Showing papers by "École Polytechnique Fédérale de Lausanne published in 1993"

The physics of simple metal clusters: experimental aspects and simple models

Abstract: The study of simple metal clusters has burgeoned in the last decade, motivated by the growing interest in the evolution of physical properties from the atom to the bulk solid, a progression passing through the domain of atomic clusters. On the experimental side, the rapid development of new techniques for producing the clusters and for probing and detecting them has resulted in a phenomenal increase in our knowledge of these systems. For clusters of the simplest metals, the alkali and noble metals, the electronic structure is dominated by the number of valence electrons, and the ionic cores are of secondary importance. These electrons are delocalized, and the electronic system exhibits a shell structure that is closely related to the well-known nuclear shell structure. In this article the results from a broad range of experiments are reviewed and compared with theory. Included are the behavior of the mass-abundance spectra, polarizabilities, ionization potentials, photoelectron spectra, optical spectra, and fragmentation phenomena.

A user's guide to tabu search

Abstract: We describe the main features of tabu search, emphasizing a perspective for guiding a user to understand basic implementation principles for solving combinatorial or nonlinear problems. We also identify recent developments and extensions that have contributed to increasing the efficiency of the method. One of the useful aspects of tabu search is the ability to adapt a rudimentary prototype implementation to encompass additional model elements, such as new types of constraints and objective functions. Similarly, the method itself can be evolved to varying levels of sophistication. We provide several examples of discrete optimization problems to illustrate the strategic concerns of tabu search, and to show how they may be exploited in various contexts. Our presentation is motivated by the emergence of an extensive literature of computational results, which demonstrates that a well-tuned implementation makes it possible to obtain solutions of high quality for difficult problems, yielding outcomes in some settings that have not been matched by other known techniques.

Chaos shift keying: modulation and demodulation of a chaotic carrier using self-synchronizing Chua's circuits

Abstract: Describes a technique for transmitting digital information using a chaotic carrier. Each symbol to be transmitted is coded as an attractor in Chua's circuit. The symbols are detected at the receiver by cascaded self-synchronizing Chua's circuit subsystems. A proof of the synchronization effect is demonstrated using weak assumptions on the statistical behavior of the chaotic carrier to be transmitted. Furthermore a bound for the average time of synchronization is given. Results of both practical experimentation and simulations are presented which verify the approach. >

Probabilistic modelling of microstructure formation in solidification processes

Abstract: A new approach to the modelling of grain structure formation in solidification processes is proposed. Based upon a two-dimensional cellular automata technique, the model includes the mechanisms of heterogeneous nucleation and of grain growth. Nucleation occurring at the mould wall as well as in the liquid metal are treated by using two distributions of nucleation sites. The location and the crystallographic orientation of the grains are chosen randomly among a large number of cells and a certain number of orientation classes, respectively. The growth kinetics of the dendrite tip and the preferential 〈100〉 growth directions of cubic metals are taken into account. The model is then applied to small specimens of uniform temperature. The columnar-to-equiaxed transition, the selection and extension of columnar grains which occur in the columnar zone and the impingement of equiaxed grains are clearly shown by this technique. The calculated effect of the alloy concentration and cooling rate upon the resultant microstructure agree with experimental observations.

Parallel iterative search methods for vehicle routing problems

Abstract: This paper presents two partition methods that speed up iterative search methods applied to vehicle routing problems including a large number of vehicles. Indeed, using a simple implementation of taboo search as an iterative search method, every best-known solution to classical problems was found. The first partition method (based on a partition into polar regions) is appropriate for Euclidean problems whose cities are regularly distributed around a central depot. The second partition method is suitable for any problem and is based on the arborescence built from the shortest paths from any city to the depot. Finally, solutions that are believed to be optimum are given for problems generated on a grid. © 1993 by John Wiley & Sons, Inc.

Particle tracking velocimetry in three-dimensional flows

Abstract: Particle Tracking Velocimetry (PTV) is a well-known technique for the determination of velocity vectors within an observation volume. However, for a long time it has rarely been applied because of the intensive effort necessary to measure coordinates of a large number of flow marker particles in many images. With today's imaging hardware in combination with the methods of digital image processing and digital photogrammetry, however, new possibilities have arisen for the design of completely automatic PTV systems. A powerful 3D PTV has been developed in a cooperation of the Institute of Geodesy and Photogrammetry with the Institute of Hydromechanics and Water Resources Management at the Swiss Federal Institute of Technology. In this paper hardware components for 3D PTV systems wil be discussed, and a strict mathematical model of photogrammetric 3D coordinate determination, taking into account the different refractive indices in the optical path, will be presented. The system described is capable of determining coordinate sets of some 1000 particles in a flow field at a time resolution of 25 datasets per second and almost arbitrary sequence length completely automatically after an initialization by an operator. The strict mathematical modelling of the measurement geometry, together with a thorough calibration of the system provide for a coordinate accuracy of typically 0.06 mm in X, Y and 0.18 mm in Z (depth coordinate) in a volume of 200 × 160 × 50 mm3.

Association of integrase, matrix, and reverse transcriptase antigens of human immunodeficiency virus type 1 with viral nucleic acids following acute infection

Abstract: We have examined components of the preintegration complex of human immunodeficiency virus type 1 (HIV-1) and have analyzed features which govern the association of these components. HIV-1 nucleoprotein complexes, isolated from nuclear and cytoplasmic extracts of CD4+ cells after acute virus infection, contained viral RNA and DNA in association with viral matrix (MA), integrase (IN), and reverse transcriptase (RT) antigens but not capsid (CA) antigens and possessed integration activity in vitro. Association of IN but not RT or MA antigens with viral DNA was detergent-stable. Analysis of viral DNA synthesis and nuclear import of viral nucleoprotein complexes in the presence of a reversible RT inhibitor demonstrated that reverse transcription of viral RNA could be completed entirely in the host cell nucleus. Our studies demonstrate structural and functional features of the nucleoprotein (preintegration) complex of HIV-1 which are pertinent to the understanding of early events in the lentiviral life cycle.

Public participation in decision making: A three-step procedure

Abstract: This article introduces a novel model of public particpation in political decisions. Structured in three consecutive steps, the model is based on the view that stakeholders, experts, and citizens should each contribute to the planning effort their particular expertise and experience. Stakeholders are valuable resources for eliciting concerns and developing evaluative criteria since their interests are at stake and they have already made attempts to structure and approach the issue. Experts are necessary to provide the data base and the functional relationships between options and impacts. Citizens are the potential victims and benefactors of proposed planning measures; they are the best judges to evaluate the different options available on the basis of the concerns and impacts revealed through the other two groups. The three-step model has been developed and frequently applied as a planning tool in West Germany. We compare this experience with the model's first application in the United States, and conclude that the three-step procedure offers a limited, but promising future for democratizing policy making in the United States.

Regularity properties and pathologies of position-space renormalization-group transformations: Scope and limitations of Gibbsian theory

Abstract: We reconsider the conceptual foundations of the renormalization-group (RG) formalism, and prove some rigorous theorems on the regularity properties and possible pathologies of the RG map. Our main results apply to local (in position space) RG maps acting on systems of bounded spins (compact single-spin space). Regarding regularity, we show that the RG map, defined on a suitable space of interactions (=formal Hamiltonians), is always single-valued and Lipschitz continuous on its domain of definition. This rules out a recently proposed scenario for the RG description of first-order phase transitions. On the pathological side, we make rigorous some arguments of Griffiths, Pearce, and Israel, and prove in several cases that the renormalized measure is not a Gibbs measure for any reasonable interaction. This means that the RG map is ill-defined, and that the conventional RG description of first-order phase transitions is not universally valid. For decimation or Kadanoff transformations applied to the Ising model in dimension d⩾3, these pathologies occur in a full neighborhood {β>β0, ¦h¦

New Photolabeling and Crosslinking Methods

Abstract: INTRODUCTION AND SUMMARY 483 PHOTOLABILE GROUPS AND REACTIVE INTERMEDIATES . . . . . . . . . . . . 485 Generators of Carbenes and Nitrenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . 486 Arylketones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 489 Reagents for Target Site-Selective Activation . . . . . . . . . . . . . . . . . . . . . . . 489 PHOTOLABELlNG OF THE APOLAR CORE OF MEMBRANES . . . . . . . . . . . 490 Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 490 Design and Properties of Reagents .. . . ...... . . . . . ... . .. . . . . .... 491 General Features of Hydrophobic Photolabeling of Membranes. . . . . . . . . . . . 494 Hydrophobic Photolabeling of Integral Proteins . . . . . . . . . . . . . . . . . . . . . 496 Recent Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 500 Photo-activatable Phospholipids as Photo-affinity Probes . . . . . . . . . . . . . . . . 502 APPROACHES TO "SITE-SPECIFIC" PHOTOCROSSLINKING . . . . . . . . . . . . 503 Synthetic Peptides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 503 Photo-activatable Proteins Through Modification of Reactive Amino Acid Residues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505 Site-Directed, Biosynthetic Incorporation of Photo-activatable Amino Acids Into Proteins . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 506 CONCLUSIONS AND PROSPECTS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 509

Ecology, metabolite production, and substrate utilization in endophytic fungi.

Abstract: Endophytic fungi are a taxonomically and ecologically heterogenous group of organisms, mainly belonging to the Ascomycotina and Deuteromycotina. The isolation methods affect the species composition of the endophyte assemblage in a given host. The number of endophyte taxa isolated from a host species is usually large; however, only few, normally host specific species or strains are dominant. Endophyte assemblages are specific at the host species level, but species composition and frequencies are significantly affected by site-specific conditions. Moreover, the relative importance and number of endophytic species vary among individuals within sites. In some cases, each individual could be considered a separate ecosystem. In general, however, 40 individuals with 30 to 40 sampling units per organ and individual should be enough to detect 80% of taxa present in a given host at one site. Endophytes usually produce the enzymes necessary for the colonization of plant tissues. Substrate utilization studies and isozyme analysis have demonstrated that most endophytes are able to utilize most plant cell components. The production of growth promoting factors and of metabolites useful in the pharmaceutical and agricultural industry is widespread among endophytic fungi. The usefulness of endophytes in agricultural and pharmaceutical research is briefly discussed.

Mobile Robot Miniaturisation: A Tool for Investigation in Control Algorithms

Abstract: The interaction of an autonomous mobile robot with the real world critically depends on the robots morphology and on its environment Building a model of these aspects is extremely complex, making simulation insufficient for accurate validation of control algorithms

Magnetic moments of iron clusters with 25 to 700 atoms and their dependence on temperature.

Abstract: Magnetic moments μ(N) of iron clusters in a molecular beam, with temperatures ranging from 100 to 1000 K, are investigated from their Stern-Gerlach deflections. We find that at a temperature of 120 K, μ (25≤N≤130) is 3μ B per atom, decreasing to about the bulk value (2.2μ B per atom) near N=500. For all sizes, μ decreases with increasing temperature, and is approximately constant above a temperature T C (N). For example, T C (130) is about 700 K, and T C (550) is about 550 K (T C bulk=1043 K). Limitations of the superparamagnetic model due to rotational erects are discussed

Improved myocardial tagging contrast

Abstract: Myocardial tagging is a new noninvasive MRI method that allows the study of myocardial motion with high accuracy. However, with conventional tagging techniques tagging contrast is impaired at later heart phases due to longitudinal relaxation. An improved method, called Complementary SPAtial Modulation of Magnetization (CSPAMM), which separates the component of the magnetization with the tagging information from the relaxed component by subtraction of two measurements with first a positive and then a negative tagging grid, is presented. This technique improves the grid contrast and greatly facilitates the automatic evaluation of the myocardial motion. Thus the motion assessment of the entire heart cycle becomes possible. The improvements are documented by numerical simulations and by experiments on phantoms and on human volunteers.

Lightning-induced voltages on overhead lines

Abstract: A modeling procedure that permits calculation of lightning-induced voltages on overhead lines starting from the channel-base current is discussed. The procedure makes use of a coupling model already presented in the literature, based on transmission line theory, for field-to-overhead line coupling calculations. Both models are discussed and compared with experimental results. The hypothesis of perfect conducting ground, used to analyze the voltages induced on an overhead line by a nearby lightning return stroke with a striking point equidistant from the line terminations, and the limits of its validity are determined. A comparison shows that peak value and maximum front steepness of the induced voltages calculated using other lightning return-stroke models differ. It is also shown that another coupling model used in the power-lightning literature by several other authors may result in a less accurate estimation of the induced voltages. >

Controls on Sonic Velocity in Carbonates

Abstract: Compressional and shear-wave velocities (V p and V s ) of 210 minicores of carbonates from different areas and ages were measured under variable confining and pore-fluid pressures. The lithologies of the samples range from unconsolidated carbonate mud to completely lithified limestones. The velocity measurements enable us to relate velocity variations in carbonates to factors such as mineralogy, porosity, pore types and density and to quantify the velocity effects of compaction and other diagenetic alterations.

The role of microorganisms in mobilization and fixation of phosphorus in sediments

Abstract: Cycling of phosphorus (P) at the sediment/water interface is generally considered to be an abiotic process. Sediment bacteria are assumed to play only an indirect role by accelerating the transfer of electron from electron donors to electron acceptors, thus providing the necessary conditions for redoxand pH-dependent, abiotic sorption/desorption or precipitation/dissolution reactions.

Collective dipole oscillations in small silver clusters embedded in rare-gas matrices.

Abstract: The optical-absorption spectra of small mass-selected ${\mathrm{Ag}}_{\mathit{N}}$ clusters (N=2\char21{}21) embedded in solid argon are measured in the energy range 2.5\char21{}6.2 eV. Investigation of a continuous range of cluster sizes reveals the size development of the photoabsorption behavior. The main features of the spectra are in agreement with predictions based on the Nilsson-Clemenger shell model. The measured photoabsorption cross sections are consistent with a sum-rule calculation involving only s electrons. In addition, the medium influence on the spectra is studied by changing the matrix gas from argon to krypton and xenon for the cluster sizes N=7, 11, 15, and 21.

Welch’s Bound and Sequence Sets for Code-Division Multiple-Access Systems

Abstract: Welch’s bound for a set of M complex equi-energy sequences is considered as a lower bound on the sum of the squares of the magnitudes of the inner products between all pairs of these sequences. It is shown that, when the sequences are binary (±-1 valued) sequences assigned to the M users in a synchronous code-division multiple-access (S-CDMA) system, precisely such a sum determines the sum of the variances of the interuser interference seen by the individual users. It is further shown that Welch’s bound, in the general case, holds with equality if and only if the array having the M sequences as rows has orthogonal and equi-energy columns. For the case of binary (±-1 valued) sequences that meet Welch’s bound with equality, it is shown that the sequences are uniformly good in the sense that, when used in a S-CDMA system, the variance of the interuser interference is the same for all users. It is proved that a sequence set corresponding to a binary linear code achieves Welch’s bound with equality if and only if the dual code contains no codewords of Hamming weight two. Transformations and combination of sequences sets that preserve equality in Welch’s bound are given and used to illustrate the design and analysis of sequence sets for non-synchronous CDMA systems.

Solving real-life vehicle routing problems efficiently using tabu search

Abstract: This paper presents a tabu search based method for finding good solutions to a real-life vehicle routing problem. The problem considered deals with some new features beyond those normally associated with the classical problems of the literature: in addition to capacity constraints for vehicles and time windows for deliveries, it takes the heterogeneous character of the fleet into account, in the sense that utilization costs are vehicle-dependent and that some accessibility restrictions have to be fulfilled. It also deals with the use of trailers. In spite of the intricacy of the problem, the proposed tabu search approach is easy to implement and can be easily adapted to many other applications. An emphasis is placed on means that have to be used to speed up the search. In a few minutes of computation on a personal workstation, our approach obtains solutions that are significantly better than those previously developed and implemented in practice.