Recent developments in X-UV optics and X-UV diagnostics

Metrology of XUV beams (X-ray lasers, high-harmonic generation and VUV free-electron lasers) is of crucial importance for the development of applications. We have thus developed several new optical systems enabling us to measure the optical properties of XUV beams. By use of a Michelson interferometer working as a Fourier-transform spectrometer, the line shapes of different X-ray lasers have been measured with a very high accuracy (/10^-6). Achievement of the first XUV wavefront sensor has enabled us to measure the beam quality of laser-pumped as well as discharge-pumped X-ray lasers. A capillary discharge X-ray laser has demonstrated a very good wavefront allowing us to achieve an intensity as high as 3×10¹⁴ Wcm^-2 by focusing with a f=5 cm mirror. The sensor accuracy has been measured using a calibrated spherical wave generated by diffraction. The accuracy has been estimated to be as good as /120 at 13 nm. Commercial developments are underway. At Laboratoire dOptique Appliquée, we are setting up a new beamline based on high-harmonic generation in order to start the femtosecond, coherent XUV optic . PACS 07.85.Nc;32.70.Jz;41.50.+h;42.15.Dp;42.55.Vc;52.70.La     

Universal sound absorption in low-temperature amorphous solids

A general elastohydrodynamic theory is developed based on the phenomenological assumption of a sharp decrease of shear relaxation time at large wave vectors k>k(xi), where k(xi) is of order of inverse of several interatomic distances a. This theory describes the low-energy excitations of glassy and amorphous solids, which contribute to anomalous linear-in-temperature specific heat and limit phonon thermal conductivity. The ratio of the wavelength of the phonon, lambda, to its mean free path, l, which is the universal property of sound absorption in glasses, is derived in this theory to be lambda/l=(2/3)(c(t)/c(l))(2)(k(xi)a)(3), where c(t) and c(l) are transverse and longitudinal sound velocities correspondingly.     

Dynamic alignment in driven magnetohydrodynamic turbulence

Motivated by recent analytic predictions, we report numerical evidence showing that in driven incompressible magnetohydrodynamic turbulence the magnetic- and velocity-field fluctuations locally tend to align the directions of their polarizations. This dynamic alignment is stronger at smaller scales with the angular mismatch between the polarizations decreasing with the scale lambda approximately as theta(lambda) is proportional to lambda(1/4). This can naturally lead to a weakening of the nonlinear interactions and provide an explanation for the energy spectrum E(k) is proportional to k(-3/2) that is observed in numerical experiments of strongly magnetized turbulence.     

Spectrum of magnetohydrodynamic turbulence

We propose a phenomenological theory of strong incompressible magnetohydrodynamic turbulence in the presence of a strong large-scale external magnetic field. We argue that in the inertial range of scales, magnetic-field and velocity-field fluctuations tend to align the directions of their polarizations. However, the perfect alignment cannot be reached; it is precluded by the presence of a constant energy flux over scales. As a consequence, the directions of shear-Alfvén fluid and magnetic-field fluctuations at each scale lambda become effectively aligned within the angle phi(lambda) proportional to lambda (1/4), which leads to scale-dependent depletion of the nonlinear interaction and to the field-perpendicular energy spectrum E(k(perpendicular)) proportional to k(perpendicular)(-3/2). Our results may be universal, i.e., independent of the external magnetic field, since small-scale fluctuations locally experience a strong field produced by large-scale eddies.     

Spectral energy dynamics in magnetohydrodynamic turbulence

Spectral direct numerical simulations of incompressible MHD turbulence at a resolution of up to 1024(3) collocation points are presented for a statistically isotropic system as well as for a setup with an imposed strong mean magnetic field. The spectra of residual energy, E(R)k=|E(M)k - E(K)k|, and total energy, Ek=E(K)k+E(M)k, are observed to scale self-similarly in the inertial range as E(R)k approximately k(-7/3), E(k)approximately k(-5/3) (isotropic case) and E(R)(k(perpendicular) approximately k(-2)(perpendicular), E(k(perpendicular))approximately k(-3/2)(perpendicular) (anisotropic case, perpendicular to the mean field direction). A model of dynamic equilibrium between kinetic and magnetic energy, based on the corresponding evolution equations of the eddy-damped quasinormal Markovian closure approximation, explains the findings. The assumed interplay of turbulent dynamo and Alfvén effect yields E(R)k approximately kE2(k), which is confirmed by the simulations.     

一般幂指数中心势作用下的波包回归和部分回归

利用WKB近似和自关联函数方法,我们研究了一般幂指数中心势V(r)=rk (-20)作用下波包的回归和部分回归。对于排斥势(>0, k>0), 势是一长程势,量子化能级结构中只有一个量子数,波包的回归结构和一维幂指数势的情况类似。这一结果表明能级结构相同的体系具有相同的波包回归结构。 对于吸引势,能级结构中有两个量子数, 当 k取不同的值时,波包的回归结构不同。对于库仑吸引势,波包回归和部分回归出现; 但是对于其它的k值, 经过一段时间后,波包出现坍塌。本文的研究对于探讨里德堡原子和分子中电子运动的经典极限提供了一个新的方法。     

一般幂指数中心势作用下的波包回归和部分回归

利用WKB近似和自关联函数方法,我们研究了一般幂指数中心势V(r)=γrk (-20)作用下波包的回归和部分回归.对于排斥势(γ>0, k>0), 势是一长程势,量子化能级结构中只有一个量子数,波包的回归结构和一维幂指数势的情况类似.这一结果表明能级结构相同的体系具有相同的波包回归结构.对于吸引势,能级结构中有两个量子数, 当 k取不同的值时,波包的回归结构不同.对于库仑吸引势,波包回归和部分回归出现;但是对于其它的k值, 经过一段时间后,波包出现坍塌.本文的研究对于探讨里德堡原子和分子中电子运动的经典极限提供了一个新的方法.     

Subcritical bifurcations and nonlinear balloons in faraday waves

Bicritical points at wave numbers k(b) larger than the critical wave numbers k(c) are found in parametric surface waves (Faraday waves) using both numerical simulations and nonlinear analysis. Because k(b)-k(c) is small, it is argued that subcritical bifurcations at k>k(b) can be easily observed in experiments. In the second part we present a generic argument predicting the existence of nonlinear states resembling a balloon outside the instability region. The prediction is confirmed in simulations and it is argued to apply to other systems with similar instability curves.     

Universality and self-similarity of an energy-constrained sandpile model with random neighbors

We study an energy-constrained sandpile model with random neighbors. The critical behavior of the model is in the same universality class as the mean-field self-organized criticality sandpile. The critical energy E(c) depends on the number of neighbors n for each site, but the various exponents are independent of n. A self-similar structure with n-1 major peaks is developed for the energy distribution p(E) when the system approaches its stationary state. The avalanche dynamics contributes to the major peaks appearing at E(p(k))=2k/(2n-1) with k=1,2,...,n-1, while the fine self-similar structure is a natural result of the way the system is disturbed.     

Pressure spectrum in homogeneous turbulence

The pressure spectrum in homogeneous steady turbulence is studied using direct numerical simulation with resolution up to 1024(3) and the Reynolds number R(lambda) between 38 and 478. The energy spectrum is found to have a finite inertial range with the Kolmogorov constant K = 1.65+/-0.05 followed by a bump at large wave numbers. The pressure spectrum in the inertial range is found to be approximately P(k) = B(p)epsilon;(4/3)k(-7/3) with B(p) = 8.0+/-0.5, and followed by a bump of nearly k(-5/3) at higher wave numbers. Universality and a new scaling of the pressure spectrum are discussed.     

Energy spectrum of quasigeostrophic turbulence

We consider the energy spectrum of a quasigeostrophic model of forced, rotating turbulent flow. We provide a rigorous a priori bound E(k)相似文献   

Limited path percolation in complex networks

We study the stability of network communication after removal of a fraction q=1-p of links under the assumption that communication is effective only if the shortest path between nodes i and j after removal is shorter than al(ij)(a> or =1) where l(ij) is the shortest path before removal. For a large class of networks, we find analytically and numerically a new percolation transition at p(c)=(kappa(0)-1)((1-a)/a), where kappa(0) [triple bond] / and k is the node degree. Above p(c), order N nodes can communicate within the limited path length al(ij), while below p(c), N(delta) (delta<1) nodes can communicate. We expect our results to influence network design, routing algorithms, and immunization strategies, where short paths are most relevant.     

Boson representations of symplectic algebras

A reduction of the boson representation of the algebra of the noncompact groupSp(4k, R), k>0, to its subgroupSU(k) is realized. The reduction scheme has two main branches: one through the totally symmetric unitary representations of the maximal compact subalgebra u(2k); the other through the ladder representations of the noncompact subalgebrau(k, k). Both reductions are accomplished by means of the same set of Hermitian operators, but taken in different order. The case ofk=3, for the groupSp(12,R), used in the interacting vector boson model, is discussed in more detail.     

Anomalous self-similarity in a turbulent rapidly rotating fluid

Our velocity measurements on quasi-two-dimensional turbulent flow in a rapidly rotating annulus yield self-similar (scale-independent) probability distribution functions for longitudinal velocity differences, deltav(l) = v(x+l)-v(x). These distribution functions are strongly non-Gaussian, suggesting that the coherent vortices play a significant role. The structure functions <[deltav(l)](p)> approximately l(zeta)p exhibit anomalous scaling: zeta(p) = p / 2 rather than the expected zeta(p) = p / 3. Correspondingly, the energy spectrum is described by E(k) approximately k(-2) rather than the expected E(k) approximately k(-5/3).     

Magnetic penetration depth measurements of Pr2-xCexCuO4-delta films on buffered substrates: evidence for a nodeless gap

We report measurements of the inverse squared magnetic penetration depth, lambda(-2)(T), in Pr(2-x)Ce(x)CuO(4-delta) (0.115< or =x < or =0.152) superconducting films grown on SrTiO3 (001) substrates coated with a buffer layer of insulating Pr2CuO4. lambda(-2)(0), T(c), and normal-state resistivities of these films indicate that they are clean and homogeneous. Over a wide range of Ce doping, 0.124< or =x < or =0.144, lambda(-2)(T) at low T is flat: it changes by less than 0.15% over a factor of 3 change in T, indicating a gap in the superconducting density of states. Fits to the first 5% decrease in lambda(-2)(T) produce values of the minimum superconducting gap in the range of 0.29< or =Delta(min)/k(B)T(c)< or =1.01.     

Neural mechanism for binaural pitch perception via ghost stochastic resonance

We present a physiologically plausible binaural mechanism for the perception of the pitch of complex sounds via ghost stochastic resonance. In this scheme, two neurons are driven by noise and a different periodic signal each (with frequencies f(1)=kf(0) and f(2)=(k+1)f(0), where k>1), and their outputs (plus noise) are applied synaptically to a third neuron. Our numerical results, using the Morris-Lecar neuron model with chemical synapses explicitly considered, show that intermediate noise levels enhance the response of the third neuron at frequencies close to f(0), as in the cases previously described of ghost resonance. For the case of an inharmonic combination of inputs (f(1)=kf(0)+Deltaf and f(2)=(k+1)f(0)+Deltaf) noise is also seen to enhance the rates of most probable spiking for the third neuron at a frequency f(r)=f(0)+[Deltaf(k+12)]. In addition, we show that similar resonances can be observed as a function of the synaptic time constant. The suggested ghost-resonance-based stochastic mechanism can thus arise either at the peripheral level or at a higher level of neural processing in the perception of pitch.     

Evidence for the two-pole structure of the lambda(1405) resonance

The K- p --> pi0pi0sigma0 reaction is studied within a chiral unitary model. The distribution of pi0sigma0 states forming the lambda(1405) shows, in agreement with a recent experiment, a peak at 1420 MeV and a relatively narrow width of gamma = 38 MeV. The mechanism for the reaction is largely dominated by the emission of a pi0 prior to the K- p interaction leading to the lambda(1405). This ensures the coupling of the lambda(1405) to the K- p channel, thus maximizing the contribution of the second state found in chiral unitary theories, which is narrow and of higher energy than the nominal lambda(1405). This is unlike the pi- p --> K0pisigma reaction, which gives more weight to the pole at lower energy and with a larger width. The data of these two experiments, together with the present theoretical analysis, provide firm evidence of the two-pole structure of the lambda(1405).     

Orange,red and deep-red flashlamp-pumped Pr3+:LiYF4 laser with improved output energy and efficiency

Three laser transitions (607.2 nm, 639.5 nm, 720.9 nm) in a flashlamp-pumped Pr³⁺:LiYF₄ laser at room temperature are investigated. Cerium ions (Ce³⁺) doped into the fused-quartz envelope of the flashlamp efficiently transfer strong UV radiation from pumping light into the absorption region of Pr³⁺ ions ( = 420–480 nm), so that the slope efficiency of all three laser emissions could be increased by almost 100% compared to excitation with a pure quartz flashlamp. At a pump energy of 30 J the output energy of these three laser emissions reached 4.7 mJ, 87 mJ and 30 mJ.     

Bianchi Type Ⅲ String Cosmological Model with Bulk Viscosity

The Bianchi type Ⅲ cosmological model for a cloud string with bulk viscosity are presented. To obtain a determinate model, an equation of state ρ=kλ and a relation between metric potentials B = C^n are assumed. The physical and geometric aspects of the model are also discussed. The model describes a shearing non-rotating continuously expanding universe with a big-bang start, and the relation between the coefficient of bulk viscosity and the energy density is ζ∝ρ^1/2.