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Inertial migration has proven effective for high-throughput manipulation of tiny particles in confined flows. However, complex and even controversial relationships between hydrodynamic forces and flow conditions hinder the development of an explicit formula for inertial lift acting on finite-sized particles at Reynolds numbers in the hundreds. Herein, we reveal the different scaling laws for shear gradient-induced inertial lift and wall-induced inertial lift by separating the contributions of pressure stress and viscous stress. A direct result of this treatment is a new explicit formula for the total inertial lift, valid for Reynolds numbers up to 400 and particle blockage ratios up to 0.25. This study provides an alternative way to understand inertial lift, thereby greatly facilitating the prediction of particle migration in practical microfluidic applications. [ABSTRACT FROM AUTHOR]

The orthorhombic O and O′ structure phases of ABO3-type perovskites possess the same symmetry and quite different physical properties. In this work, a pressure induced isostructural O′–O phase transition with volume expansion and less distortion in DyFeO3 orthoferrite was experimentally observed and studied by high-pressure x-ray diffraction and Raman spectroscopy measurements. Up to 35.2 GPa, DyFeO3 crystallizes in an orthorhombic Pbnm symmetry structure. With no change in the symmetry group, distinct O′-phase and O-phase were verified below 13.1 GPa and above 15.1 GPa, respectively. Meanwhile, the lattice becomes less distorted and metrically pseudo-cubic on the high-pressure side. The cell volume expands by a step in the intermediate pressure window accompanied by a decrease in spontaneous strain. It is suggested that FeO6 octahedra are more compressible than DyO12 sites under pressure. This work claimed the physical mechanism underlying phonon dynamics and variation of local structure symmetry of DyFeO3, which can be considered as the basic view for RFeO3 perovskites. [ABSTRACT FROM AUTHOR]

Beta gallium oxide (β-Ga2O3) is an emerging ultrawide bandgap (∼4.8 eV) semiconductor with attractive properties for future power and radio frequency (RF) electronics, optoelectronics, and sensors for detecting gases and solar-blind ultraviolet radiation. Beyond such promises, β-Ga2O3 crystal possesses excellent mechanical properties, making it pertinent as a material for micro/nanoelectromechanical systems (M/NEMS). Here, we present an overview and perspective on the emerging β-Ga2O3 M/NEMS and their roles in supplementing Ga2O3 power and RF electronics. We review the development of β-Ga2O3 micro/nanomechanical devices and precise extraction of mechanical properties from these devices. We evaluate the design for frequency scaling up to over 4 GHz by tuning the device geometry and dimensions. Toward technological applications, β-Ga2O3 M/NEMS are analyzed in two aspects: β-Ga2O3 vibrating channel transistors for potential integration with β-Ga2O3 power and RF electronics with operating frequency beyond 1 GHz, and β-Ga2O3 resonant transducers for photon radiation detection with scaling of responsivity and response time. With analytical prediction, we envision challenges and propose strategies and schemes in efficient electromechanical transduction engineering, frequency scaling, and design and fabrication for future development of β-Ga2O3 M/NEMS. [ABSTRACT FROM AUTHOR]

Reciprocity is a basic property existing in wave phenomena characterized by complex transmission coefficients. Reciprocity in a duct describes the relationship between the direct response at the duct outlet to a source applied at the inlet and the reverse response at the inlet to a source at the outlet. The reciprocity has been adopted to simplify the methodology for predicting the reverse transmission coefficient from the direct coefficient and vice versa. The existing methods, however, cannot predict the complex form of the coefficient because they only consider the reciprocity in the moduli but neglect the reciprocity in the phases. The study on the reciprocity in a duct with a temperature gradient has hardly been seen. The present work derived a reciprocal equation with a time domain method to characterize the reciprocity in a duct with spatially varying temperature. In addition to considering the reciprocity in the moduli of the direct and reverse transmission coefficients, the proposed method invented a correction term to consider the reciprocity in their phases as well. The correction term was developed from the time delay between the incident and the transmitted pulses in the duct solved with the time domain method. The modified reciprocal equation using the correction term was derived and used to predict the reverse transmission coefficient for calculating exhaust noise of an engine connecting to the duct. The result has shown that the proposed time domain method is an effective approach to accurately describe the phase relationship of waves in flow ducts. [ABSTRACT FROM AUTHOR]

We report on the nondestructive measurement of Young's modulus of thin-film single crystal beta gallium oxide (β-Ga2O3) out of its nanoscale mechanical structures by measuring their fundamental mode resonance frequencies. From the measurements, we extract the Young's modulus in the (100) plane, EY,(100) = 261.4 ± 20.6 GPa, for β-Ga2O3 nanoflakes synthesized by low-pressure chemical vapor deposition (LPCVD), and in the [010] direction, EY,[010] = 245.8 ± 9.2 GPa, for β-Ga2O3 nanobelts mechanically cleaved from bulk β-Ga2O3 crystal grown by the edge-defined film-fed growth (EFG) method. The Young's moduli extracted directly on nanomechanical resonant device platforms are comparable to theoretical values from first-principle calculations and experimentally extracted values from bulk crystal. This study yields important quantitative nanomechanical properties of β-Ga2O3 crystals and helps pave the way for further engineering of β-Ga2O3 micro/nanoelectromechanical systems (M/NEMS) and transducers. [ABSTRACT FROM AUTHOR]

Insertion loss (IL) is a more useful acoustic index than noise reduction and transmission loss for muffler design, but it is harder to predict because the prediction of IL requires knowing the characteristics of the muffler itself and the acoustic source. A muffler with porous material and complex meanflow makes the prediction even more challenging. Although calculation of noise reduction and transmission loss with the transient computational fluid dynamics (CFD) can naturally solve the flow–acoustic interaction problem, the published CFD methods have not been adopted to evaluate the IL of the dissipative muffler with exhaust flow. In this paper, a time-domain simulation method is developed to predict the IL of a complex engine muffler with porous material. The noise reduction and load impedance of the muffler were first calculated using a three-dimensional CFD model, in which the porous material was simulated by the governing equations modified with the material porosity and flow resistivity. The IL was then predicted from the noise reduction, the load impedance, and the engine source extracted from the measured exhaust noise. This developed method explores the application of the three-dimensional CFD method in calculating noise reduction, load impedance, and insertion loss of complex dissipative mufflers with exhaust flow. [ABSTRACT FROM AUTHOR]

We report on device physics modeling, assisted by experimental measurement results, for all-electronic transduction of resonant electromechanical motion of nanoscale β-Ga2O3 vibrating channel transistors (VCTs), including both direct readout of high frequency transmission current (without frequency down-conversion) and frequency modulation (FM) down-conversion techniques. The β-Ga2O3 VCTs under consideration have fundamental mode resonance frequencies at ∼25 MHz–1 GHz, with quality factors (Qs) of ∼100–5000 and transconductance gm at ∼0.2 nS–5 μS. In analysis of signal transduction with varying device parameters, the transistor's gate trench depth z0 and transconductance gm play key roles in improving the electromechanical coupling and device performance. Reduction of trench depth z0 can engender a major enhancement in readout current. Reducing channel thickness h and increasing channel length L can improve the readout current while downshifting the resonance frequency at the same time. We design β-Ga2O3 VCT with a suspended modulation doped field effect transistor structure for efficient operation in the GHz range. This study paves the way for future engineering of all-electronic transduction and integration of high frequency β-Ga2O3 resonators on chip with β-Ga2O3 electronics and optoelectronics. [ABSTRACT FROM AUTHOR]

Among transition-metal nitrides, the mononitride γ-MoN with a rock-salt structure has drawn particular attention because it has been predicted to possess excellent mechanical and electronic properties, especially the high superconducting temperature around 30 K. However, synthesis of bulk γ-MoNx with the nitrogen concentration, x, more than 0.5 is still challenging, leading to contradictions on its phase stability and properties. In this work, we formulated a high-pressure synthesis reaction for the formation of single-crystal γ-MoNx with a remarkably high nitrogen concentration value of x ≈ 0.67. This nitride possesses a high asymptotic hardness of ∼24 GPa, which is so far the second hardest among metal nitrides. Impressively, the expected superconductivity is absent in the as-synthesized product. We further performed density functional theory calculations to clarify the structural stability and the absence of superconductivity in stoichiometric γ-MoN. We find that the ground state of γ-MoN is theoretically explored to be a Mott insulator with an antiferromagnetic phase, while a paramagnetic configuration is adopted at the ambient conditions. Such magnetic properties would explain the structural stability and the absence of superconductivity in the as-synthesized γ-MoNx with a high nitrogen concentration. [ABSTRACT FROM AUTHOR]

When a rotating wave carrying orbital angular momentum undergoes an extreme Doppler shift, the observed frequency will become negative. The negative frequency is the prerequisite condition for rotational superradiance, which is principally in connection with the black hole radiance. This condition has been observed in a published experiment using two rotating microphones, which mainly obtained the spectral results. However, the experiment showed a few graphical details of the wave shape. We propose a numerical method not only to demonstrate the Doppler shift of the observed frequency but also to show the formation of the acoustic rotating waves in the time domain. The numerical model adopts a circular array of momentum sources to generate the rotating waves and an array of static monitoring points to simulate the rotating microphones. The time domain observation shows that a negative frequency can be explained with either the handedness reversal of orbital angular momentum or the time reversal of the signal. This point of view has never been reported. The proposed method is not only instructive for designing an experiment with static microphones which are more easily assembled than rotating microphones but also useful in exploring the physical insight of the supperadiance phenomenon. [ABSTRACT FROM AUTHOR]

This paper introduces a numerical method of reconstructing three-dimensional (3D) ultrasound fields from their two-dimensional back-projections to understand the intensity distribution of nonplanar waves. The horizontal planes for line-focused ultrasound fields were roughly elliptical, whereas those for point-focused ultrasound fields were nearly circular. Experimental and simulated results indicated that the ultrasound intensity at the central axis can be calibrated from the light intensity by 3D reconstruction of the ultrasound field. The difference between the measured ultrasound intensity for nonplanar waves in schlieren imaging and the simulation results were discussed. From this work, the 3D ultrasound field becomes possible to be reconstructed. [ABSTRACT FROM AUTHOR]

Orthogonal fluidic connections are essential for developing multilayered microfluidic devices. At present, most orthogonal connectors are realized by a horizontal channel and a vertical channel in different plates. Therefore, some extra alignment and adhesion processes for precise plate assembly are required. In this paper, the method of injection molding is proposed to make a one-body-type orthogonal connector in a single plastic plate. The connector was composed of a cantilevered tube and the other in the substrate. An injection mold was developed in which a side core-pulling mechanism and an ejection mechanism of push-pipes were combined to form the mold for an orthogonal connector. Both the type and the location of gate were optimized for the mold. The results showed that the fan gate in the middle position of the plate was the most suitable in term of both defect control and practicability. The effect of melt temperature was numerically investigated and then verified experimentally. With the optimized parameters, the relative length and the relative wall thickness of a cantilevered tube in the plastic part can reach 98.89% and 99.80%, respectively. Furthermore, using the plastic part as a cover plate, a three-layer plastic microfluidic device was conveniently fabricated for electrochemical detection. [ABSTRACT FROM AUTHOR]

In this paper, the electrokinetic ion breakdown in a nanochannel is investigated. The Poisson-Nernst-Planck equations are employed to simulate the influence of the voltage on the concentration. Both theoretical research and experiments show that increasing the voltage can promote the ion concentration, but high voltage will break up the repulsion effect of the electric double layer and bring the concentration down. For a given micro-nanochannel, the ion concentration has a peak value corresponding with a peak voltage. Narrowing the width of a nanochannel improves the peak voltage and the peak concentration. The results will be beneficial to research the internal discipline of electrokinetic concentration. [ABSTRACT FROM AUTHOR]

Silicon insert is a promising tool for microinjection moulding (MIM). However, its fracture problem induced by impact in MIM creates a bottleneck for application. The purpose of this paper is to investigate the impact behaviour in MIM and the effect on the fracture of silicon inserts. The finite element method is utilised to calculate the crack propagation of silicon inserts with pressure load and thermal load in the MIM process. The simulation result shows that the crack propagation is more easily induced by the increase of pressure load, while the temperature change has little effect on the crack propagation. An experimental platform, including a novel rotatable insert mould, is developed to investigate the dynamic pressure in the MIM process. The result shows that both the maximum pressure and the maximum loading rate occur in the initial period of MIM process. It indicates that the silicon insert is more prone to fracture at the beginning of the MIM process, and spatial pressure peaks are observed in the cavity as well. The nearly consistent distribution between the peak positions and the insert fracture zones shows that the pressure distribution is quite relevant to the fracture of the silicon insert. The result is helpful because it reveals the fracture phenomenon of silicon inserts. [ABSTRACT FROM AUTHOR]

Mechanoluminescence (ML) refers to the photon emission from materials subjected to mechanical stimuli. A general concept concerning ML materials is that they cannot directly record a stress history (mechanical record; MR). Here, we report an approach that retrieves a quantitative recording of stress history through the afterglow characteristics analysis of a specific multi-piezo ML material Li0.12Na0.88NbO3:Pr (LNNO), which has been designated to exist near the structural phase boundary of rhombohedral R3c and orthorhombic P21ma. Contrary to the general expectation that the afterglow characteristics are irrelevant to mechanical loading history, LNNO was strongly correlated with the stress distribution after UV excitation. This enabled us to read the recorded stress precisely from the afterglow image of the material. Furthermore, the spatial distribution of the MR was also realized, showing the capability of the readout of stress history in both time and space. This unusual MR function is attributed to eliminating shallow trap sites, which affected the afterglow characteristics upon applying mechanical load. [ABSTRACT FROM AUTHOR]

The rotational Doppler shift in acoustic waves carrying orbital angular momentum (OAM) has been applied in experiments to demonstrate the Zel'dovich amplification, which is analogously linked with the black hole radiance. However, rotating microphones used in the published experiments are difficult to assemble and intrinsically weak in the frequency response at frequencies around 0 Hz. To circumvent the limitations, this paper proposes an experimental method to study the rotational Doppler shift in acoustic OAM waves using static microphones. The Doppler shifted signals viewed by a rotating observer were extracted from the original acoustic signals at frequencies around 100 Hz, where the microphones had flat frequency response. The measured results not only demonstrated the extreme Doppler shift to a negative frequency but also showed that the acoustic amplitudes were very low when the Doppler shifted frequency was 0 Hz. This phenomenon was explained by showing in a time-domain numerical simulation that the wave was co-rotating with the observer. In addition, the characteristics of the simulated pressure contours of the rotating wave have been discussed. The proposed methods can further be used to study the physics related to the wave carrying OAM, such as the acoustic super-radiance. [ABSTRACT FROM AUTHOR]

In order to analyze the sharpness of combustion noise of the gasoline direct injection (GDI) engine, the in-cylinder pressure and noise of different loads and speeds of a three-cylinder GDI engine are measured. An adaptive cycle Wiener filter method is proposed to separate the combustion noise. The result indicated that the proposed method can overcome the drawbacks of the original Wiener filter method and is able to separate high-frequency combustion noise components more accurately and effectively. The sharpness of the combustion noise is calculated by the Zwicker method and Aures method, and the comparison results show that the Aures method is more suitable for evaluating combustion noise. The influence of different loads and speeds on the sharpness of combustion noise is investigated by the proposed method systematically. The results indicated that at low speed (1500 r/min), the combustion noise and sharpness will increase with the load while at medium (3000 r/min) and high (5500 r/min) speeds, the combustion noise under half load (50%) has the highest sharpness. Excessive high load (100%) and too low load (0%) both suppress the generation of combustion noise. As for the same engine load, a higher engine speed will lead to higher combustion noise and sharpness. [ABSTRACT FROM AUTHOR]

Photoelectron spectroscopy was used to investigate poly(3-hexylthiophene) (P3HT), [6,6]-phenyl C61 butyric acid methyl ester (PCBM), and their blends on various conductive substrates. The study shows a P3HT-rich layer at the top of the P3HT:PCBM blend films. The energy level alignment of the top P3HT changes with the work function of the substrate and the PCBM concentration at the bottom surface of the blend film. The results can be explained using the integer charge transfer model. [ABSTRACT FROM AUTHOR]

In this work, fabrication of organic thin film transistors (OTFTs) using a phenyltrimethoxysilane (PhTMS) modified SiO2 insulator greatly improves the device electrical properties over those with plain or octadecyltrichlorosilane (OTS) modified SiO2, particularly improves the carrier mobility, the subthreshold slope, and channel resistance resulted from reduced density of charge trapping states at the semiconductor/insulator interface. The pentacene OTFTs with modification from PhTMS (3.5‰ v/v) achieves carrier mobility of 1.03 cm2/V s, on/off current ratio of 1.98×105, and subthreshold slope of 0.20 V/decade. This work renders a new, simple approach to significantly improve the OTFT performance. [ABSTRACT FROM AUTHOR]

We report the realization of the dynamic image of stress distribution by developing a remarkably strong mechanoluminescence (ML) material of Sr[sub 0.975]Al[sub 2]O[sub 3.985]:Eu[sub 0.01], which can emit four orders of magnitude larger intensity than that of the reported strong ML material of quartz crystal. This ML material can be mixed in the target composite or coated on the surface to sense stress by emitting visible light. This method is applicable to the dynamic visualization of stress distribution in a solid not only in the atmosphere but also in an aqueous environment. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The concept that classic waves reflected from a rotating absorbing cylinder will be amplified is analogously linked to the Penrose superradiance that may extract energy from a rotating black hole. The superradiance of acoustic waves carrying orbital angular momentum (OAM) from a sound absorber has been demonstrated in a recently published experiment using two rotating microphones. The experiment showed that the waves were amplified by up to 30% when the rotation rate of the absorber satisfied the Zel'dovich condition. Here, we proposed an experimental method to detect acoustic superradiance by static microphones. We demonstrated that the acoustic waves transmitted through a perforated absorber were amplified (by up to 1000%) even when the Zel'dovich condition was not satisfied. The experiment also showed that when the Doppler-shifted frequency of an observer was 0 Hz, the acoustic amplitude was very weak regardless of the rotation rate of the absorber. Our work was worthwhile not only in the proposed experimental method that was well-suited for observing the acoustic OAM wave but also in the high amplification achieved at a low rotation rate of the perforated absorber, showing a great prospect in practical applications such as amplifying the information-carrying OAM waves for high-speed acoustic communication. [ABSTRACT FROM AUTHOR]

Electron-surface interaction plays a fundamental role in surface science, which could evolve into an electronic avalanche under a high electrical field, resulting in devastating device failures. In the present study, using a thin polymer film approach, the effective surface layer, whose thickness is estimated to be about 200 nm, where the electron-surface interaction occurs is directly probed. The morphological evolution of thin polymer films with thicknesses from nanometers to micrometers is investigated with a focus on its influence on the electron avalanche process (or flashover) under a dc electric field in vacuum. It is found that the film thickness dependence of flashover is divided into three parts, i.e., fast increase, slow increase, and saturation, each of which has a dominant microscopic mechanism. The results indicate that the secondary electron emission (SEE) yield decreases significantly even when a discontinuous polymer layer is deposited, which varies little afterward. In contrast, the shallow surface traps develop into deep ones with the film thickness. The density ratio of deep traps increases exponentially after a continuous film is formed. The clear transition from SEE dominated to surface charge trap dominated flashover and their unique dependence on film morphology provide a deeper insight into the electron-surface interaction, which can be used for theoretical modeling, surface modification, and advanced functional devices. [ABSTRACT FROM AUTHOR]

Although artificial micromotors with unconventional shapes are emerging as a powerful tool in various applications, little research has been undertaken to clarify their propulsion mechanism, especially how the shape effect alters the bubble dynamics and hydrodynamic flows. In this study, we fabricated two types of bowl-shaped micromotors to investigate the distinct dynamics due to the shape effect of concave and convex surfaces, by coating a platinum (Pt) layer on either the concave surface or the convex surface of the micromotor. In the single-bubble propulsion mode at low fuel concentration, the concave-surface-Pt-coated micromotor moved unexpectedly slower than the convex-surface-Pt-coated micromotor, and the bubble growth on the concave surface was also much slower than that on the convex surface. It was elucidated that the confinement effect of the concave surface hindered fuel replenishment and thus the catalytic reaction. We further introduced the Kelvin impulse to explain why the concave shape eventually weakened the propulsion from hydrodynamic jet flows caused by bubble collapse. In the multi-bubble propulsion mode at high fuel concentration, the interaction among bubbles rendered a "more is less" phenomenon—increase in the fuel concentration did not enhance the maximum instantaneous propulsion speed. These findings inspire the development of new manipulation strategies utilizing the unconventional shape effect in micromotors. [ABSTRACT FROM AUTHOR]

Highly efficient fluorescence organic light-emitting diodes (OLEDs) based on the mixed 4,4',4''- tris[3-methylphenyl(phenyl)amino]triphenylamine:tris-[3-(3-pyridyl)mesityl]borane (1:1) system are reported. The electroluminescence due to the exciplex emission is red shifted when the thickness of the electron-transporting layer increases. The prepared OLEDs achieve a low turn-on voltage of 2.1V, a high current efficiency of 36.79 cd/A, and a very high luminescence of 17 100 cd/m², as well as a low efficiency roll-off. The current efficiency of the optimized OLED is maintained at more than 28.33 cd/A up to 10 000 cd m-2. The detailed recombination mechanism of the prepared OLEDs is investigated by the transient electroluminescence method. It is concluded that there are no contributions from trapped charges and annihilations of triplet-triplet excitons to the detected electroluminescence. [ABSTRACT FROM AUTHOR]

Liquid control in microchannels is quite important in microfluidic devices used in, for example, lab-on-a-chip and point-of-care applications. Capillary microfluidics, being self-powered, is especially advantageous for use in passive devices, and has attracted significant attention. In this paper, capillary flows in rectangular microchannels with spacers are studied experimentally and theoretically; in particular, capillary flow synchronization (or waiting) behavior is identified and investigated. Based on changes of channel walls, two basic synchronization modes are proposed for flows isolated by spacers in a channel. Experimental results show that the velocities of faster capillary flows are reduced by the liquid pinning effect and that the time delay between two capillary flows is automatically balanced. The synchronization behavior of capillary flows is explained by analyzing the time delay, contact angle variation, and capillary forces. In addition, the quantity of liquid flowing out of the waiting channels is estimated and verified. Then a model for the change in contact angle during synchronization is derived and verified. Finally, we conceive a series of studies of the control of capillary flows for different spacer designs and conduct an experiment to study the dynamic behaviors of a number of capillary flows by adding many spacers in a microchannel. This study expands the applications of capillary microfluidics. [ABSTRACT FROM AUTHOR]

We demonstrate a highly efficient inverted bulk heterojunction polymer solar cell based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate (Cs2CO3). This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of 11.17 mA/cm2, open-circuit voltage of 0.59 V, and fill factor of 63% under AM1.5G 100 mW/cm2 irradiation. This result is comparable to the previous regular structure device on the same system. Ultraviolet photoelectron spectroscopy shows that the work function of annealed Cs2CO3 layer decreases from 3.45 to 3.06 eV. Further x-ray photoelectron spectroscopy results reveal that Cs2CO3 can decompose into low work function, doped cesium oxide Cs2O upon annealing, which is accountable for the work-function reduction and device efficiency improvement. [ABSTRACT FROM AUTHOR]

An acoustic metasurface with a sub-wavelength thickness can manipulate acoustic wavefronts freely by the introduction of abrupt phase variation. However, the existence of a narrow bandwidth and a low transmittance limits further applications. Here, we present a broadband and highly transparent acoustic metasurface based on a frequency-independent generalized acoustic Snell's law and pentamode metamaterials. The proposal employs a gradient velocity to redirect refracted waves and pentamode metamaterials to improve impedance matching between the metasurface and the background medium. Excellent wavefront manipulation based on the metasurface is further demonstrated by anomalous refraction, generation of non-diffracting Bessel beam, and sub-wavelength flat focusing. [ABSTRACT FROM AUTHOR]

UV-blue light was obtained from a thin-film electroluminescence device using Gd[sub 3]Ga[sub 5]O[sub 12]:Ag as a light-emitting layer, which was deposited by using electron-beam evaporation. The crystal composition and structure of Gd[sub 3]Ga[sub 5]O[sub 12]:Ag were studied by x-ray powder diffraction. The Gd[sub 3]Ga[sub 5]O[sub 12]:Ag has a photoluminescence emission which peaked at around 397 and 467 nm, which were attributed to the oxide vacancies and Ag[sup +], respectively. The brightness of 32 cd/m2 was obtained when an alternating voltage of 130 V at 1 kHz was applied. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]