Pulsed Nonlinear Surface Acoustic Waves in Crystals

Abstract: Predictions from a recent theory for the propagation of nonlinear surface waves in anisotropic solids are comp~ed with measurements of laser-generated surface-wave pulses in silicon. These are the first reported comparisons of theory and experiment for the nonlinear evolution of surface waves in a crystal. THEORY AND EXPERIMENT A theoretical model was developed recently [1] that describes the propagation of plane nonlinear surface waves in anisotropic media. The spectral equations for the jth vector component (j = x, y, z) of the particle velocity in the surface wave are where x is the direction of propagation, z the coordinate normal to the traction-free surface of the solid, w the fundamental angular frequency and k the corresponding wavenurnber in the expansion; tinj (z) are eigenfunctions of the linear problem, and Rlm is a nonlinearity matrix that is known explicitly in terms of the second-and third-order elastic constants of the material. The coupled equations for Vn(x) are solved numerically, with the ad hoc absorption coefficients a~= n2Q1 intreduced for numerical stability when shocks are formed. Numerical simulations were reported for propagation in several real tryst als, and for different surface cuts and propagation directions [2]. This previous work [1,2] focused on radiation from monofrequency sources. Our purpose here is to report comparison of the theory with measurements of pulsed nonlinear surface waves in crystalline silicon. The present work thus extends an earlier comparison of theory and experiment [3] for nonlinear Rayleigh waves in an isotropic solid. Generation and detection of the surface waves were accomplished with Nd:YAG l~ers [3][4][5]. Laser radiation of wavelength 1064 nm, pulse duration 7 ns, and energy up to 50 mJ generated the surface waves. The radiation was focused with a cylindrical lens into a thin strip 6 mm by 50 pm on the surface of crystalline silicon cut along its (11 1) plane. To enhance the conversion of optical to elastic energy, a liquid layer having a large optical absorption coefficient was deposited on the surface of the silicon in the excitation region. The surface waves were detected by the deflection of a probe laser beam (diode pumped Nd:YAG, wavelength 532 nm, power 40 mW) that irradiated spots of approximately 4 pm in diameter on the surface of the silicon at distances 5 mm and 21 mm from the excitation region. The reflected probe signals were detected by two photodiodes, the output from which is proportional to the vertical velocity component v. at the surface. Surface wave pulses in these experiments had durations of 20-40 ns and peak strains of order 10-2. RESULTS Figures 1(a)-(c) show the measured waveforms and peak-normalized spectrum at distance x = 5 mm from the excitation region. Propagation of the surface wave was in the (112) direction of the (111) plane, and because of symmetry VV= O. Linear theory was used to compute the horizontal velocity VZ [1(b)] from the measured vertical velocity v= [1(a)]. The measurement at z = 5 mm was used w the starting condition for the computations, and the resulting predictions for the waveforms and spectra at z = 21 mm are shown as dashed lines i

Search for top quark partners with charge 5/3 in proton-proton collisions at √s=13 TeV

A search for the production of heavy partners of the top quark with charge 5/3 (X-5/3) decaying into a top quark and a W boson is performed with a data sample corresponding to an integrated luminosity of 2.3 fb(-1), collected in proton-proton collisions at a center-of-mass energy of 13 TeV with the CMS detector at the CERN LHC. Final states with either a pair of same-sign leptons or a single lepton, along with jets, are considered. No significant excess is observed in the data above the expected standard model background contribution and an X-5/3 quark with right-handed (left-handed) couplings is excluded at 95% confidence level for masses below 1020 (990) GeV. These are the first limits based on a combination of the same-sign dilepton and the single-lepton final states, as well as the most stringent limits on the X-5/3 mass to date

Measurement of ZZ production in pp collisions at root s=7 TeV and limits on anomalous ZZZ and ZZ gamma couplings with the ATLAS detector

"A measurement of the ZZ production cross section in proton-proton collisions at root s = 7 TeV using data recorded by the ATLAS experiment at the Large Hadron Collider is presented. In a data sample corresponding to an integrated luminosity of 4.6 fb(-1) collected in 2011, events are selected that are consistent either with two Z bosons decaying to electrons or muons or with one Z boson decaying to electrons or muons and a second Z boson decaying to neutrinos. The ZZ((*)) -> l(+)l(-)l'(+)l'(-) and ZZ -> l(+)l(-) nu(nu) over bar cross sections are measured in restricted phase-space regions. These results are then used to derive the total cross section for ZZ events produced with both Z bosons in the mass range 66 to 116 GeV, sigma(tot)(ZZ) = 6.7 +\/- 0.7 (stat.) (+0.4)(-0.3) (syst.) +\/- 0.3 (lumi.) pb, which is consistent with the Standard Model prediction of 5.89(-0.18)(+0.22) pb calculated at next-to-leading order in QCD. The normalized differential cross sections in bins of various kinematic variables are presented. Finally, the differential event yield as a function of the transverse momentum of the leading Z boson is used to set limits on anomalous neutral triple gauge boson couplings in ZZ production.