Critical dynamics of cluster algorithms in the dilute Ising model

Autocorrelation times for thermodynamic quantities atT _C are calculated from Monte Carlo simulations of the site-diluted simple cubic Ising model, using the Swendsen-Wang and Wolff cluster algorithms. Our results show that for these algorithms the autocorrelation timesdecrease when reducing the concentration of magnetic sites from 100% down to 40%. This is of crucial importance when estimating static properties of the model, since the variances of these estimators increase with autocorrelation time. The dynamical critical exponents are calculated for both algorithms, observing pronounced finite-size effects in the energy autocorrelation data for the algorithm of Wolff. We conclude that, when applied to the dilute Ising model, cluster algorithms become even more effective than local algorithms, for whichincreasing autocorrelation times are expected.     

Jet reconstruction in heavy ion collisions

We examine the problem of jet reconstruction at heavy-ion colliders using jet-area-based background subtraction tools as provided by FastJet. We use Monte Carlo simulations with and without quenching to study the performance of several jet algorithms, including the option of filtering, under conditions corresponding to RHIC and LHC collisions. We find that most standard algorithms perform well, though the anti-k _t and filtered Cambridge/Aachen algorithms have clear advantages in terms of the reconstructed p _t offset and dispersion.     

Relaxation Model and Mapping of Magnetic Field Gradients in MRI

The purpose of the work is development of algorithms for separate mapping of T ₂ relaxation time and gradients, using gradient recalled echo (GRE) sequence. Application of three-dimensional (3D) model of gradients and their volumetric averaging within a voxel lead to analytical model of relaxation function, which is consistent with experimental data for both regular macroscopic and randomized micro- and mesoscopic gradients. The model is verified by fitting into experimental data obtained on specially made phantoms. Verification of algorithms is completed by comparing gradient maps obtained on specially made cylindrical phantoms with theoretical maps of their exact 3D electro-dynamic solutions. Analytical model of relaxation function proved to be in good agreement with experimental relaxation curves. On the basis of this model, fast and unambiguous fittingless algorithms were developed. Gradient maps measured on special cylindrical phantoms are in good qualitative agreement with theory. 3D statistical model and fittingless algorithms provide the basis for separating the GRE signal into two meaningful parameters—T ₂ and gradients, thus doubling information from magnetic resonance imaging.     

Jet algorithms in electron–positron annihilation: perturbative higher order predictions

This article gives results on several jet algorithms in electron–positron annihilation: Considered are the exclusive sequential recombination algorithms Durham, Geneva, Jade-E0 and Cambridge, which are typically used in electron–positron annihilation. In addition also inclusive jet algorithms are studied. Results are provided for the inclusive sequential recombination algorithms Durham, Aachen and anti-k _t , as well as the infrared-safe cone algorithm SISCone. The results are obtained in perturbative QCD and are N³LO for the two-jet rates, NNLO for the three-jet rates, NLO for the four-jet rates and LO for the five-jet rates.     

B-spline Galerkin solutions for the equal width equation

The equal width equation has been solved numerically using the Galerkin finite-element method, based on the Adams-Moulton method for time integration and quadratic/cubic B-splines for space integration. The two proposed algorithms are tested on the problems of propagation of a solitary wave and interaction of two solitary waves. For the first test problem, the convergence rate is computed for the proposed algorithms and the L _?? error norm is used to measure the differences between the exact and numerical solutions. The three conserved quantities of motion are calculated to determine the conservation properties of the two proposed algorithms for both test problems.     

Structural, energetic, and electronic properties of Sin, Gen, and SinGen clusters

Results of a theoretical study of the properties of Si_n, Ge_n, and Si_nGe_n clusters are presented. An approximate density-functional method in combination with genetic algorithms have been used in an unbiased determination of the structures of the lowest total energy. The resulting structural, energetic, and electronic properties are analysed and compared with each other for the different systems.     

Convex relaxation based sparse algorithm for hyperspectral target detection

Target detection in hyperspectral images is an important task. In this paper, we propose a sparsity based algorithm for target detection in hyperspectral images. In sparsity model, each hyperspectral pixel is represented by a linear combination of a few samples from an overcomplete dictionary, and the weighted vector for such reconstruction is sparse. This model has been applied in hyperspectral target detection and solved with several greedy algorithms. As conventional greedy algorithms may be trapped into a local optimum, we consider an alternative way to regularize the model and find a more accurate solution to the model. The proposed method is based on convex relaxation technique. The original sparse representation problem is regularized with a properly designed weighted ?₁ minimization and effectively solved with existing solver. The experiments on synthetic and real hyperspectral data suggest that the proposed algorithm outperforms the classical sparsity-based detection algorithms, such as Simultaneous Orthogonal Matching Pursuit (SOMP) and Simultaneous Subspace Pursuit (SSP) and conventional ?₁ minimization.     

Use of Cm and cross product coefficients in influence coefficient algorithms for quantitative XRF analysis

The use of influence coefficients in algorithms for the correction of matrix effects in quantitative XRF analysis is discussed. In principle, multi‐element influence coefficients should be used. In practice, many algorithms are based on the use of binary influence coefficients, even when dealing with multi‐component specimens. This involves approximations which may affect the accuracy of the analysis. It is shown that the effects of this approximation can be reduced, provided that the binary influence coefficients are calculated for the correct composition (this involves the use of C_m, where C_m is the sum of the concentrations of the matrix elements, i.e. C_m = 1 ? C_i) and by applying cross‐product coefficients. Copyright © 2004 John Wiley & Sons, Ltd.     

信号处理改善波长调制光谱灵敏度的实验研究

基于可调谐半导体激光吸收光谱，提出了一种新的信号处理方法用于提高吸收光谱测量的灵敏度. 研究结果表明：采用信号处理（DSP）可以使波长调制光谱的信噪比提高一个数量级,且测量CO₂分子的时候发现一条新谱线. 这些信号处理包括多次平均、数字低通滤波以及最小二乘法拟合. 由于该方法不需要增加设备的复杂性，所以比其他噪声抑制技术更容易在实验中实施、有更实际的应用价值. 关键词： 波长调制 信号处理 半导体激光器 吸收光谱     

QEPAS for chemical analysis of multi-component gas mixtures

A gas sensor based on quartz-enhanced photoacoustic spectroscopy and using near-infrared, fiber-coupled diode lasers as an excitation source was developed for chemical analysis of gas mixtures containing H₂S, CO₂, and CH₄ at concentrations from 0 to 100%. Analysis of physical phenomena affecting the sensor operation is performed, the sensor performance is evaluated, and simple algorithms are developed to derive concentrations of the gases from detected electrical signals.     

Optically—thick accretion discs with advection

The structures of optically-thick accretion discs with radial advection have been investigated by the iteration and integration algorithms. The advective cooling term changes mostly the inner part of disc solution, and even results in an optically-thick advection-dominated accretion flow (ADAF). Three distinct branches－the outer Shakura－Sunyaev disc (SSD), the inner ADAF and the middle transition layer－are found for a super-Eddington disc. The SSD－ADAF transition radius can be estimated as 18(\dot{M}/\dot{M}_E)R_G where R_G is the Schwarzschild radius, \dot{M} is the mass accretion rate and \dot{M}_E is the Eddington accretion rate. SSD solutions calculated with the iteration and integration methods are identical, while ADAF solutions obtained by these two methods differ greatly. Detailed algorithms and their differences have been analysed. The iteration algorithm is not self-consistent, since it implies that the dimensionless advection factor ξ is invariant, but in the inner ADAF region the variation of ξ is not negligible. The integration algorithm is always effective for the whole region of an optically-thick disc if the accretion rate is no smaller than 10^-4\dot{M}_E. For optically-thin discs, the validity of these two algorithms is different. We suggest that the integration method be employed to calculate the global solution of a disc model without assuming ξ to be a constant. We also discuss its application to the emergent continuum spectrum in order to explain observational facts.     

An algorithmic approach to solving polynomial equations associated with quantum circuits

In this paper we present two algorithms for reducing systems of multivariate polynomial equations over the finite field F ₂ to the canonical triangular form called lexicographical Gröbner basis. This triangular form is the most appropriate for finding solutions of the system. On the other hand, the system of polynomials over F ₂ whose variables also take values in F ₂ (Boolean polynomials) completely describes the unitary matrix generated by a quantum circuit. In particular, the matrix itself can be computed by counting the number of solutions (roots) of the associated polynomial system. Thereby, efficient construction of the lexicographical Gröbner bases over F ₂ associated with quantum circuits gives a method for computing their circuit matrices that is alternative to the direct numerical method based on linear algebra. We compare our implementation of both algorithms with some other software packages available for computing Gröbner bases over F ₂.     

Estimating the efficiency of ensemble quantum computing

A straightforward method to estimate the efficiency of ensemble quantum computing using nuclear magnetic resonance spectroscopy is presented by defining a quantum computing efficiency factor (QC_ε). This method allows critical evaluation of the performance of quantum gates and algorithms and provides a way to design efficient quantum computer.     

Nondestructive Quantification of Foliar Chlorophyll in an Apple Orchard by Visible/Near-Infrared Reflectance Spectroscopy and Partial Least Squares

Chlorophylls respond rapidly to the current physiological status of a tree and reflect nutrient availability. Visible/near-infrared spectroscopy was attempted to determine foliar chlorophyll content in an apple orchard. Backward interval partial least squares and genetic algorithms were sequentially applied to select an optimized spectral interval and an optimized combination of spectral regions selected from informative regions in model calibration. Backward interval partial least squares was used to remove the noninformative regions, which significantly reduced the number of variables. The subsequent application of genetic algorithms-partial least squares to this reduced domain could lead to an efficient and refined model. The performance of the final model was back-evaluated according to root mean square error of calibration (RMSEC) and the correlation coefficient (R _c ) in the calibration set, and was then tested by root mean square error of prediction (RMSEP) and the correlation coefficient (R _p ) in the prediction set. The optimal backward interval partial least squares-genetic algorithms model was obtained with 5 partial least squares factors with 3 spectral regions and 71 variables selected. The measurement results of the final model were achieved as follows: RMSEC = 0.26, R _c  = 0.91 in the calibration set; and RMSEP = 0.22, R _p  = 0.91 in the prediction set. This experiment showed that visible/near-infrared spectroscopy and backward interval partial least squares-genetic algorithms are useful tools for nondestructively assessing foliar chlorophyll content and may have potential application for field assessments in decision-making and operational fertilizer management programs for apple orchards.     

Molecular many-body dissociation: Data reduction strategies in translational spectroscopy

Advanced time- and position-sensitive multi-hit detectors allow to study molecular breakup processes into two, three, and more massive fragments by translational spectroscopy. We discuss the feasibility to perform kinematically complete final state analysis of complex molecular dissociation processes using such detectors. We have developed new algorithms to determine - for an arbitrary number of fragments - the fragment momentum vectors in the center-of-mass frame from the measured positions and arrival time differences. These algorithms can easily be implemented to perform online data reduction in coincidence experiments. We have tested the new data reduction strategies in an experimental study and in Monte-Carlo simulations of realistic experimental conditions. We show that the new algorithms can discriminate between two-, three-, and four-body decay of a four-atomic molecule and can uniquely determine the momentum vectors of all fragments. For two-body decay, we find that the accuracy of the new algorithm is superior to the frequently used approximate formula introduced by DeBruijn and Los. We demonstrate this improvement in the evaluation of experimental data for the decay of laser-excited triatomic hydrogen H₃ 3s (N=1,K=0) into H + H₂(v,J) fragment pairs. Received 12 April 2000     

Monte Carlo simulation of magnetic ordering in the Gd3Fe5O12 Ising ferrite with garnet structure

The simulation of magnetic properties in extended quantum spin networks can be done in good conditions with Ising models within Monte Carlo–Metropolis algorithms, as our systematic studies, employing original computer codes, proved. The present analysis provides interesting insights into the exchange interactions governing the magnetic behavior of the Gd₃Fe₅O₁₂ system, taken as a prototype for ferrites with garnet structure. Effective exchange interaction parameters are estimated by fitting the computed with experimental compensation temperature and temperature dependence of the magnetization.     

Algorithms for extraction of spacing information from a head/tape interface

Various algorithms for the extraction of spacing information from an interferometric fringe pattern of the head/tape interface have been proposed. In this paper, a comparison of three algorithms is presented: tape unloading, polynomial fitting and Fourier transform. The relative merits of each method are discussed, together with implementation details. In an experiment with a transparent head and CrO₂ tape, it was found that all methods give a similar result to within ±5 nm, with none unilaterally preferable to the others. The Fourier transform method, which has not been used for this application before, gives smooth spacing data for high fringe orders, and is less sensitive to fringe order errors, but produces noisier spacing data in the contact region.     

Global structure optimization study on Au 2-20

The geometries and electronic properties of the most stable small Au_n clusters with n=2 to 20 are presented. An intensive search for low-energy minima of Au_n clusters was carried through using a density-functional tight-binding method combined with genetic algorithms for an unbiased global structure optimization. The structural and energetic properties of the small gold clusters are compared with those of planar Au_n clusters with n=5 to 15.     

Effective dipole moment of rotational transitions of X 2 Y molecules

Contributions determining the rotational dependence of the effective dipole moment of molecules are calculated for the ground state of H₂S and H₂O molecules. The calculation is carried out in various ordering algorithms of perturbation theory. It is shown that the convergence of the effective dipole moment for the ground state of an H₂O molecule in the polynomial representation is rather slow in the rotational operator J _z (the convergence radius is K*≤17). Nonpolynomial forms of the dipole moment as a function of rotational operators are discussed.     

Global search algorithms in surface structure determination using photoelectron diffraction

Three different algorithms to effect global searches of the variable-parameter hyperspace are compared for application to the determination of surface structure using the technique of scanned-energy mode photoelectron diffraction (PhD). Specifically, a new method not previously used in any surface science methods, the swarm-intelligence-based particle swarm optimisation (PSO) method, is presented and its results compared with implementations of fast simulated annealing (FSA) and a genetic algorithm (GA). These three techniques have been applied to experimental data from three adsorption structures that had previously been solved by standard trial-and-error methods, namely H₂O on TiO₂(110), SO₂ on Ni(111) and CN on Cu(111). The performance of the three algorithms is compared to the results of a purely random sampling of the structural parameter hyperspace. For all three adsorbate systems, the PSO out-performs the other techniques as a fitting routine, although for two of the three systems studied the advantage relative to the GA and random sampling approaches is modest. The implementation of FSA failed to achieve acceptable fits in these tests.