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Oxide vapor phase epitaxy (OVPE) has attracted much attention as a highly efficient method for synthesizing high-quality bulk GaN crystals, but the mechanical properties of OVPE GaN have not been clarified. We measured the five independent elastic constants of the OVPE GaN by resonant ultrasound spectroscopy. The in-plane Young modulus E _1 and shear modulus C _66 of the OVPE GaN are smaller than those of the hydride vapor phase epitaxy GaN by 1.8% and 1.3%, respectively. These reductions agree with predictions by density functional theory calculations. We also calculated the Debye temperature, revealing that oxygen impurity decreases its magnitude.

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

(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 ± 0.7 GPa 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 ∼ mm dimensions. [ABSTRACT FROM AUTHOR]