Origin of the universal roughness exponent of brittle fracture surfaces:stress-weighted percolation in the damage zone

We suggest that the observed large-scale universal roughness of brittle fracture surfaces is due to the fracture propagation being a damage coalescence process described by a stress-weighted percolation phenomenon in a self-generated quadratic damage gradient. We use the quasistatic 2D fuse model as a paradigm of a mode I fracture model. We measure for this model, which exhibits a correlated percolation process, the correlation length exponent nu approximately 1.35 and conjecture it to be equal to that of classical percolation, 4/3. We then show that the roughness exponent in the 2D fuse model is zeta=2nu/(1+2nu)=8/11. This is in accordance with the numerical value zeta=0.75. Using the value for 3D percolation, nu=0.88, we predict the roughness exponent in the 3D fuse model to be zeta=0.64, in close agreement with the previously published value of 0.62+/-0.05. We furthermore predict zeta=4/5 for 3D brittle fractures, based on a recent calculation giving nu=2. This is in full accordance with the value zeta=0.80 found experimentally.     

Activated escape of periodically driven systems

We discuss activated escape from a metastable state of a system driven by a time-periodic force. We show that the escape probabilities can be changed very strongly even by a comparatively weak force. In a broad parameter range, the activation energy of escape depends linearly on the force amplitude. This dependence is described by the logarithmic susceptibility, which is analyzed theoretically and through analog and digital simulations. A closed-form explicit expression for the escape rate of an overdamped Brownian particle is presented and shown to be in quantitative agreement with the simulations. We also describe experiments on a Brownian particle optically trapped in a double-well potential. A suitable periodic modulation of the optical intensity breaks the spatio-temporal symmetry of an otherwise spatially symmetric system. This has allowed us to localize a particle in one of the symmetric wells. (c) 2001 American Institute of Physics.     

High-frequency magneto-oscillations in GaAs/AlGaAs quantum wells

We have observed very high-frequency, highly reproducible magneto-oscillations in modulation doped GaAs/AlGaAs quantum well structures. The oscillations are periodic in an inverse magnetic field (1/B) and their amplitude increases with temperature up to T approximately 700 mK. Being initially most pronounced around the filling factor nu=1/2, they move towards lower nu with increasing T. Front and back-gating data imply that these oscillations require a coupling to a parallel conducting layer. A comparison with existing oscillation models renders no explanation.     

Critical exponent crossovers in escape near a bifurcation point

In periodically driven systems, near a bifurcation (critical) point the period-averaged escape rate Wmacr; scales with the field amplitude A as |ln(Wmacr;| proportional, variant (A(c)-A)(xi), where A(c) is a critical amplitude. We find three scaling regions. With increasing field frequency or decreasing |A(c)-A|, the critical exponent xi changes from xi=3/2 for a stationary system to a dynamical value xi=2 and then again to xi=3/2. Monte Carlo simulations agree with the scaling theory.     

On the conversion of triple- to single-quantum coherences in MQMAS NMR

A systematic experimental and numerical evaluation of several basic approaches to multiple-quantum magic angle spinning (MQMAS) NMR is presented for spin-32 nuclei. The approaches use identical MQ excitation, via a single RF pulse of high power, and three types of methods for conversion to observable coherence: (a) nutation by strong continuous wave pulse; (b) rotation-induced adiabatic coherence transfer (RIACT), and (c) fast amplitude modulation (FAM-1). The optimization strategies and maximum achievable MQMAS efficiencies of (87)Rb in RbNO(3) and LiRbSO(4) are investigated using several coherence transfer schemes under a wide range of experimental parameters. These parameters include the strength of the RF magnetic field nu(RF), the sample rotation speed nu(R), the length of the conversion period, and the modulation frequency in FAM-1. The data provide new insights into the spin dynamics involved in these techniques and the experimental guidelines for achieving the best sensitivity. The RF requirements for maximum efficiency of conversion depend on the method to be used. In general, FAM-1 performs better than the nutation and RIACT methods in terms of efficiency and off-resonance behavior, especially when nu(RF) is small compared to the quadrupole frequency nu(Q). The experiments performed using nutation, RIACT, and FAM-1 methods yield similar resolution in the isotropic dimension, regardless of nu(RF).     

Roughness of interfacial crack fronts: stress-weighted percolation in the damage zone

We study numerically the roughness exponent zeta of an in-plane fracture front slowly propagating along a heterogeneous interface embedded in an elastic body, using a model based on the evolution of a process zone rather than a fracture line. We find zeta=0.60+/-0.05. For the first time, simulation results are in close agreement with experimental results. We then show that the roughness exponent is related to the correlation length exponent nu of a stress-weighted percolation problem through zeta=nu/(1+nu). A numerical study of the stress-weighted percolation problem yields nu=1.54 giving zeta=0.61 in close agreement with our numerical results and with experimental observations.     

The Solution of a New Kind of Integrals of Bessel Functions Used in the Power Calculation

In the evaluation of the power flow in some millimeter RF structures, a new kind of integrals of the type in which _±(kr) denotes arbitrary Bessel functions of real order ±, is met. In order to integrate it, a new transformation of modified Bessel function is build in this paper: then a general integral formula involving the products two terms, each of them can be expressed a sum of any two modified Bessel function, is obtained by means of applying a theorem on calculus to this transformation smartly, Furthermore, making use of the characteristics of modified Bessel function and the definition of Bessel function, the analytical expression of the above integral is derived.     

Translocation of a confined polymer through a hole

Based on an analogy between polymer translocation across a free energy barrier associated with polymer worming through a hole and classical nucleation and growth process, the escape time tau is predicted asymptotically to be N(N/rho)(1/3nu). N is the polymer length, rho is the monomer density prior to escape, and nu is the radius of gyration exponent. Monte Carlo simulation data collected in the high salt limit (nu approximately 3/5) are in agreement with the asymptotic law and provide vivid details of the escape.     

Spectral properties of the interference head wave

The interference head wave propagating through a sediment with a linear sound speed gradient is studied as a function of the parameter zeta, which is itself a function of acoustic frequency f, sediment sound speed and its gradient, and range. For zeta<1 the amplitude spectrum of the interference head waves goes as |S(f)|/f, where S(f) is the source spectrum. For increasing zeta beyond unity a more complicated modulation of S(f) ensues, which is explained by a channel transfer function H1(f), constructed analytically from a summation of terms involving zeroth-order refracted waves (referred to as a ray approach). For zeta greater than or approximately equal to 2 this summation compares well with a wave theory result for the interference head wave involving a fluid-fluid boundary. The amplitude spectrum of the interference head wave in the absence of sediment attenuation is |S(f)| x |H1(f)| and it is essential to know these to obtain an estimate of sediment attenuation from field observations. Examples of |S(f)| x |H1(f)| are presented for which H1(f) is computed directly using the ray approach and indirectly using the parabolic wave equation. A brief discussion on the application of these results towards the inversion of sediment attenuation is given.     

New limits on interactions between weakly interacting massive particles and nucleons obtained with CsI(Tl) crystal detectors

New limits are presented on the cross section for weakly interacting massive particle (WIMP) nucleon scattering in the KIMS CsI(T?) detector array at the Yangyang Underground Laboratory. The exposure used for these results is 24?524.3 kg·days. Nuclei recoiling from WIMP interactions are identified by a pulse shape discrimination method. A low energy background due to alpha emitters on the crystal surfaces is identified and taken into account in the analysis. The detected numbers of nuclear recoils are consistent with zero and 90% confidence level upper limits on the WIMP interaction rates are set for electron equivalent energies from 3 to 11 keV. The 90% upper limit of the nuclear recoil event rate for 3.6-5.8 keV corresponding to 2-4 keV in NaI(T?) is 0.0098 counts/kg/keV/day, which is below the annual modulation amplitude reported by DAMA. This is incompatible with interpretations that enhance the modulation amplitude such as inelastic dark matter models. We establish the most stringent cross section limits on spin-dependent WIMP-proton elastic scattering for the WIMP masses greater than 20 GeV/c2.     

Random walks on a ( 2+1)-dimensional deformable medium

A model of random walks on a deformable medium is proposed in 2+1 dimensions. The behavior of the walk is characterized by the stability parameter beta and the stiffness exponent alpha. The average square end-to-end distance l approximately equals (2nu) and the average number of visited sites approximately equals (k) are calculated. As beta increases, for each alpha there exists a critical transition point beta(c) from purely random walks ( nu = 1/2 and k approximate to 1) to compact growth ( nu = 1/3 and k = 2/3). The relationship between beta(c) and alpha can be expressed as beta(c) = e(alpha). The landscape generated by a walk is also investigated by means of the visit-number distribution N(n)(beta). There exists a scaling relationship of the form N(n)(beta)approximately n(-2)f(n/beta(z)).     

Role of parametric resonance in the inhibition of chaotic escape from a potential well

This paper shows how a periodic parametric modulation can inhibit chaotic escape of a driven oscillator from the cubic potential well that typically models a metastable system close to a fold. Melnikov analysis shows that, depending on its amplitude, period, and initial phase, a periodic parametric modulation of the linear potential term suppresses chaotic escape when certain resonance conditions are met. In particular, it is shown that chaotic escape suppression is impossible under a period-1 parametric perturbation. The effect of nonlinear damping on the inhibition scenario is also studied.     

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.     

大气激光通信中的多层空间脉冲位置幅度调制

针对现有光空间脉冲位置调制频谱效率低、激光器利用率不高等问题,将分层技术与空间脉冲位置幅度调制相结合,提出了一种适合于大气激光通信的多层空间脉冲位置幅度调制方案.通过额外增加少量几个激光器构成多层结构,并通过脉冲位置幅度调制中的脉冲位置携带比特信息,不同层通过脉冲幅度得到区分.介绍了系统中层映射、空间脉冲位置幅度映射及其逆映射的原理,并推导出该方案的误码率表达式.利用蒙特卡洛仿真方法进一步验证了该方案的正确性,并与传统空间调制系统的性能进行了对比.结果表明:与传统光空间调制系统相比,所提方案提高了系统的频谱效率,且所用激光器数目更少.在传输比特相同的条件下,相对于(32,4,128)-空间脉冲位置调制系统,(9,4,8,2)-多层空间脉冲位置幅度调制系统的频谱效率提高了16倍,当误码率为10-3时,其信噪比改善了约1dB,且所用激光器数目不到前者的1/3.其中,括号中的参数分别表示激光器数目、探测器数目、采用调制方式的阶数及层数,层数为1时忽略.     

Interference quenching of nu(")=1 vibrational line in resonant photoemission of N2: a possibility to obtain geometrical information on the core-excited state

An interference quenching of the nu(")=1 vibrational line in the resonant Auger decay of N 1s-->pi(*) core-excited N2 is observed and analyzed. The intensity ratio between the nu(")=1 and nu(")=0 vibrational levels of the X2Sigma(+)(g) final state shows a surprising nonmonotonous variation as a function of frequency detuning, going through a minimum with a complete suppression of nu(")=1. We have developed a simple model which shows a linear relation between the value of the detuning frequency for this minimum and the equilibrium bond distance R(0)(c) of the core-excited state. A new way is thus established of determining the equilibrium bond distance for the core-excited state with a precision deltaR(0)(c)<10(-3) A.     

Hybrid measurement of auditory steady-state responses and distortion product otoacoustic emissions using an amplitude-modulated primary tone

A maximum auditory steady-state response (ASSR) amplitude is yielded when the ASSR is elicited by an amplitude-modulated tone (f(c)) with a fixed modulation frequency (f(m) = 40 Hz), whereas the maximum distortion product otoacoustic emission (DPOAE) level is yielded when the DPOAE is elicited using a fixed frequency ratio of the primary tones (f2/f1 = 1.2). When eliciting the DPOAE and ASSR by the same tone pair, optimal stimulation is present for either DPOAE or ASSR and thus adequate simultaneous DPOAE/ASSR measurement is not possible across test frequency f2 or f(c), respectively. The purpose of the present study was to determine whether the ASSR and DPOAE can be measured simultaneously without notable restrictions using a DPOAE stimulus setting in which one primary tone is amplitude modulated. A DPOAE of frequency 2f1-f2 and ASSR of modulation frequency 41 Hz were measured in ten normal hearing subjects at a test frequency between 0.5 and 8 kHz (f2 = f(c)). The decrease in the DPOAE level and the loss in ASSR amplitude during hybrid mode stimulation amounted, on average, to only 2.60 dB [standard deviation (SD) = 1.38 dB] and 1.83 dB (SD = 2.38 dB), respectively. These findings suggest simultaneous DPOAE and ASSR measurements to be feasible across all test frequencies when using a DPOAE stimulus setting where the primary tone f2 is amplitude modulated.     

Top-quark mass measurement from dilepton events at CDF II

We report a measurement of the top-quark mass using events collected by the CDF II detector from pp collisions at square root of s = 1.96 TeV at the Fermilab Tevatron. We calculate a likelihood function for the top-quark mass in events that are consistent with tt --> bl(-)nu(l)bl'+ nu'(l) decays. The likelihood is formed as the convolution of the leading-order matrix element and detector resolution functions. The joint likelihood is the product of likelihoods for each of 33 events collected in 340 pb(-1) of integrated luminosity, yielding a top-quark mass M(t) = 165.2 +/- 6.1(stat) +/- 3.4(syst) GeV/c2. This first application of a matrix-element technique to tt --> bl+ nu(l)bl'- nu(l') decays gives the most precise single measurement of M(t) in dilepton events. Combined with other CDF run II measurements using dilepton events, we measure M(t) = 167.9 +/- 5.2(stat) +/- 3.7(syst) GeV/c2.     

Analysis of Rovibrational Resonances Observed in the Microwave Spectrum of FCCCN

Microwave spectrum of fluorocyanoacetylene (FCCCN) produced by a glow discharge in pentafluorobenzonitrile (C6F5CN) was observed using a source modulation spectrometer with a free-space absorption cell. Rotational transitions in the range from J = 9-8 to 53-52 were observed for the vibrationally excited states of nu4 (C&sbond;C stretch), nu5 (CCN bend), nu6 (FCC bend), nu7 (CCC bend), and their associated overtone and combination states up to about 1000 cm-1. Most of the vibrational states above 500 cm-1 are perturbed by rovibrational resonances. The effective vibration-rotation constant of the nu4 state has a negative value (-0.4 MHz), although a vibration-rotation constant generally has a positive value in the excited state of the stretching vibrational mode in a linear molecule. This anomalous behavior is interpreted as due to the rovibrational resonances between the nu4 and several nearby states. By the simultaneous analysis of the states concerned, the unperturbed vibrational energy and rotational constant of the nu4 state are obtained to be 686.50(76) cm-1 and 2068.2387(21) MHz, respectively, where the uncertainties correspond to one standard deviation. Copyright 1999 Academic Press.     

Evolving dark energy with w not = -1

Theories of evolving quintessence are constructed that generically lead to deviations from the w = -1 prediction of nonevolving dark energy. The small mass scale that governs evolution, m(phi) approximately = 10(-33) eV, is radiatively stable, and the "Why now?" problem is solved. These results rest on seesaw cosmology: Fundamental physics and cosmology can be broadly understood from only two mass scales, the weak scale nu and the Planck scale M. Requiring a scale of dark energy rho(DE)(1/4) governed by nu2/M and a radiatively stable evolution rate m(phi) given by nu4/M3 leads to a distinctive form for the equation of state w(z). Dark energy resides in the potential of a hidden axion field that is generated by a new QCD-like force that gets strong at the scale lambda approximately = nu2/M approximately = rho(DE)(1/4). The evolution rate is given by a second seesaw that leads to the axion mass m(phi) approximately = lambda2/f, with f approximately = M.     

Propagation of ultrawideband short pulses of terahertz radiation through submillimeter-diameter circular waveguides

We report experimental investigations in which quasi-optical methods were used to efficiently couple freely propagating, optoelectronically generated, subpicosecond pulses of terahertz (THz) radiation into submillimeter circular metal tubes (waveguides) and, consequently, to measure the transmitted pulses from these waveguides. We observe very dispersive low-loss propagation over the frequency band from 0.65 to 3.5 THz, with frequency-dependent group velocities nu(g) ranging from c/4 to c and phase velocities nu(p) from 4c to c , where nu(g)nu(p)=c(2) . The linearly polarized incoming THz pulses couple significantly only into the TE(11) , TM(11) , and TE(12) modes of the utilized 24- and 4-mm-long stainless-steel tubes, with inside diameters of 240 and 280microm , respectively.