Electrostrictive laser-induced gratings for time-resolved observation of translational-rotational energy transfer in H2

Electrostrictive laser-induced gratings (LIG) have been formed in H₂ of various pressures (0.5…5 bar) and with frequencies of the Brillouin modes (sound waves) of about 14 MHz. Under these conditions, the rotational degrees of freedom can only partially follow the temperature variation from the acoustic oscillation. A model is given, which completes the linearized fluid dynamical equations with equations for the rotational state relaxation, separately for the average relaxation of the states for Ortho-H₂ and Para-H₂. Using such a model, the dispersion of sound frequency is reproduced with an accuracy of a few percent. However, the total dissipation of sound and the observed strong alternation in peak heights in the temporal evolution of the LIG intensity is only approximately predicted by the model.     

Polarization-dependent laser-induced grating measurements

The formation of electrostrictive gratings during laser-induced grating (LIG) experiments is considered in an electromagnetic rather than an electrostatic approach. A different form of the relation that was used previously for the electrostrictive pressure was achieved. The theoretical findings were experimentally verified by polarization-dependent LIG spectroscopy measurements of a mixture of nitrogen with methanol vapor at high pressure with 1064-nm radiation. Conditions for suppressing the contribution of the electrostrictive grating relative to that of the simultaneously generated thermal grating signals were found theoretically and experimentally. The technique can potentially increase detection sensitivity for population gratings in high-density gas mixtures.     

Heterodyne-detected electrostrictive laser-induced gratings for gas-flow diagnostics

The signal of heterodyne-detected electrostrictive laser-induced gratings (LIGs) originates from the interference of a reference laser beam with the laser light diffracted at the counterpropagating sound-wave packets, which are generated in the overlap volume of crossed laser beams. The frequencies of the sound waves, which contain the information on the sound velocity and on the motion of the medium, can approximately be extracted from the frequencies at the maxima of the two peaks, which dominate the power spectrum of the heterodyne LIG signal intensity. In free-air jets behind a slot nozzle, flow velocities up to 60 ms^-1 were determined by quick fits from the power spectrum and by fitting the time-dependent signal intensity data to model functions. Results agree well, the standard deviations being about one-half in the latter case (1.6–2.6 ms^-1, for positions close to the nozzle). Problems arising from the sampling and from the finite observation time of the signal intensity are discussed. Furthermore, the results of the LIG measurements were compared with data provided by laser Doppler anemometry. As an application of the instantaneous and unseeded LIG measurement technique, temperatures in heated air jets were determined simultaneously with the flow velocity by quick fits from the power spectrum. PACS 42.62.-b; 47.62.+q; 43.58.+z     

Application of narrow band laser ultrasonics to the nondestructive evaluation of thin bonding layers

In this paper, a modified laser induced grating technique (LIG) has been utilized to generate narrow band surface waves in an epoxy-bonded copper-aluminum layered structure. A high performance optical interferometer system was utilized to detect the laser-generated surface waves. The dispersion of surface wave in an epoxy-bonded copper-aluminum specimen was measured and compared with the theoretical solution. An inverse algorithm based on the simplex method was then introduced to determine the bonding thickness as well as the elastic properties of the bonding layer. The inversion results demonstrated that the thickness in the microm range or the elastic properties of the bonding layer could be successfully determined.     

Top-seeded infiltration growth of Y–Ba–Cu–O bulk superconductors

A top-seeded melt-growth (TSMG) process is widely used to fabricate single domain YBa₂Cu₃O_y (Y–Ba–Cu–O) bulk superconductors. Pores are often found in the TSMG-processed Y–Ba–Cu–O samples due to the oxygen gas evolution during the molten stage. Recently developed liquid infiltration growth (LIG) process is known to be effective in suppressing the pore evolution and in refining the size of Y₂BaCuO₅ (Y211) particles dispersed in YBa₂Cu₃O_y matrix. The LIG process utilizes the liquid (Ba₃Cu₅O₈) infiltration into a pre- sintered Y211 contact and slow cooling through a peritectic temperature. In this study, we fabricated bulk Y–Ba–Cu–O superconductors by the LIG process combined with top-seeding with SmBa₂Cu₃O_y seed and confirmed that a single-domain bulk can be produced. Trapped field measurements however showed that some distortion in the field distribution was observed in the region near the seed crystal, which was attributed to Y211 density and its relatively large size.     

Visualization of the photorefractive grating in BSO

A technique based on self-imaging at finite conjugates has been experimentally demonstrated for visualization of laser-induced photorefractive gratings in BSO. A simple model theory has been used to provide the correct rules of geometrical optics of self-imaging. A novel technique has been used for photoinducing the periodic structure. An application has been made to determine the harmonic components of the nonsinusoidal refractive index modulation in the BSO for a grating period of 10 m and for external DC voltages of 6.0 and 3.0 kV. The technique is directly applicable to the visualization and study of laser-induced gratings (LIG) in thin nonlinear optical media.     

Gas-phase diagnostics by laser-induced gratings II. Experiments

In this article we review the results achieved in the past ten years at the Paul Scherrer Institute on the topic of diagnostics in the gas phase by laser-induced gratings (LIGs). The technique has been applied for thermometry in air and in flames at different pressures, for flow velocimetry, for concentration measurements, and for imaging purposes. The influence of collisional energy-transfer and relaxation processes in molecules on the temporal evolution of the LIG signals has also been investigated. It has been demonstrated that, for molecules with a low fluorescence quantum yield, excitation of laser-induced thermal gratings can be used as a sensitive spectroscopic tool. For the quantitative interpretation of the experiments shown in this work, the findings presented in the companion paper [1] have been used.     

Smoothing out negative tension brane

We propose an extension of the five-dimensional gravitational action with an external source in order to allow arbitrary smoothing of the negative tension brane in the Randall–Sundrum model. This extended action can be derived from a model with an auxiliary four form field coupled to the gravity. We point out a further generalization of our model in relation to tachyon condensation. A possible mechanism for radion stabilization in our model is also discussed.     

Mixing search on complex networks

In this article, we derive the first passage time (FPT) distribution and the mean first passage time (MFPT) of random walks from multiple sources on networks. On the basis of analysis and simulation, we find that the MFPT drops substantially when particle number increases at the first stage, and converges to the shortest distance between the sources and the destination when particle number tends to infinite. Given the fact that a Brownian particle from a high-degree node often needs a large number of steps to reach an expected low-degree node, which is the bottleneck for a single random walk, we propose a mixing search model to improve the efficiency of search processes by using random walks from multiple sources to continue the searches from high-degree nodes to destinations. We compare our model with the mixing navigation model proposed by Zhou on complex networks and find that our model converges much faster with lower hardware cost than Zhou’s model. Moreover, simulations on scale-free networks show that the search efficiency of our model is much higher than that of a single random walk, and comparable to that of multiple random walks which have much higher hardware cost than our model. Finally, we discuss the traffic cost of our model, and propose an absorption strategy for our model to recover the additional walkers in networks. Simulations indicate that this strategy reduces the traffic cost of our model effectively.     

The pressure in the Huang-Yang-Luttinger model of an interacting boson gas

This completes our study of the equilibrium thermodynamics of the Huang-Yang-Luttinger model of a boson gas with a hard-sphere repulsion. In an earlier paper we obtained a lower bound on the pressure, but our proof of an upper bound held only for a truncated version of the model. In this paper we establish an upper bound on the pressure in the full model; the upper and lower bounds coincide and provide a variational formula for the pressure. The proof relies on recent second-level large deviation results for the occupation measure of the free boson gas.     

Accelerating universe from an evolving Λ in higher dimension

We find exact solutions in 5D inhomogeneous matter dominated model with a varying cosmological constant. Adjusting arbitrary constants of integration one can also achieve acceleration in our model. Aside from an initial singularity our spacetime is regular everywhere including the centre of the inhomogeneous distribution. We also study the analogous homogeneous universe in (4 + d) dimensions. Here an initially decelerating model is found to give late acceleration in conformity with the current observational demands. We also find that both anisotropy and number of dimensions have a role to play in determining the time of flip, in fact the flip is delayed in multidimensional models. Some astrophysical parameters like the age, luminosity distance, etc., are also calculated and the influence of extra dimensions is briefly discussed. Interestingly our model yields a larger age of the universe compared to many other quintessential models.     

一种基于布朗粒子的混合搜索模型

基于网络上的布朗粒子运动基本原理,提出了一种单粒子和多粒子相结合的混合搜索模型.该模型将一次搜索过程分成单粒子搜索与多粒子搜索两个阶段,既克服了单粒子搜索效率低下的缺点,又降低了多粒子搜索的硬件代价.在各种复杂网络拓扑上实施该模型,并与混合导航模型进行比较.结果表明,混合搜索模型的平均搜索时间收敛更快,硬件代价更小.将度大优先的目标选择策略与混合搜索模型相结合,能进一步提高搜索效率.此外通过仿真发现,在无标度网络上混合搜索模型的效率远高于单粒子随机行走,与多粒子随机行走的效率相当,但硬件代价远小于多粒子行走.最后针对该模型给出了一种能有效降低负载的"吸收"策略.     

A realistic model for complex networks with local interaction,self-organization and order

In this paper, a new mechanism for the emergence of scale-free distribution is proposed. It is more realistic than the existing mechanism. Based on our mechanism, a model responsible for the scale-free distribution with an exponent in a range of 3-to-5 is given. Moreover, this model could also reproduce the exponential distribution that is discovered in some real networks. Finally, the analytical result of the model is given and the simulation shows the validity of our result.     

A new bistatic model for electromagnetic scattering from perfectly conducting random surfaces: numerical evaluation and comparison with SPM

This paper is a companion to our previous contribution deriving a new approximate bistatic model for electromagnetic scattering from perfectly conducting rough surfaces. We evaluate this model numerically and compare it with an 'exact' numerical solution of the scattering problem. This comparison shows good agreement between our approximation and numerical solution for a wide range of incident and scattering angles. However, for horizontal-incident horizontal-scattered polarization (HH-pol), the model exhibits strong deviation from the 'exact' solution for near-grazing scattering angles. The model shows a similar divergence at HH-pol when compared with the small-perturbation method (SPM). The cause of this divergence is explained. During the SPM comparison, we noticed that the integral equation method model also does not reproduce the SPM limit except for forward and backscatter geometries. We propose in this paper a simple modification of our model to ensure agreement with the bistatic SPM approximation when applicable, and show that the modified model also yields close agreement with numerical computations even when the surface roughness does not satisfy the SPM condition.     

Variational Bayesian Inference for Nonlinear Hawkes Process with Gaussian Process Self-Effects

Traditionally, Hawkes processes are used to model time-continuous point processes with history dependence. Here, we propose an extended model where the self-effects are of both excitatory and inhibitory types and follow a Gaussian Process. Whereas previous work either relies on a less flexible parameterization of the model, or requires a large amount of data, our formulation allows for both a flexible model and learning when data are scarce. We continue the line of work of Bayesian inference for Hawkes processes, and derive an inference algorithm by performing inference on an aggregated sum of Gaussian Processes. Approximate Bayesian inference is achieved via data augmentation, and we describe a mean-field variational inference approach to learn the model parameters. To demonstrate the flexibility of the model we apply our methodology on data from different domains and compare it to previously reported results.     

Hexadecapolar Kondo effect in URu2Si2?

We derive the coupling of a localized hexadecapolar mode to conduction electrons in tetragonal symmetry. The derivation can be easily adapted to arbitrary multipoles in an arbitrary environment. We relate our model to the two-channel Kondo (2CK) model and show that for an f(2) configuration a relevant crystal field splitting in addition to the 2CK interaction is intrinsic to tetragonal symmetry. We discuss possible realizations of a hexadecapolar Kondo effect in URu(2)Si(2). Solving our model we find good agreement with susceptibility and specific heat measurements in Th(1-x)U(x)Ru(2)Si(2) (x?1).     

Geographical influences of an emerging network of gang rivalries

We propose an agent-based model to simulate the creation of street gang rivalries. The movement dynamics of agents are coupled to an evolving network of gang rivalries, which is determined by previous interactions among agents in the system. Basic gang data, geographic information, and behavioral dynamics suggested by the criminology literature are integrated into the model. The major highways, rivers, and the locations of gangs’ centers of activity influence the agents’ motion. We use a policing division of the Los Angeles Police Department as a case study to test our model. We apply common metrics from graph theory to analyze our model, comparing networks produced by our simulations and an instance of a Geographical Threshold Graph to the existing network from the criminology literature.