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The rate of change e ( k , t ) = ∂ E ( k , t ) ∕ ∂ t of the energy spectrum E ( k , t ) of a weak turbulence in a system with a decay dispersion relation is investigated with particular interests in the initial stage of evolution such as the first several tens of periods. The theoretical predictions given by the kinetic equation of Hasselmann and that of Janssen which have been derived by the weak turbulence theory are compared with the results of direct numerical simulation (DNS) with a great accuracy. It is shown that e ( k , t ) predicted by Janssen’s equation correctly reproduces the rapid variation of e ( k , t ) with the linear time-scale which is observed in DNS, while that predicted by Hasselmann’s equation does not. It is also shown that the transient behavior of the asymptotic approach of Janssen’s e ( k , t ) to Hasselmann’s one strongly depends on the wavenumber. For wavenumbers smaller than the spectral peak, the approach is exponential, while for wavenumbers larger than the spectral peak it contains a damped oscillation whose amplitude decays in time like 1 ∕ t . A reasonable explanation for the origin of this damped oscillation is given in terms of the frequency mismatch of the three-wave “sum” interactions.

A powerful propulsion device that enables ultimate simplification is one of the core technologies of microswimmers and microrobots. Here, we report a high-speed swimmer that is self-propelled by th...

A metachronal motion produces a net flow in a low Reynolds regime and is thus important for the development of artificial cilia. In this study, we report that a thermally actuated double pendulum exhibits a metachronal motion using a self-propulsion mechanism caused by spontaneous asymmetrical heat transfer. Specifically, by using a multiple pendulum structure in which three beams of different lengths are hung in parallel in order of length on a U-shaped nichrome heater, we demonstrate that phase differences can be produced among beams. Our device can be applied to biomedical microfluidic systems or microrobots in water.

Mass production-ready technologies of AlGaN/AlN/GaN high-electron-mobility transistor (HEMT) structures on 200 mm diameter silicon substrates are developed using a large-scale metal-organic chemical vapor deposition system. High-yield epitaxial substrates on large-diameter wafers are required for reducing the cost of industrializing power device applications. Through multi-point vertical leakage current measurements, it was confirmed that the AlGaN/AlN/GaN HEMT, under optimum growth conditions, showed high yield characteristics such as a highly uniform leakage current and a low number of breaking points over the entire 200 mm diameter wafer. On introducing an AlN spacer layer between the AlGaN Schottky barrier and the GaN channel, a lower on-state resistance for power devices can be expected. Cross-sectional transmission electron microscopy images revealed that the thin AlN spacer layer was grown between the AlGaN and GaN layers with atomically abrupt and flat interfaces. HEMT structures with an AlN spacer layer exhibited a considerably high two-dimensional-electron-gas mobility of 2000 cm2 V−1 s−1 at room temperature and 10 700 cm2 V−1 s−1 at 77 K. The AlN spacer layer between the AlGaN and GaN layers was successfully fabricated and suppressed the alloy disorder scattering effect.

Microorganisms can generate net flows effectively even in a low Reynolds number regime by using asymmetric beating motion of hair-like cilia. Here, we demonstrate a high-speed novel artificial cilium driven by the heat engine using a self-propelled swing motion in a nucleate- to film-boiling regime. In particular, we report the asymmetrical motion of the thermal cilium with an asymmetric joint structure. We believe that our thermal cilium opens a new way to develop a microfluidic circuit., Article, JOURNAL OF THE PHYSICAL SOCIETY OF JAPAN.89(11):114402(2020)

Interaction between surface and interfacial waves with continuous energy spectra in a two-layer density stratified fluid system is investigated numerically. For an initial wave field which consists only of the surface waves all propagating in the same direction, it is confirmed that the spectra $S_{s}(k)$ of the surface waves and $S_{i}(k)$ of the interfacial waves change significantly due to the recently found class 3 triad resonance. When the bulk of the surface wave spectrum $S_{s}(k)$ is initially located well above the critical wavenumber $k_{crit}$, below which the class 3 triad resonance is prohibited, $S_{s}(k)$ downshifts gradually toward the lower wavenumber during the initial stage of evolution. However, this downshift halts when the peak of $S_{s}(k)$ reaches around $k_{crit}$, and after that a steep peak forms in $S_{s}(k)$ around $k_{crit}$. It is confirmed that the timescale of the spectral evolution is of $O(1/{\it\epsilon}^{2})$ (${\it\epsilon}$ is a characteristic non-dimensional wave amplitude) in most of the $k$ space, consistent with the prediction of the wave turbulence theory for a system with a decay-type dispersion relation. However, it is also found that the timescale of the formation and growth of the sharp peak in $S_{s}(k)$ around $k_{crit}$ is of $O(1/{\it\epsilon})$, i.e. the timescale of the deterministic three-wave resonance.

This paper presents an intensive discussion on conducted electromagnetic interference (EMI) emissions from an inverter-driven air conditioner. The common-mode voltages produced by the pulse-width-modulation (PWM) inverter may cause a high-frequency leakage current. The use of a spectrum analyzer and a line-impedance stabilization network (LISN) makes it possible to refer the leakage current in the time domain to the conducted-EMI emissions in the frequency domain, which are prescribed by the EMI regulations. Estimating the conducted-EMI emissions by means of computer simulation in advance is beneficial for engineers who try to efficiently design a cost-effective EMI-filter intended for meeting the regulations. This paper achieves modeling of common-mode voltages, leakage-current propagation paths, and an EMI-filter for an inverter-driven air conditioner. The simulation circuit based on the developed models makes it possible to predict conducted-EMI emissions. Simulated results agree well with experimental ones in both cases of the EMI-filter being connected and disconnected.

A series of numerical studies has been performed to investigate the statistical characteristics of fluctuations of energy s = | a k | 2 of each component wave and its time derivative s = d | a k | 2 / d t in a wave turbulence for a Hamiltonian system with a non-decay type dispersion relation. It has been found that the probability density functions (PDF) of s and s evaluated among independent realizations are almost equivalent to the PDF’s evaluated among nearby modes in the k space in a single realization. Therefore, the averaging over the realizations and that over the nearby modes in a single realization are interchangeable. It has also been found that the PDF of s evolves in time toward the χ 2 distribution with two degrees of freedom, while that of s evolves toward the Laplace distribution (or the two-sided exponential distribution) at any wavenumber k , both in the peripheral of the spectrum (large k region) and in the energy-containing range (small k region).

AlGaN films and deep ultraviolet light-emitting diode (UV-LED) structures with AlGaN multi-quantum wells (MQWs) were grown directly on 2-in.-diameter epitaxial AlN/sapphire template (AlN template) by metalorganic chemical vapor deposition (MOCVD). It was confirmed that crack-free and good-crystal-quality AlGaN films and UV-LED structures can be grown on AlN template without the use of any buffer layer technique such as superlattice structures or pulsed atomic layer deposition. This seemed to be mainly due to the in-plane compressive stress caused by using AlN template as underlying substrates. Deep UV-LED devices were successfully fabricated using MOCVD-grown wafers. Electroluminescence spectra for those LEDs exhibited sharp near-band emissions ranging from 265 to 277 nm, depending on their Al compositions in MQWs. It was also confirmed that the insertion of a thin AlN layer between the active region and the p-type cladding layer can drastically suppress a parasitic sub-band emission around 320 nm.

A stress control method was proposed to suppress crack generation in the hydride vapor phase epitaxy (HVPE) of a thick AlN film on an AlN/sapphire template, i.e., a high-compressive-strain (HCS) AlN/sapphire template was employed as a substrate. The AlN films on the HCS AlN/sapphire template were crack-free and very smooth. However, the AlN films on the normal AlN/sapphire template (with a reduced or fully relaxed compressive strain) contained many cracks. This is because the HCS in the AlN/sapphire template may balance some of the thermal tensile strain resulting from the different thermal expansion coefficients of AlN and sapphire and thus suppress the crack generation. The typical full width at half maximum (FWHM) values of X-ray rocking curves for the (0002), (1012), and (1010) diffractions of the HVPE-grown AlN film on the HCS template were 127, 381, and 520 arcsec, respectively. Accordingly, by atomic force microscopy (AFM), clear atomic steps were observed on the HVPE-grown AlN films on the HCS template, and the root-mean-square (RMS) roughness value of the AlN films was 0.361 nm.

The temporal evolution of the energy spectrum of a field of random surface gravity waves in deep water is investigated by means of direct numerical simulations of the deterministic primitive equations. The detected rate of change of the spectrum is shown to be proportional to the cubic power of the energy density and to agree very well with the nonlinear energy transfer Snl as predicted by Hasselmann. Despite the fact that use of various asymptotic relations that are valid only for t → ∞ or integration with respect to time over a time scale much longer than O[period × (ak)−2] is necessary in the derivation of Hasselmann’s Snl, it is clearly demonstrated that the rate of change of the spectrum given by the numerical simulation agrees very well with Hasselmann’s Snl at every instance of ordinary time scale comparable to the period. The result implies that the four-wave resonant interactions control the evolution of the spectrum at every instant of time, whereas nonresonant interactions do not make any significant contribution even in a short-term evolution. It is also pointed out that the result may call for a reexamination of the process of derivation of the kinetic equation for the spectrum.

We study the self organized growth of GaN quantum dots deposited by molecular beam epitaxy on Al x Ga 1-x N, with x varying between 1 and 0.21. As a result of the specific strain relaxation mechanism of Al x Ga 1-x N on AlN, it is found that dots can be formed for an Al content as low as about 30%. Monopolar devices have been realized by depositing Schottky contacts on n-type doped Al 0.5 Ga 0.5 N. Finally, electroluminescence of Eu-doped GaN quantum dots embedded in n-type Al 0.5 Ga 0.5 N has been demonstrated, emphasizing the potentialities of such an active layer for room temperature visible light emission.

We report on the growth of Tb-doped GaN quantum dots deposited by plasma-assisted molecular beam epitaxy. It was found that Tb ions enhance the desorption process of Ga ad-atom on the growing surface. This effect becomes more important with increasing growth temperature, and above a certain amount of Tb, the QDs formation is completely inhibited. Transitions from the 5D4 excited state were found intense and temperature stable, while the transitions from 5D3 excited state were only observed at low temperature. A comparative optical study with Tb-doped AlN thick layer indicates that Tb ions are incorporated in GaN QDs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on a comparative optical study of molecular beam epitaxy grown GaN:Eu quantum dots (QDs) and GaN:Eu layer. In the GaN:Eu layer the narrow emission lines of the 5D0-7F2 transition allow to identify two different Eu locations corresponding to near the sample surface and lying deeper in the bulk. Eu3+ emission from QDs has been found to be at the same spectral position than that assigned to Eu3+ ions located in the bulk of the GaN:Eu layer. Eu emission in the temperature range 4-300 K is thermally stable in doped QDs, but exhibits a strong thermal quenching in the doped layer, particularly for Eu3+ ions located near the surface. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Silicon doped AlxGa1–xN with high Al mole fraction was grown using epitaxial AlN as a underlying layer by low-pressure metalorganic vapor phase epitaxy. n-type conductivity control of high quality Si-doped AlxGa1–xN was achieved successfully. Structural, electrical and optical properties were investigated for various Al mole fractions, flow rates of silicon, and AlxGa1–xN thickness, respectively. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on a comparative optical study of InGaN:Eu quantum dots (QDs) and GaN:Eu layer grown by molecular beam epitaxy (MBE). Analysis of the 5D0 → 7F2 transition as a function of the excitation wavelength shows that Eu3+ ions in InGaN:Eu QDs are located inside InGaN QDs and also in the GaN barrier layer. The existence of Eu3+ ions in the GaN barrier layer is explained by Eu segregation/diffusion during growth. For Eu3+ ions located inside InGaN QDs the photoluminescence (PL) shows only a slight decrease with temperature from 5 K to 300 K. In contrast, the PL from Eu3+ ions in the GaN barrier layer or in GaN thick layer shows a much more pronounced thermal quenching.

We report on the MBE growth of GaN and InGaN quantum dots (QDs) doped with rare earth ions, namely Eu, Tm and Tb exhibiting red, blue and green luminescence, respectively. Intense photoluminescence/cathodoluminescence is observed, resulting from the spatial localization of rare earth ions in dots combined with the confinement properties of the carriers. White light emission has been produced by combining the three rare earths in a multilayer sample of stacked GaN QD planes. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Different-Al-content AlxGa1-xN/GaN (0.26≤x≤0.52) high-electron-mobility transistor (HEMT) structures were grown on large-area 100-mm-diameter sapphire substrates by metalorganic vapor phase epitaxy (MOVPE). A low sheet resistance of approximately 380 Ω/sq. and an in-wafer total variation of 7.6% with a standard deviation of 1.6% were obtained for the sample with the highest Al content of 0.52. The two-dimensional electron gas (2DEG) density and room-temperature Hall mobility of the Al0.52Ga0.48N/GaN HEMT structures were 1.76×1013/cm2 and 971 cm2/Vs with in-wafer total variations of 7.8% and 8.1%, respectively. HEMTs were successfully fabricated and they exhibited good dc characteristics. The dependence of maximum source–drain current density on Al content was consistent with the Al content dependence of sheet carrier concentration. The highest source–drain current density of 1033 mA/mm with a maximum extrinsic transconductance of 228 mS/mm was obtained for 2-µm-gate-length Al0.52Ga0.48N/GaN HEMTs.

AlInN layers were directly grown on AlN/α-Al 2 O 3 (0 0 0 1) epitaxial templates by metalorganic vapor phase epitaxy. Indium incorporation coefficient was decreased with increasing the growth temperature. The crystalline quality and the composition of the grown films were strongly affected with the growth temperature and large compositional splitting was observed at 750 °C. Both photoluminescence and cathodoluminescence showed very complicated spectra in the photon-energy range of 2.4–4.3 eV, and the origin of these emissions were caused by a potential minima formed by compositional inhomogeneity. Absorption edge for the Al 0.91 In 0.09 N was larger than 4.3 eV.

We demonstrate Eu, Tm and Er doping of GaN quantum dots. In Er-doped samples, two very sharp blue lines were observed. These lines were dominant over the green lines reported in the case of Er-doped GaN thick layers. However, intensities were very low in comparison with Eu-doped and Tm-doped GaN QDs. Intense blue emission from Tm-doped GaN QDs was observed. Temperature-dependent photoluminescence was studied, and the intensity only decreased to 27% between room temperature and 10 K. Structural studies on Tm-doped samples showed that the kinetic role of Tm atoms was different from that of Eu atoms.

The effects of AlN overgrowth on the structural properties of GaN nanostructures (quantum wells and quantum dots) grown by plasma-assisted molecular beam epitaxy have been investigated using Rutherford backscattering spectroscopy, transmission electron microscopy, and reflection high-energy electron diffraction. The capping process induces a remarkable change in the dimensions of the nanostructures. The overgrowth process implies a thinning of the GaN quantum well and an isotropic reduction of the GaN island size. We demonstrate that this thickness/size reduction affects only the top GaN/AlN interface. The phenomenon is attributed to an exchange mechanism between Al atoms from the cap layer and Ga atoms in the nanostructures. We also demonstrate that this exchange is thermally activated and depends on the strain state of the nanostructures.

The effects of discretization of the spectrum on the evolution of weak turbulence of surface gravity waves are investigated by direct numerical simulations. Discretization of the spectrum is implemented by imposing periodic boundary conditions on the horizontal plane. Initial wave fields are constructed so that they have the JONSWAP spectrum, and the nonlinear energy transfer among component waves is estimated by following their evolution deterministically according to the primitive governing equations. It is found that the discretized system can produce the same nonlinear energy transfer as that predicted theoretically for a continuous spectrum, even when the distribution of component waves on the wavenumber plane is so sparse that the corresponding area of the water surface on the horizontal plane is just 4λp×4λp, with λp being the wavelength corresponding to the peak of the spectrum, provided that the ensemble averaging is taken over a sufficient number of realizations. The regime of “frozen” turbulence, which is known to appear in the weak turbulence of capillary waves with a discretized spectrum, is not observed in the case of gravity waves. This difference in the effects of discretization of the spectrum on the weak turbulence of capillary waves and that of gravity waves is discussed in relation to the number of trios or quartets of component waves which satisfy the conditions for quasi-resonant nonlinear interactions.

Conductivity control of 1-μm-thick Si-doped Al0.5Ga0.5N using an epitaxial AlN underlying layer wasrealized by introducing CH3SiH3 during low-pressure metalorganic vapor phase epitaxy. Cathodoluminescence measurements indicated that luminescence from a deep-level transition was caused by silicon doping. X-ray diffraction measurements showed that fluctuations of the tilt component of the c-axis increased with an increasing the silicon dopant flow rate. The electron concentration increased linearly with increasing the CH3SiH3 flow rate from 3.5 × 1017 cm−3 to 9.3 × 1017 cm−3. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We demonstrate the growth of GaN quantum dots doped with Eu by plasma-assisted MBE. Intense emission from the intra-atomic 4f transition of Eu3+ was observed at 622 nm. GaN quantum dots (QDs) doped with 1% Eu showed the maximum emission. The full-width at half maximum (FWHM) of the CL spectrum of Eu-doped GaN QDs was 2.1 nm at room temperature. Eu doping increased the size of QDs by 4 times and decreased the density of QDs by one order of magnitude. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We propose a procedure to grow GaN quantum dots (QDs) on AlN by using the Ga surfactant effect in plasma-assisted molecular beam epitaxy. Self-formed GaN islands were spontaneously generated under vacuum, after evaporation of the Ga bilayer stabilizing the two-dimensional GaN layer grown under Ga-rich conditions. Island characteristics (size and density) are studied as a function of the nominal amount of GaN deposited. We demonstrate that the QD density can be controlled in the 3×1010 cm−2–2×1011 cm−2 range. It is shown that beyond a given amount of GaN nominally deposited, there is a coexistence between elastic and plastic relaxation, with GaN islands being formed on a partially relaxed two-dimensional GaN layer thicker than two monolayers.

High-quality GaN films were grown on epitaxial AlN/sapphire templates by metal-organic vapor phase epitaxy. The Hall mobility as high as 790 cm2/V s with the carrier concentration of 7.6×1016 cm−3 at 300 K along with low dislocation density of 5×107 cm−2 have been achieved. The X-ray rocking curve full-width at half-maximum were 61 and 232 arcsec for the (0 0 0 4) and (2024) reflections, respectively. Atomic force microscopy images showed smooth surface morphology and clear step formation. Additionally, pit terminations could be hardly found. Photoluminescence measurement revealed a higher intensity near the band edge.

The temporal evolution of nonlinear wave fields of surface gravity waves is studied by large-scale direct numerical simulations of primitive equations in order to verify Hasselmann's theory for nonlinear energy transfer among component gravity waves. In the simulations, all the nonlinear interactions, including both resonant and non-resonant ones, are taken into account up to the four-wave processes. The initial wave field is constructed by combining more than two million component free waves in such a way that it has the JONSWAP or the Pierson–Moskowitz spectrum. The nonlinear energy transfer is evaluated from the rate of change of the spectrum, and is compared with Hasselmann's theory. It is shown that, in spite of apparently insufficient duration of the simulations such as just a few tens of characteristic periods, the energy transfer obtained by the present method shows satisfactory agreement with Hasselmann's theory, at least in their qualitative features.

A-plane (1210) AlN films were deposited on the off-angle R-plane (1 102) sapphire substrates, in which the surface plane was tilted towards the [1101] sapphire direction, by metalorganic chemical vapor deposition in order to clarify off-angle effects. The polar direction of the AlN film was inverted by changing the sign of the off-angle and the off-angle was effective in restraining the generation of inverted twins in the AlN. A minus off-angle was found to improve total crystal quality of the AlN from X-ray diffraction results. High-resolution transmission electron microscopy images in the vicinity of interfaces between the AlN and the sapphire indicated that atomic arrangement at an initial AlN growth stage influenced the total crystal quality.

Linearly aligned tetranuclear Pt(II)-Cr(II)-Cr(II)-Pt(II) complexes were prepared using a tridentate pyphos ligand (pyphos = 6-diphenylphosphino-2-pyridonate). The bonding between Cr(II) and Pt(II) was discussed on the basis of magnetic measurements. A dichromium complex, [Cr2(pyphos)4] (1), was prepared by the reaction of [Cr2(OAc)4] with the sodium salt of pyphos ligand. The complex 1 has four phosphorus atoms capable of holding transition metals at both of the axial positions of the Cr2 moiety. Three tetranuclear Pt(II)-Cr(II)-Cr(II)-Pt(II) complexes, i.e., [Cr2Pt2Me4(pyphos)4] (2), [Cr2Pt2Cl4(pyphos)4] (3), and [Cr2Pt2Cl2Me2(pyphos)4] (4), have been prepared by the addition of the respective platinum species, PtIIMe2, PtIICl2, and PtIIMeCl, to the axial positions of the Cr2 moiety of 1. Magnetic measurements of 1-4 together with a crystallographic study for the complexes 1 and 2 revealed that the quadruply-bonded Cr-Cr distance was delicately altered by the nature of the Pt atoms, i.e., the kind of su...

A method of studying nonlinear random fields of surface gravity waves by direct numerical simulation is developed by combining the “high-order spectral method” of West et al. (J. Geophys. Res. 92 (1987) 11,803–11,824) and the complex amplitude function b( k ) introduced by Zakharov (J. Appl. Mech. Tech. Phys. (Engl. Transl.) 2 (1968) 190–194) in his Hamiltonian formalism of the water wave problem. By computing the rate of change of b( k ) for a given wave field by both the high-order spectral method and Zakharov's equation, it is confirmed that the high-order spectral method, when the order of nonlinearity is appropriately chosen, gives exactly the same result as Zakharov's equation within a much shorter computation time. Methods of evaluating the frequency spectrum, directional spectrum and the component phase speed are given. Results of some preliminary computations are also presented to prove the great potentialities of the present approach as a tool for the examination of various characteristics of nonlinear random wave fields with continuous spectra, such as ocean waves.

Effect of ion beam etching on laser-induced surface damage was investigated in CsLiB6O10 (CLBO) crystaland fused silica. In high-power UV lasers, these optical surfaces were damaged due to absorption of thepolishing compound embedded inside the surface. From SIMS analysis, the polishing compounds were foundto be embedded in the CLBO and fused silica surfaces to a depth of 60 nm. These polishing compounds couldbe removed by using Ar ion beam etching. No ion beam-induced surface damage was observed when theapplied ion beam voltage was less than 200 V. Due to the effect of polishing compounds removal on damageresistivity, the laser-induced surface damage threshold of CLBO was improved up to 16 J/cm2 (wavelength: 355 nm, pulse width: 0.85 ns) as compared with that of the as-polished surface 11 J/cm2. The laser-inducedsurface damage of fused silica also increased from 7.5 J/cm2 to 14.5 J/cm2.

The structure of freeze-dried atactic polystyrene (a-PS) prepared from dilute benzene and cyclohexane solutions was examined by using Fourier transform infra-red spectroscopy, wide-angle X-ray scattering and differential scanning calorimetry (d.s.c.). The phenyl–phenyl correlation in the freeze-dried a-PS is weak compared with that in the reference sample cast from a 5 wt% solution in tetrahydrofuran. The data suggest an expanded chain conformation for the specific structure of freeze-dried a-PS, which can easily be formed from a good solvent at high temperature, and with high-molecular-weight a-PS. The d.s.c. data of freeze-dried a-PS showed an endothermic peak just below the glass transition temperature, indicating an enthalpy difference between the freeze-dried and melt states.