Metal-neon compound ions in slow field evaporation

A magnetic sector atom-probe has been employed to study slow field evaporation of most of the transition metals in vacuum, in neon and in a mixture of neon and hydrogen. Various metals were found to form metal-neon molecular ions. Slow field evaporation and the presence of hydrogen are favorable for their formation. All the experiments were done at 78 K. The metals that evaporate as nei'des abundantly are Ti, Zn, Zr, Nb, Mo, Pd and Ta, of which Ti, Nb and Pd produce neïde ions as much as 80 to 90%. There were also some neïdes with W, Re, Ir and Rh, definitely above the detection limit estimated to be 3% of the field evaporating metal ions. The role of hydrogen is thought to be two-fold: At the surface, hydrogen adsorption is assumed to cause a stronger metal-neon bond, while the electron shower from free space ionization of the auxiliary gas excites or ionizes the complex by electron impact to allow evaporation at a reduced field.     

Aging of quenched polypropylene

Structural changes occurring during the aging of polypropylene quenched from the melt have been investigated by measurements of gas transport parameters and mechanical properties. Major changes in diffusion and sorption behavior during the first 30 hr of aging at room temperature were found to be strongly dependent on the atomic size of the inert gas penetrants. Tensile mechanical properties increased with aging time, while resilience and mechanical damping showed more complex behavior. The interpretation of the combined results suggests that the aging process may involve molecular rearrangements generally comparable to those for secondary crystallization processes, but within domains comparable in size to that of the smaller penetrant species. The relaxation of a residual stress distribution related to nonuniform plastic flow during quenching may be superimposed on the rearrangement phenomena.     

Random lifts of graphs: Independence and chromatic number

For a graph G, a random n‐lift of G has the vertex set V(G)×[n] and for each edge [u, v] ∈ E(G), there is a random matching between {u}×[n] and {v}×[n]. We present bounds on the chromatic number and on the independence number of typical random lifts, with G fixed and n→∞. For the independence number, upper and lower bounds are obtained as solutions to certain optimization problems on the base graph. For a base graph G with chromatic number χ and fractional chromatic number χ_f, we show that the chromatic number of typical lifts is bounded from below by const ? and also by const ? χ_f/log²χ_f (trivially, it is bounded by χ from above). We have examples of graphs where the chromatic number of the lift equals χ almost surely, and others where it is a.s. O(χ/logχ). Many interesting problems remain open. © 2002 John Wiley & Sons, Inc. Random Struct. Alg., 20, 1–22, 2002     

Estimating Missing Values Using Neural Networks

The problem of missing values is common in statistical analysis. One approach to deal with missing values is to delete the incomplete cases from the data set. This approach may disregard valuable information, especially in small samples. An alternative approach is to reconstruct the missing values using the information in the data set. The major purpose of this paper is to investigate how a neural network approach performs compared to statistical techniques for reconstructing missing values. The backpropagation algorithm is used as the learning method to reconstruct missing values. The results of back-propagation are compared with results from two methods, viz., (1) using averages, and (2) using iterative regression analysis, to compute missing values. Experimental results show that backpropagation consistently outperforms other methods in both the training and the test data sets, and suggest that the neural network approach is a useful tool for reconstructing missing values in multivariate analysis.     

Stability and largeness of core for symmetric games

Largeness of the core is sufficient for stability of the core. In general the necessity is not known. In this paper we answer affirmatively the necessity for symmetric games. We also prove its equivalence to n specified vectors being imputations and also to the convexity of the lower boundary of the set of all acceptable pay-off vectors of the game. In this paper we establish the equivalence of a condition given by Shapley to the newly evolved condition, thereby give an alternate proof to Shapley’s condition. Received: January 1997/final version: March 1999     

Random Graph Coverings I: General Theory and Graph Connectivity

In this paper we describe a simple model for random graphs that have an n-fold covering map onto a fixed finite base graph. Roughly, given a base graph G and an integer n, we form a random graph by replacing each vertex of G by a set of n vertices, and joining these sets by random matchings whenever the corresponding vertices are adjacent in G. The resulting graph covers the original graph in the sense that the two are locally isomorphic. We suggest possible applications of the model, such as constructing graphs with extremal properties in a more controlled fashion than offered by the standard random models, and also "randomizing" given graphs. The main specific result that we prove here (Theorem 1) is that if is the smallest vertex degree in G, then almost all n-covers of G are -connected. In subsequent papers we will address other graph properties, such as girth, expansion and chromatic number. Received June 21, 1999/Revised November 16, 2000 RID="*" ID="*" Work supported in part by grants from the Israel Academy of Aciences and the Binational Israel-US Science Foundation.     

Synthesis and characterization of nanostructured (Pb1− x Sr x )TiO3 thin films by a modified chemical route

The coating solutions of nanostructured (Pb_{1– x} Sr _x )TiO₃ (PST) thin films have been prepared by the sol–gel combined metallo-organic decomposition method. The coating solutions were deposited on Pt/Ti/SiO₂/Si substrates using a spin-coating technique with spinning speed of 4300 rpm and annealed at 650°C. The effect of Sr content in reducing the grain size and tetragonal distortion of PST films has been studied. The optimum conditions for crystalline phase formation in the films have been analyzed by thermogravimetric, differential thermal analysis and Fourier transform infrared spectroscopy. The phase and microstructure of the films were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The XRD pattern shows that the PST thin films are crystallized into tetragonal structures without any impurity phase and the distortion ratio reduces with increasing Sr concentration. The AFM results indicate an increase in grain size with increasing annealing temperature of the film and reduction in grain size with increasing Sr concentration.     

Investigation on wavelength re-modulated bi-directional passive optical network for different modulation formats

In this paper, the cost effective bi-directional passive optical network architecture with wavelength remodulated scheme is investigated. To realize the cost-effective PON, remodulation scheme is used, in which the downstream optical signal is reused as a carrier for the upstream transmission as it eliminates the need for an extra laser source at optical network units. The performance of proposed passive optical network is analyzed and compared for various modulation formats such as Non Return to Zero (NRZ), Return to Zero (RZ) and On–Off Keying (OOK) with 64 optical networks units (ONUs) at different traffic speed for downlink and uplink, respectively. It has been observed that the most suitable data format for proposed PON network is NRZ. Further the proposed system performance is compared with the current state-of-the-art PON architectures.     

Theoretical analysis of pit formation in GaAs surfaces in picosecond and femtosecond laser ablation regimes

In the present work, the effect of pit formation in laser-ablated GaAs surfaces is analyzed theoretically. The formation of pits has been previously observed during laser ablation experiments in the picosecond (35 ps) and the femtosecond (100 fs) regimes. In the present work, it is shown theoretically that the thermal conductivity of the laser-ablated GaAs surface is changed both in the picosecond and the femtosecond regimes as compared to the unexposed surface. The variation in thermal conductivity of an ablated surface depends upon the fraction of the thermal conductivity of the cracks (), and the fraction of the cross-sectional area of the cracks (γ). In the picosecond regime (35 ps), the thermal conductivity of the ablated GaAs surface decreases (both for single and multiple laser pulses) for all the values of and γ (between 0.1 and 0.9) as compared to the thermal conductivity of a smooth film with no pits. In the femtosecond regime (100 fs), the thermal conductivity of the ablated surface increases or decreases (both for single and multiple laser pulses) depending upon the value of and γ.