Showing papers by "Delft University of Technology published in 2002"

Electron transport through double quantum dots

Abstract: Electron transport experiments on two lateral quantum dots coupled in series are reviewed. An introduction to the charge stability diagram is given in terms of the electrochemical potentials of both dots. Resonant tunneling experiments show that the double dot geometry allows for an accurate determination of the intrinsic lifetime of discrete energy states in quantum dots. The evolution of discrete energy levels in magnetic field is studied. The resolution allows one to resolve avoided crossings in the spectrum of a quantum dot. With microwave spectroscopy it is possible to probe the transition from ionic bonding (for weak interdot tunnel coupling) to covalent bonding (for strong interdot tunnel coupling) in a double dot artificial molecule. This review is motivated by the relevance of double quantum dot studies for realizing solid state quantum bits.

Enhanced gilbert damping in thin ferromagnetic films.

Abstract: The precession of the magnetization of a ferromagnet is shown to transfer spins into adjacent normal metal layers. This ``pumping'' of spins slows down the precession corresponding to an enhanced Gilbert damping constant in the Landau-Lifshitz equation. The damping is expressed in terms of the scattering matrix of the ferromagnetic layer, which is accessible to model and first-principles calculations. Our estimates for permalloy thin films explain the trends observed in recent experiments.

Green, Catalytic Oxidations of Alcohols

Abstract: Three methods are described, in the context of the guiding principles of green chemistry, for the catalytic oxidation of alcohols. The first employs a recyclable oligomeric TEMPO catalyst (PIPO) and sodium hypochlorite as the oxidant in a bromide-free and chlorinated hydrocarbon solvent-free system. The second involves a ruthenium/TEMPO catalyst and oxygen as the oxidant. The third consists of a recyclable water-soluble palladium-diamine complex in conjunction with air as the oxidant in an aqueous biphasic system. The mechanisms of the ruthenium/TEMPO- and palladium-catalyzed oxidations are discussed, and the mechanism of the former is compared with that of the analogous copper/TEMPO catalyst.

General model for representing variable speed wind turbines in power system dynamics simulations

Abstract: A tendency to erect ever more wind turbines can be observed in order to reduce the environmental consequences of electric power generation. As a result of this, in the near future, wind turbines may start to influence the behavior of electric power systems by interacting with conventional generation and loads. Therefore, wind turbine models that can be integrated into power system simulation software are needed. In this contribution, a model that can be used to represent all types of variable speed wind turbines in power system dynamics simulations is presented. First, the modeling approach is commented upon and models of the subsystems of which a variable speed wind turbine consists are discussed. Then, some results obtained after incorporation of the model in PSS/E, a widely used power system dynamics simulation software package, are presented and compared with measurements.

Robust Positioning Algorithms for Distributed Ad-Hoc Wireless Sensor Networks

Abstract: A distributed algorithm for determining the positions of nodes in an ad-hoc, wireless sensor network is explained in detail. Details regarding the implementation of such an algorithm are also discussed. Experimentation is performed on networks containing 400 nodes randomly placed within a square area, and resulting error magnitudes are represented as percentages of each node’s radio range. In scenarios with 5% errors in distance measurements, 5% anchor node population (nodes with known locations), and average connectivity levels between neighbors of 7 nodes, the algorithm is shown to have errors less than 33% on average. It is also shown that, given an average connectivity of at least 12 nodes and 10% anchors, the algorithm performs well with up to 40% errors in distance measurements.

Spin pumping and magnetization dynamics in metallic multilayers

Abstract: We study the magnetization dynamics in thin ferromagnetic films and small ferromagnetic particles in contact with paramagnetic conductors. A moving magnetization vector causes ‘‘pumping’’ of spins into adjacent nonmagnetic layers. This spin transfer affects the magnetization dynamics similar to the Landau-LifshitzGilbert phenomenology. The additional Gilbert damping is significant for small ferromagnets, when the nonmagnetic layers efficiently relax the injected spins, but the effect is reduced when a spin accumulation build-up in the normal metal opposes the spin pumping. The damping enhancement is governed by ~and, in turn, can be used to measure! the mixing conductance or spin-torque parameter of the ferromagnet‐normal-metal interface. Our theoretical findings are confirmed by agreement with recent experiments in a variety of multilayer systems.

Mercury, Fish Oils, and the Risk of Myocardial Infarction

Abstract: Background It has been suggested that mercury, a highly reactive heavy metal with no known physiologic activity, increases the risk of cardiovascular disease. Because fish intake is a major source of exposure to mercury, the mercury content of fish may counteract the beneficial effects of its n–3 fatty acids. Methods In a case–control study conducted in eight European countries and Israel, we evaluated the joint association of mercury levels in toenail clippings and docosahexaenoic acid (C22:6n–3, or DHA) levels in adipose tissue with the risk of a first myocardial infarction among men. The patients were 684 men with a first diagnosis of myocardial infarction. The controls were 724 men selected to be representative of the same populations. Results The average toenail mercury level in controls was 0.25 μg per gram. After adjustment for the DHA level and coronary risk factors, the mercury levels in the patients were 15 percent higher than those in controls (95 percent confidence interval, 5 to 25 percent). ...

The non-trivial choice between innovation indicators

Abstract: We discuss the strengths and weaknesses of five alternative innovation indicators: R&D, patent applications, total innovation expenditure and shares in sales taken by imitative and by innovative products as they were measured in the 1992 Community Innovation Survey (CIS) in the Netherlands. We conclude that the two most commonly used indicators (R&D and patent applications) have more (and more severe) weaknesses than is often assumed. Moreover, our factor analysis suggests that there is little correlation between the various indicators. This underlines the empirical relevance of various sources of bias of innovation indicators as discussed in this paper.

Inorganic scintillators in medical imaging.

Abstract: A review of medical diagnostic imaging methods utilizing x-rays or gamma rays and the application and development of inorganic scintillators is presented.

Shot-boundary detection: unraveled and resolved?

Abstract: Partitioning a video sequence into shots is the first step toward video-content analysis and content-based video browsing and retrieval. A video shot is defined as a series of interrelated consecutive frames taken contiguously by a single camera and representing a continuous action in time and space. As such, shots are considered to be the primitives for higher level content analysis, indexing, and classification. The objective of this paper is twofold. First, we analyze the shot-boundary detection problem in detail and identify major issues that need to be considered in order to solve this problem successfully. Then, we present a conceptual solution to the shot-boundary detection problem in which all issues identified in the previous step are considered. This solution is provided in the form of a statistical detector that is based on minimization of the average detection-error probability. We model the required statistical functions using a robust metric for visual content discontinuities (based on motion compensation) and take into account all (a priori) knowledge that we found relevant to shot-boundary detection. This knowledge includes the shot-length distribution, visual discontinuity patterns at shot boundaries, and characteristic temporal changes of visual features around a boundary. Major advantages of the proposed detector are its robust and sequence-independent performance, while there is also the possibility to detect different types of shot boundaries simultaneously. We demonstrate the performance of our detector regarding two most widely used types of shot boundaries: hard cuts and dissolves.

Nanotechnology: Carbon nanotubes with DNA recognition

Abstract: Since the discovery of their one-dimensional electronic band structure1, the leading candidate that has emerged for nanodevice applications is single-walled carbon nanotubes (SWNTs) . Here we unite their unique properties with the specific molecular-recognition features of DNA by coupling SWNTs to peptide nucleic acid (PNA, an uncharged DNA analogue2) and hybridizing these macromolecular wires with complementary DNA. Our findings provide a new, versatile means of incorporating SWNTs into larger electronic devices by recognition-based assembly, and of using SWNTs as probes in biological systems by sequence-specific attachment.

The Reductive Amination of Aldehydes and Ketones and the Hydrogenation of Nitriles: Mechanistic Aspects and Selectivity Control

Abstract: This review deals with two of the most commonly used methods for the preparation of amines: the reductive amination of aldehydes and ketones and the hydrogenation of nitriles. There is a great similarity betweenthese two methods, sin ce both have the imine as intermediate. However, due to the high reactivity of this intermediate, primary, secondary and/or tertiary amines are obtained (oftensimultan eously). The relationof the selectivity to different substrate structures and reaction condi- tions is briefly summarised, the main focus being on the catalyst as it is the most significant factor that governs the selectivity. Different mechanisms are discussed with the view to correlate the structure of the catalyst and, more particularly, the nature of the metal and the support with selectivity. The crucial point is the presumed location of the condensation and hydrogenation steps.

Analysis of complex dielectric spectra. I. One-dimensional derivative techniques and three-dimensional modelling

Abstract: Several strategies for the analysis of isochronal and isothermal spectra as well as complete three-dimensional (3D) spectra are outlined and applied to data of (glass forming) liquids and polymers Conduction-free loss spectra are calculated using a compact solution of the Kramers–Kronig transformation and an approximation based on oe 0 =o ln x The usefulness of the dielectric modulus for data analysis is also evaluated Apart from oe 0 =o ln x, other useful derivatives in the frequency and temperature domain are treated as well Finally, we describe the evaluation of 3Drelaxation maps by means of 3D fitting 2002 Elsevier Science BV All rights reserved

Bagging, Boosting and the Random Subspace Method for Linear Classifiers

Abstract: Recently bagging, boosting and the random subspace method have become popular combining techniques for improving weak classifiers. These techniques are designed for, and usually applied to, decision trees. In this paper, in contrast to a common opinion, we demonstrate that they may also be useful in linear discriminant analysis. Simulation studies, carried out for several artificial and real data sets, show that the performance of the combining techniques is strongly affected by the small sample size properties of the base classifier: boosting is useful for large training sample sizes, while bagging and the random subspace method are useful for critical training sample sizes. Finally, a table describing the possible usefulness of the combining techniques for linear classifiers is presented.

Equilibrium lithium transport between nanocrystalline phases in intercalated TiO2 anatase

Abstract: Microcrystalline TiO2 with an anatase crystal structure is used as an anode material for lithium rechargeable batteries1,2, and also as a material for electrochromic3,4,5,6 and solar-cell devices7,8. When intercalated with lithium, as required for battery applications, TiO2 anatase undergoes spontaneous phase separation into lithium-poor (Li0.01TiO2) and lithium-rich (Li0.6TiO2) domains on a scale of several tens of nanometres9. During discharge, batteries need to maintain a constant electrical potential between their electrodes over a range of lithium concentrations. The two-phase equilibrium system in the electrodes provides such a plateau in potential, as only the relative phase fractions vary on charging (or discharging) of the lithium. Just as the equilibrium between a liquid and a vapour is maintained by a continuous exchange of particles between the two phases, a similar exchange is required to maintain equilibrium in the solid state. But the time and length scales over which this exchange takes place are unclear. Here we report the direct observation by solid-state nuclear magnetic resonance of the continuous lithium-ion exchange between the intermixed crystallographic phases of lithium-intercalated TiO2. We find that, at room temperature, the continuous flux of lithium ions across the phase boundaries is as high as 1.2 × 1020 s-1 m-2.

Linearly concatenated cyclobutane lipids form a dense bacterial membrane

Abstract: Lipid membranes are essential to the functioning of cells, enabling the existence of concentration gradients of ions and metabolites. Microbial membrane lipids can contain three-, five-, six- and even seven-membered aliphatic rings, but four-membered aliphatic cyclobutane rings have never been observed. Here we report the discovery of cyclobutane rings in the dominant membrane lipids of two anaerobic ammonium-oxidizing (anammox) bacteria. These lipids contain up to five linearly fused cyclobutane moieties with cis ring junctions. Such 'ladderane' molecules are unprecedented in nature but are known as promising building blocks in optoelectronics. The ladderane lipids occur in the membrane of the anammoxosome, the dedicated intracytoplasmic compartment where anammox catabolism takes place. They give rise to an exceptionally dense membrane, a tight barrier against diffusion. We propose that such a membrane is required to maintain concentration gradients during the exceptionally slow anammox metabolism and to protect the remainder of the cell from the toxic anammox intermediates. Our results further illustrate that microbial membrane lipid structures are far more diverse than previously recognized.

A maximum variance cluster algorithm

Abstract: We present a partitional cluster algorithm that minimizes the sum-of-squared-error criterion while imposing a hard constraint on the cluster variance. Conceptually, hypothesized clusters act in parallel and cooperate with their neighboring clusters in order to minimize the criterion and to satisfy the variance constraint. In order to enable the demarcation of the cluster neighborhood without crucial parameters, we introduce the notion of foreign cluster samples. Finally, we demonstrate a new method for cluster tendency assessment based on varying the variance constraint parameter.

The combining classifier: to train or not to train?

Abstract: When more than a single classifier has been trained for the same recognition problem the question arises how this set of classifiers may be combined into a final decision rule. Several fixed combining rules are used that depend on the output values of the base classifiers only. They are almost always suboptimal. Usually, however, training sets are available. They may be used to calibrate the base classifier outputs, as well as to build a trained combining classifier using these outputs as inputs. It depends on various circumstances whether this is useful, in particular whether the training set is used for the base classifiers as well and whether they are overtrained. We present an intuitive discussion on the use of trained combiners, relating the question of the choice of the combining classifier to a similar choice in the area of dissimilarity based pattern recognition. Some simple examples are used to illustrate the discussion.

A generalized kernel approach to dissimilarity-based classification

Abstract: Usually, objects to be classified are represented by features. In this paper, we discuss an alternative object representation based on dissimilarity values. If such distances separate the classes well, the nearest neighbor method offers a good solution. However, dissimilarities used in practice are usually far from ideal and the performance of the nearest neighbor rule suffers from its sensitivity to noisy examples. We show that other, more global classification techniques are preferable to the nearest neighbor rule, in such cases.For classification purposes, two different ways of using generalized dissimilarity kernels are considered. In the first one, distances are isometrically embedded in a pseudo-Euclidean space and the classification task is performed there. In the second approach, classifiers are built directly on distance kernels. Both approaches are described theoretically and then compared using experiments with different dissimilarity measures and datasets including degraded data simulating the problem of missing values.

Spin battery operated by ferromagnetic resonance

Abstract: Precessing ferromagnets are predicted to inject a spin current into adjacent conductors via Ohmic contacts, irrespective of a conductance mismatch with, for example, doped semiconductors. This opens the way to create a pure spin source (``spin battery'') by the ferromagnetic resonance. We estimate the spin current and spin bias for different material combinations.

Grain nucleation and growth during phase transformations.

Abstract: The mechanical properties of polycrystalline materials are largely determined by the kinetics of the phase transformations during the production process. Progress in x-ray diffraction instrumentation at synchrotron sources has created an opportunity to study the transformation kinetics at the level of individual grains. Our measurements show that the activation energy for grain nucleation is at least two orders of magnitude smaller than that predicted by thermodynamic models. The observed growth curves of the newly formed grains confirm the parabolic growth model but also show three fundamentally different types of growth. Insight into the grain nucleation and growth mechanisms during phase transformations contributes to the development of materials with optimal mechanical properties.

An overview of constraint-based path selection algorithms for QoS routing

Abstract: Constraint-based path selection aims at identifying a path that satisfies a set of quality of service (QoS) constraints. In general, this problem is known to be NP-complete, leading to the proposal of many heuristic algorithms. We provide an overview of these algorithms, focusing on restricted shortest path and multi-constrained path algorithms.