Singular field decomposition based on path-independent integrals

A method is developed by which the field generated by a source (measured or numerically evaluated) is decomposed in a series of singular self-equilibrating linear elastic field components. These elementary sources form an elastic equivalent of the real source, the set of their magnitudes representing a ‘spectral decomposition’ of the investigated field. The decomposition is performed using path–independent interaction integrals computed through the field of interest, far from the source. The method may be used to investigate internal field sources, such as dislocations, as well as sources of field perturbation, such as crack tips or wedges. The method is applied to the analysis of the core structure of an edge dislocation in aluminium. The dislocation is represented in an atomistic simulation and a model core is sought in the form of a series of multipoles or elementary linear elastic field sources. The field of the dislocation is composed from the Volterra solution and higher–order components induced by the nonlinear behaviour of the material in the core region. The magnitude of the component elementary sources is determined from the numerically evaluated field far from the core. This expansion characterizes the core structure. Its variation associated with the core deformation as the dislocation is loaded against the Peierls barrier is investigated.     

Synthetic Impedance Circuits in Semi-Passive Vibration Control with Piezo-Ceramics - Efficiency and Limitations

Electrical circuits that represent complex impedance connected to the plates of piezo element affect the frequency response function of compound electromechanical systems. The external network consisting of inductance results in absorbing a certain frequency of vibrations. The performance can significantly be increased using negative capacitance. All the negative impedance elements are achievable via semi-passive way with the use of an electronic gyrator realized by operational amplifiers that require power supply. Its performance is limited due to the maximum voltage operational amplifiers can produce. The higher excitation forces require higher output voltages of amplifiers. The aim of the paper is to present the relation between the required voltage on synthetic impedance and force excitation in 1DOF vibrating system regarding parameters of piezo element and mechanical system for absorbing case. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

High-efficiency 60 W TEM(00) Nd:YVO(4) oscillator pumped at 888 nm

We propose a technique for pumping Nd:YVO(4) with high optical power at 888nm while making absorption independent of the pump light polarization state. This is especially suitable for systems end pumped by high-power, high-brightness fiber-coupled diode sources associated with long vanadate crystals to effectively spread the heat load in a large volume. A compact 60 W output, 55% optical efficiency cw TEM(00) oscillator was demonstrated.     

47 W, 6 ns constant pulse duration, high-repetition-rate cavity-dumped Q-switched TEM(00) Nd:YVO(4) oscillator

We report on a cavity-dumped Q-switched TEM(00) Nd:YVO(4) oscillator offering a unique combination of high power, constant short pulse duration, and high repetition rate, suppressing the gain dependence of pulse duration in classical Q-switched oscillators. Its performance is compared with that of the same oscillator operated in a classical Q-switched regime, demonstrating the much higher peak powers achievable with this technique, especially at high repetition rates. Up to 31 W of 532 nm green light was generated by frequency doubling in a noncritical phase matched LBO crystal, corresponding to 70% conversion efficiency.     

Raman characterization of passivated GaAs surfaces

The technique of Raman scattering at room temperature, is used to investigate the effect of H₂S passivation of the surface of n-type GaAs. Well-defined LO, L^- and L⁺ features are distinguished in spectra which have been recorded in z(x, y)[ovbar|zovbar] scatte ring orientation. It is observed that the ratio of the LO to L^- peak is reduced by the effects of the passivation process and that the shift of the L⁺ feature from the laser line is decreased. This latter effect, it is suggested, is caused by a decrease in free-carrier concentration due to donor neutralization by hydrogen during passivation. This neutralization effect will also affect the LO to L^- ratio and so complicate a quantitative analysis of the influence of passivation on the surface barrier potential.     

A Linear Bound On The Diameter Of The Transportation Polytope*

We prove that the combinatorial diameter of the skeleton of the polytope of feasible solutions of any m×n transportation problem is at most 8(m+n−2). * Research for this paper was done while the second and third author were visiting the Isaac Newton Institute for Mathematical Sciences, Cambridge, U.K. All authors were supported by the TMR Network DONET of the European Community ERB TMRXCT98-0202. † Research partially funded by the Dutch BSIK/BRICKS project.     

An Inverse-Ackermann Type Lower Bound For Online Minimum Spanning Tree Verification*

Given a spanning tree T of some graph G, the problem of minimum spanning tree verification is to decide whether T = MST(G). A celebrated result of Komlós shows that this problem can be solved with a linear number of comparisons. Somewhat unexpectedly, MST verification turns out to be useful in actually computing minimum spanning trees from scratch. It is this application that has led some to wonder whether a more flexible version of MST verification could be used to derive a faster deterministic minimum spanning tree algorithm. In this paper we consider the online MST verification problem in which we are given a sequence of queries of the form “Is e in the MST of T ∪{e}?”, where the tree T is fixed. We prove that there are no linear-time solutions to the online MST verification problem, and in particular, that answering m queries requires Ω(mα(m,n)) time, where α(m,n) is the inverse-Ackermann function and n is the size of the tree. On the other hand, we show that if the weights of T are permuted randomly there is a simple data structure that preprocesses the tree in expected linear time and answers queries in constant time. * A preliminary version of this paper appeared in the proceedings of the 43rd IEEE Symposium on Foundations of Computer Science (FOCS 2002), pages 155–163. † This work was supported by Texas Advanced Research Program Grant 003658-0029-1999, NSF Grant CCR-9988160, and an MCD Graduate Fellowship.