300 K. From the temperature dependence of the optical and acoustic phonon frequencies, we found that low-frequency phonon modes below 100 cm−1 have large relative temperature coefficients (total anharmonicity values). The longitudinal acoustic mode, whose Brillouin light scattering has not been reported thus far, was found to have the largest negative total anharmonicity in a wide temperature range, indicating the possibility that the acoustic modes, as well as the low-frequency optical modes that are associated with the motions of WO4 and ZrO6 polyhedra, should contribute to the negative thermal expansion of ZWO.

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.

Adachi K., Ogi H., Nagakubo A., et al. Piezoelectric coefficients of GaN determined by hopping conduction of carriers. Applied Physics Letters, 109(18), 182108 (2016) https://doi.org/10.1063/1.4966995., Using resonant ultrasound spectroscopy, we monitored the resonance-frequency and internal-friction behaviors of a GaN monocrystal at elevated temperatures. An internal-friction peak appears with increasing temperature, at which reduction of frequency occurs. The frequency shift reflects the disappearance of the apparent piezoelectricity due to hopping conduction of carriers, allowing us to accurately determine the piezoelectric coefficients eij. We measured the frequency decrements of eight vibrational modes to inversely determine three independent eij: Our values are e15=-0.22±0.02 C/m2, e31=-0.14±0.02 C/m2, e33=1.15±0.05 C/m2.

Diamond shows high Debye temperature (~2200 K) and small Gruneisen parameter (~1), which represent its small anharmonicity. Therefore, the temperature dependence of elastic constants is very small, preventing accurate measurement. Recently, we measured the temperature dependence of C 11 of diamond by using picosecond ultrasonics. [A. Nagakubo et al., Appl. Phys. Lett. 108, 221902 (2016).]. Our results indicate that diamond has a further higher Debye temperature than previous measurements, and we found that elastic constants of high-Debye-temperature materials should be measured over a wide temperature range to extract the Debye temperature and the Gruneisen parameter, accurately. We also calculate the temperature dependence of C 11 by an ab-initio method, which agree with our measurement. In this study, we discuss the temperature dependence of other elastic constants. We found that careful consideration is required for bond-bending and bond-stretching resistance to evaluate the temperature dependence of i...

Adachi K., Ogi H., Nagakubo A., et al. Elastic constants of GaN between 10 and 305 K. Journal of Applied Physics, 119(24), 245111 (2016) https://doi.org/10.1063/1.4955046., Using the antenna-transmission resonant ultrasound spectroscopy, we measured the elastic constants of GaN between 10 and 305 K using 72 resonance frequencies. The mode Grüneisen parameter is determined from temperature dependence of each elastic constant, which is larger along the c axis than along the a axis, showing anisotropy in lattice anharmonicity. The zero-temperature elastic constants, determined using the Einstein-oscillator model, yield the Debye characteristic temperature of 636 K. The ab-initio calculation is carried out for deducing the elastic constants, and comparison between calculations and measurements at 0 K reveals that the local-density-approximation potential is preferable for theoretically evaluating characteristics of GaN. The theoretical calculation also supports the anisotropy in lattice anharmonicity.

Nagakubo A., Arita M., Ogi H., et al. Elastic constant C 11 of 12C diamond between 10 and 613 K. Applied Physics Letters, 108(22), 221902 (2016) https://doi.org/10.1063/1.4952613., We measured the temperature dependence of the elastic constant C11 of a 12C diamond monocrystal using picosecond ultrasonics between 10 and 613 K. We found that C11 is almost temperature independent below room temperature; the temperature coefficient around 300 K is -6.6 MPa/K. Our results show a significantly higher Einstein temperature than reported values by ∼30%, indicating that diamond has a larger zero-point energy, which remains dominant around ambient temperature. We also calculated the temperature dependence of the elastic constants using ab-initio methods, resulting in good agreement with measurements. Our study shows that below-ambient-temperature measurements are not sufficient to extract the Debye temperature and the Grüneisen parameter of high-Debye-temperature materials.

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.

Optical properties of Si are important and widely studied. However, temperature dependence of the refractive index for visible light at low temperatures has not been measured. In this study, we measured the complex refractive index of Si at 400 nm between 10 and 300 K by picosecond ultrasound spectroscopy. The measured refractive index at room temperature agrees well with reported values, confirming the accuracy of our measurement, and we found that the refractive index at 10 K is smaller than reported values.

Nagakubo A., Ogi H., Ishida H., et al. Temperature behavior of sound velocity of fluorine-doped vitreous silica thin films studied by picosecond ultrasonics. Journal of Applied Physics, 118(1), 014307 (2015) https://doi.org/10.1063/1.4923353., Vitreous silica (v-SiO2) shows anomalous temperature dependence of velocity, including positive temperature coefficient of velocity (TCV) and velocity minimum around 70 K. The former characteristic allows its application in acoustic-resonator devices as a temperature compensating material. In this paper, we study the temperature dependence of velocity of fluorine-doped v-SiO2 (v-SiO2-xFx) thin films using picosecond ultrasonic spectroscopy. To correct the temperature increase caused by irradiation with light pulses, we calculated the steady temperature increase in the measuring volume with a finite volume method, considering the temperature dependence of thermal conductivity, and find that temperature in the measurement region remains high even when the back surface is cryogenically cooled. Using the corrected temperature, we determine TCV of v-SiO2-xFx thin films for 0 < x < 0.264, which increases as x increases and is smaller than reported bulk values by a factor of 0.5 - 0.7. The velocity minimum is absent for the film with x = 0, but it is clearly observed at 70 K for the film with the highest fluorine concentration of x = 0.264. These temperature behaviors are attributed to the change in the Si-O-Si bond angle caused by the fluorine doping.

Nagakubo A., Arita M., Yokoyama T., et al. "Acoustic properties of co-doped AlN thin films at low temperatures studied by picosecond ultrasonics", Japanese Journal of Applied Physics, 54(7), 07HD01 (2015) https://doi.org/10.7567/JJAP.54.07HD01., (Mg0.5Zr0.5)xAl1-xN and (Mg0.5Hf0.5)xAl1-xN thin films are AlN-base piezoelectric materials, and their piezoelectric coefficients are higher than those of pure AlN, being promising materials for acoustic devices. However, their acoustic properties remain unknown because of measurement difficulty for deposited thin films. In this study, we measure their longitudinal-wave elastic constants C33 and their temperature coefficients using picosecond ultrasound spectroscopy for 0 < x < 0.13; we obtain C33 = 398.2 ± 0.7GPa for pure AlN, and it largely decreases by doping Mg, Zr, and Hf, leading to a minimum values of 316.8 ± 1.6 GPa for (Mg0.5Zr0.5)0.126Al0.874N.

Nagakubo A., Ogi H., Sumiya H., et al. Elasticity and hardness of nano-polycrystalline boron nitrides: The apparent Hall-Petch effect. Applied Physics Letters, 105, 8, 081906. https://doi.org/10.1063/1.4894377., Nano-polycrystalline boron nitride (BN) is expected to replace diamond as a superhard and superstiff material. Although its hardening was reported, its elasticity remains unclear and the as-measured hardness could be significantly different from the true value due to the elastic recovery. In this study, we measured the longitudinal-wave elastic constant of nano-polycrystalline BNs using picosecond ultrasound spectroscopy and confirmed the elastic softening for small-grain BNs. We also measured Vickers and Knoop hardness for the same specimens and clarified the relationship between hardness and stiffness. The Vickers hardness significantly increased as the grain size decreased, while the Knoop hardness remained nearly unchanged. We attribute the apparent increase in Vickers hardness to the elastic recovery and propose a model to support this insight. © 2014 AIP Publishing LLC.

Nagakubo A., Ogi H., Sumiya H., et al. Elastic constants of cubic and wurtzite boron nitrides. Applied Physics Letters, 102(24), 241909 (2013) https://doi.org/10.1063/1.4811789., We synthesized pure polycrystalline cubic boron nitride (cBN) and wurtzite boron nitride (wBN) by the direct conversion method from hexagonal boron nitride, and measured their longitudinal-wave elastic constants CL between 20 and 300 K using picosecond ultrasound spectroscopy. Their room-temperature values are 945 ± 3 GPa and 930 ± 18 GPa for cBN and wBN, respectively. The shear modulus G of cBN was also determined by combining resonance ultrasound spectroscopy and micromechanics calculation as G = 410 GPa. We performed ab-initio calculations and confirmed that the generalized gradient approximation potential fails to yield correct elastic constants, which indicated the necessity of a hybrid-functional method. © 2013 AIP Publishing LLC.

We measured the longitudinal-wave velocity and its attenuation in SrTiO3 between 20 and 300 K using picosecond ultrasound spectroscopy. From the temperature dependence of the velocity and attenuation, we monitored the cubic–tetragonal phase transition of SrTiO3 near 100 K, whereas no more transitions were indicated below 100 K. From the measured attenuation coefficients, we estimate the relaxation time τ. Because of the ultrahigh frequency measurements, the product ωτ is larger than unity, where the traditional theory for phonon–phonon interaction fails to explain the relaxation time. We then derived the relationship between the relaxation time and attenuation for ωτ>1.