High frequency dynamics in a monatomic glass

The high frequency dynamics of glassy selenium has been studied by inelastic x-ray scattering at beam line BL35XU (SPring-8). The high quality of the data allows one to pinpoint the existence of a dispersing acoustic mode for wave vectors (Q) of 1.5相似文献   

Refracted arrival waves in a zone of silence from a finite thickness mixing layer

Refracted arrival waves which propagate in the zone of silence of a finite thickness mixing layer are analyzed using geometrical acoustics in two dimensions. Here, two simplifying assumptions are made: (i) the mean flow field is transversely sheared, and (ii) the mean velocity and temperature profiles approach the free-stream conditions exponentially. Under these assumptions, ray trajectories are analytically solved, and a formula for acoustic pressure amplitude in the far field is derived in the high-frequency limit. This formula is compared with the existing theory based on a vortex sheet corresponding to the low-frequency limit. The analysis covers the dependence on the Mach number as well as on the temperature ratio. The results show that both limits have some qualitative similarities, but the amplitude in the zone of silence at high frequencies is proportional to omega(-1/2), while that at low frequencies is proportional to omega(-3/2), omega being the angular frequency of the source.     

Quantum-mechanical nonperturbative response of driven chaotic mesoscopic systems

Consider a time-dependent Hamiltonian H(Q,P;x(t)) with periodic driving x(t) = Asin(Omegat). It is assumed that the classical dynamics is chaotic, and that its power spectrum extends over some frequency range |omega|A(prt), where A(prt) approximately Planck's over 2pi, the system may have a relatively strong response for Omega>omega(cl) due to QM nonperturbative effect.     

Momentum distributions in fragmentation reactions with relativistic heavy ions

The momentum distribution (or angular distribution) in inelastic heavy ion reactions is calculated by using a two-step model (abrasion and ablation). First, all nucleons in the volume element where projectile and target overlap spatially during the collision are sheared away. The remaining prefragment (the projectile minus the sheared off nucleons) has a recoil momentum proportional to the Fermi momentum. The prefragment is left in an excited state and emits nucleons, the recoil momentum given to the final fragment is proportional to the nuclear temperature. This two-step model reproduces the overall trend and the isotopic dependence for the widths of the experimental momentum distributions. Contrary to previous theoretical studies we find that surface and friction phenomena lead to an anisotropy: The momentum distributions in transverse direction are always broader than in the longitudinal direction by about 5 to 10%.     

Determination of the short-wavelength propagation threshold in the collective excitations of liquid ammonia

The dynamics structure factor S(Q,E) of liquid ammonia l-NH3 at T = 200 K and at its vapor pressure has been measured by inelastic x-ray scattering (IXS) in the 1-15 nm(-1) momentum transfer ( Q) range. Contrary to previous IXS studies on other associated liquids and glasses, in l-NH3 a large inelastic signal is observed up to Q = 15 nm(-1). This, enabling S(Q,E) measurements as a function of Q at constant E transfer, allows us to demonstrate experimentally the transition from a propagating dynamics regime, where the acoustic excitation energy linearly disperses with Q, to a high-Q regime, where it is no longer possible to observe a dominant excitation in the S(Q,E).     

Universal scaling of the conductivity at the superfluid-insulator phase transition

The scaling of the conductivity at the superfluid-insulator quantum phase transition in two dimensions is studied by numerical simulations of the Bose-Hubbard model. In contrast to previous studies, we focus on properties of this model in the experimentally relevant thermodynamic limit at finite temperature T. We find clear evidence for deviations from omega k scaling of the conductivity towards omega k/T scaling at low Matsubara frequencies omega k. By careful analytic continuation using Padé approximants we show that this behavior carries over to the real frequency axis where the conductivity scales with omega/T at small frequencies and low temperatures. We estimate the universal dc conductivity to be sigma* = 0.45(5)Q2/h, distinct from previous estimates in the T = 0, omega/T > 1 limit.     

Three-dimensional reconstruction of biological objects' internal structure heterogeneity from the set of ultrasonic tomograms

The paper presents the method of the three-dimensional reconstruction of biological objects' internal structure heterogeneity based on the ultrasonic examination of a woman's breast biopsy phantom. The phantom is made of quasi-homogeneous dense gel in which drops of lesions, characterized by fixed sizes and two different acoustic impedances, were dipped at random. For the purpose of this research a special measurement setup was elaborated, enabling a non-invasive in vitro imaging of biological objects' internal structure in cross-sections for fixed levels, by means of ultrasound transmission tomography (UTT) using the parallel-ray projection geometry of scanning. The two-dimensional images of the local values of ultrasonic wave's propagation velocity in the phantom's internal structure (ultrasonic tomograms) were reconstructed for fixed levels (by using the convolution and back-projection algorithm) from the measurements of average values of ultrasonic signals' runtime propagated from many directions around the object dipped in water. Analyzing the values of particular pixels and using an appropriate image processing technique, in effect the three-dimensional image of heterogeneity boundaries in the examined phantom's internal structure was computer-reconstructed. The obtained results are compatible with the specification provided by the phantom's producer in terms of sizes and acoustic parameters of lesions, which can simulate pathological changes and of the gel imitating the healthy tissue. It means that the method presented, after an appropriate modification and development of the measurement setup with an aim to accelerate the object scanning process and thus provide an opportunity for non-invasive in vivo examinations, could be applied for detecting and diagnosing tumors in women's breasts.     

High-precision pulsed photoacoustic spectroscopy in NO2-N2

A method for high-precision pulsed photoacoustic spectroscopy applied to a simple system for detection of NO₂ traces in nitrogen is presented. The acoustic signal from a closed cell containing NO₂/N₂ samples irradiated by a pulsed visible laser is analyzed in the frequency domain. A signal-processing method to obtain a high-resolution Fourier spectrum of the signal was developed. An accurate fitting of the resonance peaks with Lorentzian profiles gives high-precision determination of the amplitude and width of the resonance peaks. The resonance maximum is proportional to the absorbed energy; therefore, the choice of the laser wavelength, linewidth and frequency stability are critical for a precise calibration due to the fine structure of the NO₂ optical spectrum. The method also allows high-accuracy measurement of the Q of the acoustic cavity. The dependence of Q on the buffer gas pressure is characteristic of an acoustic cavity where energy losses near the walls predominate. Consequently, an important enhancement of sensitivity takes place at high N₂ pressure. Received: 1 June 2001 / Revised version: 27 July 2001 / Published online: 7 November 2001     

Josephson junction as a detector of poissonian charge injection

We propose a scheme of measuring the non-Gaussian character of noise by a hysteretic Josephson junction in the macroscopic quantum tunneling regime. We model the detector as an (under)damped LC resonator. It transforms Poissonian charge injection into current through the detector, which samples the injection statistics over a floating time window of length approximately Q/omega(J), where Q is the quality factor of the resonator and omega(J) its resonance frequency. This scheme ought to reveal the Poisson character of charge injection in a detector with realistic parameters.     

Experimental study of the Doppler shift generated by a vibrating scatterer

We report an experimental study of the backscattering of a sound wave of frequency f by a surface vibrating harmonically at frequency F (F < f) and amplitude A in the regime where the Doppler effect overcomes bulk nonlinear effects. When the duration to of the analyzed time series of the scattered wave is small compared to the vibration period, the power spectrum of the backscattered wave is proportional to the probability density function of the scatterer velocity, which presents two peaks shifted from f by roughly 2fAomega/c (omega = 2piF). On the contrary, when t0 > F(-1), sidebands at frequencies f +/- nF (n integer) appear in the power spectrum, which are due to the phase modulation of the backscattered wave induced by its reflection on a moving boundary. We use the backscattered power spectrum to validate the phase modulation theory of the Doppler effect in the latter case for 2kA < 1 and 2kA approximately > 1 (k = 2pif/c, where c is the wave velocity) and we test the validity of an acoustic nonintrusive estimator of A as a function of power spectrum bandwidth and of A itself.     

Capillary waves at the transition from propagating to overdamped behavior

We measure the dispersion relation of capillary waves on a liquid surface by heterodyne x-ray photon correlation spectroscopy near the transition from propagating to overdamped dynamic behavior. A strong deviation of the propagation frequency from the small-damping result omega(p) proportional, variant k(3/2) is observed long before the actual transition where ( partial differential omega(p)/ partial differential k)<0 and omega(p) tends to zero. This behavior is successfully described by expressions derived within linear response theory.     

通道宽度和初始流量对颗粒稀疏流-密集流转变临界开口的影响

通过实验研究斜面上二维颗粒流，当出口尺寸减小到临界值Dc时，发生稀疏流到密集流的突变.发现临界尺寸Dc与初始流量和通道宽度有关，通道宽度一定的情况下，临界开口尺寸Dc近似随初始流量Q0的平方根增大.在初始流量Q0一定时，临界开口尺寸Dc随通道宽度W近似线性增大.给出了这些关系的表达式，理论计算与实验观测结果一致.同时，也讨论了通道宽度影响临界开口尺寸的原因. 关键词： 颗粒物质 颗粒流     

Q235钢板局部腐蚀声发射信号特性研究

为解决腐蚀声发射源特征提取和识别的难题,直接从理论上证明了腐蚀声发射监测的有效性并推导出声发射信号特征。气泡破裂声发射信号幅值近似与气泡半径平方和液位高度成正比,频率与气泡半径成反比;钢板及其腐蚀产物开裂声发射信号幅值与开裂位置局部应力强度成正比,频率与裂纹扩展速度成正比,与裂纹扩展距离成反比。用低频和高频两套声发射系统,同时长时间监测Q235钢板在10%FeCl₃·6H₂O、10%FeCl₃·6H₂O加0.01 mol/L HCl混合液、5%CuSO₄·5H₂O溶液中的腐蚀情况,辅以监测钢板及钢板腐蚀产物开裂作为验证实验。通过对声发射信号的参数及谱分析,得出不同声发射源可以通过撞击数及功率谱在频域的分布来有效区分。实验结果与理论分析相吻合,研究结果对腐蚀声发射监测技术具有重要指导意义。      

Evaluation of Expansion Coefficients from Optimal Fitting Parameters for the Analysis of Spectra of Diatomic Molecules and an Application to LiH

To evaluate individual expansion coefficients composing fitting parameters of the Born-Oppenheimer corrections to Dunham's coefficients Y(ij) that have been given analytically with the Delta(B) and Delta(omega) formalism, we examined the consistency of analytic expressions for those corrections with Watson's assertion of the experimental inseparability of nonadiabatic corrections Q(a, b)(r) for a molecule AB. Derived analytic expressions in terms of optimal fitting parameters for the corrections are essential to evaluate individual expansion coefficients. These expressions also reveal redundancies between empirical correction parameters Delta(ij). A method of evaluating nonadiabatic vibrational corrections Q(a, b)(r) and adiabatic corrections S(a, b)(r) separately consistent with Watson's assertion of inseparability is presented and is applied to an analysis of spectral data of LiH. Functions Q(a, b) and S(a, b) for LiH are thus successfully evaluated; S(H, Li)(r) values agree well with those predicted simply by wobble-stretch theory. Experimental values for optimal fitting parameters r(H)(1q) and r(H)(2q) are nearly equal to those of r(Li)(1q) and r(Li)(2q), respectively, in agreement with a theoretical relation r(a)(iq)=r(b)(iq). Copyright 2001 Academic Press.     

OH/D A2sigma(+)-X2pi(i) rovibronic transitions in multibubble sonoluminescence

Multibubble sonoluminescence spectra were recorded in the 300-350 nm wavelength range in the case of H(2)O/Ar, D(2)O/Ar and H(2)O/Kr solutions (acoustic frequency: 20 kHz; spectral resolution optimized to 0.34 nm). Three groups of rotational components (R(1)/R(2), Q(1)/Q(2) and P(1)/P(2)) were identified in the OH/D A2sigma(+)-X2pi(i) (0,0) transitions via the substitution of H(2)O for D(2)O. The congestion of bands and the origin of a red shading extending up to 350 nm are broached.     

Electron tunneling from a metallic TS2 layer underneath an ultra-thin MS layer with semiconducting properties for misfit-layer compounds

The effects of electron tunneling from the underlying TS₂ (H) layer on the scanning tunneling microscopy (STM) images of the uppermost MS (Q) layer have been studied for the misfit-layer compounds which are represented by the chemical formula {(MS)_1+x}_m{TS₂}_n. Systematic STM observations have been carried out under ultra-high vacuum (UHV) conditions for the 1Q/1H, 1.5Q/1H and 2Q/1H types of misfit-layer compounds. As Q layer thickness increases from about 6 to 12 Å while going from the 1Q/1H type to the 2Q/1H type, pseudo-tetragonal arrays of bright spots as expected from the atomic arrangement of a Q layer are observed more easily and more distinctly. It is found that tunneling electrons from the underlying H layer play an important role on the STM observations of the 1Q/1H and 1.5Q/1H types of compounds. Fast Fourier transform (FFT) analyses give clear evidences for electron tunneling from the underlying H layer and scattering by surface atoms of the uppermost Q layer and a mutual modulation structure peculiar to the compounds.     

Dynamics and steady states in excitable mobile agent systems

We study the spreading of excitations in 2D systems of mobile agents where the excitation is transmitted when a quiescent agent keeps contact with an excited one during a nonvanishing time. We show that the steady states strongly depend on the spatial agent dynamics. Moreover, the coupling between exposition time (omega) and agent-agent contact rate (CR) becomes crucial to understand the excitation dynamics, which exhibits three regimes with CR: no excitation for low CR, an excited regime in which the number of quiescent agents (S) is inversely proportional to CR, and, for high CR, a novel third regime, model dependent, where S scales with an exponent xi-1, with xi being the scaling exponent of omega with CR.     

Nonlinear piezoelectricity in PZT ceramics for generating ultrasonic phase conjugate waves

We have succeeded in the generation of acoustic phase conjugate waves with nonlinear PZT piezoelectric ceramics and applied them to ultrasonic imaging systems. Our aim is to make a phase conjugator with 100% efficiency. For this purpose, it is important to clarify the mechanism of acoustic phase conjugation through nonlinear piezoelectricity. The process is explained by the parametric interaction via the third-order nonlinear piezoelectricity between the incident acoustic wave at angular frequency omega and the pump electric field at 2 omega. We solved the coupling equations including the third-ordered nonlinear piezoelectricity and theoretically derived the amplitude efficiency of the acoustic phase conjugation. We compared the efficiencies between the theoretical and experimental values for PZT ceramics with eight different compositions. Pb[(Zn1/3Nb2/3)(1 - x)Tix]O3 (X = 0.09, PZNT91/9) piezoelectric single crystals have been investigated for high-performance ultrasonic transducer application, because these have large piezoelectric constants, high electrical-mechanical coupling factors and high dielectric constants. We found that they have third-order nonlinear piezoelectric constants much larger than PZT and are hopeful that the material as a phase conjugator has over 100% efficiency.     

H2的振动及转动受激喇曼散射研究

本文研究了不同泵浦光偏振态及氢气气压时,氢的振动及转动受激喇曼散射的竞争效应.在5atm的氢喇曼池中,以波长为532nm的椭偏激光为泵浦源,当椭圆的压缩系数为tg26°～tg32°时,获得了60多条Q(1)和S(1)混合谱线.并在低压氢及线偏或准线偏光泵浦条件下,观察到迄今尚未见报道的受激R(1)喇曼谱线.     

Source ranging with minimal environmental information using a virtual receiver and waveguide invariant theory

A method is presented for estimating the range of an unknown broadband acoustic source in a waveguide, using a vertical array and a signal sample from another broadband source at a known location relative to the array. The method requires no modeling of the acoustic field, and little to no environmental information for flat bathymetries. Waveguide invariant theory [e.g., D'Spain and Kuperman, J. Acoust. Soc. Am 106, 2454-2468 (1999)] is applied to the "virtual receiver" [Siderius et al., J. Acoust. Soc. Am. 102, 3439-3449 (1997)] to create a "virtual aperture" (VA). In effect, the method effectively converts a source at known range r(g) into a continuum of receivers lying between ranges (1 +/- alpha/beta)*r(g), where beta is a scalar parameter called the acoustic invariant, and alpha approximately 0.1. This effective displacement is achieved by correlating the known source field, measured at frequency component omega, with the unknown source field, measured at frequency component omega + omegas. When the VA output is plotted as a function of omega and omegas, the slope of the resulting correlation contours yields the unknown source range. The concept is illustrated via both simulation and analysis of data collected from a pseudo-random noise source with 75-150-Hz bandwidth during SWellEx-3, a shallow water experiment conducted off the San Diego coast. The virtual aperture can be reformulated for range-dependent environments, if adiabatic propagation assumptions are valid, and if the bathymetry surrounding the array is known.