Showing papers by "Wrocław University of Technology published in 2006"

Strong coupling in a single quantum dot semiconductor microcavity system

Abstract: Properties of atom-like emitters in cavities are successfully described by cavity quantum electrodynamics (cQED). We report on cavity quantum electrodynamics (cQED) experiments in a single quantum dot semiconductor system. CQED, which is a very active research field in optics and solid state physics, can be divided into a weak and a strong coupling regime. In case of weak coupling, the spontaneous emission rate of an atom-like emitter, e.g. a single quantum dot exciton, can be enhanced or reduced compared to the value in vacuum in an irreversible emission process. In contrast, a reversible energy exchange between the emitter and the cavity mode takes place when the conditions for strong coupling are fulfilled. We investigate weak as well as strong coupling in a system based on a low density In0.3Ga 0.7As quantum dot layer placed as the active layer in a high quality planar AlAs/GaAs distributed Bragg reflector cavity grown by molecular beam epitaxy. Using electron beam lithography and deep plasma etching, micropillars with high Q-factors (up to 43.000 for 4 μm diameter) were realized from the planar cavity structure. Due to the high oscillator strength of the In0.3Ga 0.7As quantum dots together with a small mode volume in high finesse micropillar cavities it is possible to observe strong coupling characterized by a vacuum Rabi splitting of 140 μeV. The fabrication of high-Q micropillar cavities as well as conditions necessary to realize strong coupling in the present system are discussed in detail.

Quantitative classification of covalent and noncovalent H-bonds.

Abstract: The covalent nature of interactions within various hydrogen bonded molecular aggregates has been characterized by the two entirely different computational methods: Bader analysis of the electron density and variation-perturbation partitioning of the intermolecular interaction energy. Analysis of 34 complexes representing different types of hydrogen bonds indicates that the proton-acceptor distance ∼1.8 A and the ratio of delocalization and electrostatic terms ∼0.45 constitutes approximately a borderline between covalent and noncovalent hydrogen bonds. The latter ratio could be used to characterize quantitatively the degree of the covalent nature of transition state interactions with active site residues, a quantity essential for an enzyme catalytic activity.

Automated classification of power-quality disturbances using SVM and RBF networks

Abstract: The authors propose a new method of power-quality classification using support vector machine (SVM) neural networks. Classifiers based on radial basis function (RBF) networks was, in parallel, applied to enable proper performance comparison. Both RBF and SVM networks are introduced and are considered to be an appropriate tool for classification problems. Space phasor is used for feature extraction from three-phase signals to build distinguished patterns for classifiers. In order to create training and testing vectors, different disturbance classes were simulated (e.g., sags, voltage fluctuations, transients) in Matlab. Finally, the investigation results of the novel approach are shown and interpreted.

Point and Interval Forecasting of Spot Electricity Prices: Linear vs. Non-Linear Time Series Models

Abstract: In this paper we assess the short-term forecasting power of different time series models in the electricity spot market. In particular we calibrate AR/ARX ("X" stands for exogenous/fundamental variable—system load in our study), AR/ARX-GARCH, TAR/TARX and Markov regime-switching models to California Power Exchange (CalPX) system spot prices. We then use them for out-ofsample point and interval forecasting in normal and extremely volatile periods preceding the market crash in winter 2000/2001. We find evidence that (i) non-linear, threshold regime-switching (TAR/TARX) models outperform their linear counterparts, both in point and interval forecasting, and that (ii) an additional GARCH component generally decreases point forecasting efficiency. Interestingly, the former result challenges a number of previously published studies on the failure of non-linear regime-switching models in forecasting.

High-impedance fault detection in distribution networks with use of wavelet-based algorithm

Abstract: A new simple and effective algorithm of arcing fault detection in distribution networks with the application of a wavelet transform technique is presented in this paper. The protection algorithm developed observes the phase displacement between wavelet coefficients calculated for zero-sequence voltage and current signals at a chosen high-level frequency. The final decision in regards to feeder switching off (or alarm issuing) is met either with a deterministic logic scheme or with the use of a neural net trained especially for that purpose. The developed wavelet-based high-impedance fault (HIF) detector has been tested with Electromagnetic Transients Program-Alternative Transients Program (ATP)-generated signals, exhibiting better performance than traditionally used algorithms and methods. The protection method proposed may be used for HIF detection independent of the network neutral-point grounding mode. The scheme proved to be robust against transients generated during normal events such as feeder energizing and de-energizing as well as capacitor bank switching

Accurate location of faults on power transmission lines with use of two-end unsynchronized measurements

Abstract: This paper presents a new algorithm for locating faults on two-terminal power transmission lines. Unsynchronized two-end voltages and currents are processed for determining the sought distance to fault and the synchronization angle. The calculations are performed initially for the lumped model of a transmission line. Then, these results are used as the initial data for the Newton-Raphson method-based iterative calculations, in which the distributed parameter line model is utilized. The delivered fault location algorithm has been tested and evaluated with the fault data obtained from versatile ATP-EMTP simulations. The sample results of the evaluation are reported and discussed.

High-resolution spectrum-estimation methods for signal analysis in power systems

Abstract: The spectrum-estimation methods based on the Fourier transform suffer from the major problem of resolution. The methods were developed and are mostly applied for periodic signals under the assumption that only harmonics are present and the periodicity intervals are fixed, while periodicity intervals in the presence of interharmonics are variable and very long. A novel approach to harmonic and interharmonic analysis based on the "subspace" methods is proposed. Min-norm and music harmonic retrieval methods are examples of high-resolution eigenstructure-based methods. Their resolution is theoretically independent of the signal-to-noise ratio (SNR). The Prony method as applied for parameter estimation of signal components was also tested in the paper. Both the high-resolution methods do not show the disadvantages of the traditional tools and allow exact estimation of the interharmonic frequencies. To investigate the methods, several experiments were carried out using simulated signals, current waveforms at the output of an industrial frequency converter, and current waveforms during out-of-step operation of a synchronous generator. For comparison, similar experiments were repeated using the fast Fourier transform (FFT). The comparison proved the superiority of the new methods.

Rational Design and Synthesis of Porous Organic−Inorganic Hybrid Frameworks Constructed by 1,3,5-Benzenetriphosphonic Acid and Pyridine Synthons

Abstract: 1,3,5-Benzenetriphosphonic acid, H6BTP, 1,3,5-[(HO)2OP]3C6H3, was reacted hydrothermally with copper salts in the absence and presence of 4,4'-bipyridine (bpy) and 4,4'-trimethlyenedipyridine (tbpy) in a 1:1 molar ratio leading to three new organic-inorganic hybrid frameworks. Compound 1, {Cu6[C6H3(PO3)3]2(H2O)8} x 5.5 H2O, has three different copper ions that are interconnected by the highly charged [1,3,5-(PO3)3C6H3]6- anionic moieties. These moieties self-assemble through tetra-copper units to give a cagelike motif with two benzene rings parallel to each other at a distance of 3.531 A which extend along the a axis and link with a grouping of four-coordinated copper units in the b axis direction to give the cross-linked layered structure. In compound 2, Cu{C6H3[PO(OH)O]2[PO(OH)2]}(C10H8N2), the copper ions are in square pyramidal geometries and are interconnected via chelating and bridging BTP ligands into layers which are further cross-linked by bpy ligands into a pillared layered architecture. Compound 3, {Cu2C6H3[PO(OH)O]2[PO3](C13H14N2)} x 3 H2O x 0.5 HCON(CH3)2, contains tetra-copper units that are linked by BTP ligands and further linked by tbpy linkers in the c axis direction to produce a large channel-sized 3D framework.

Complete disentanglement by partial pure dephasing

Abstract: We study the effect of pure dephasing on the entanglement of a pair of two-level subsystems (qubits). We show that partial dephasing induced by a super-Ohmic reservoir, corresponding to well-established properties of confined charge states and phonons in semiconductors, may lead to complete disentanglement. We show also that the disentanglement effect increases with growing distance between the two subsystems.

Molecular Interactions between Rubber and Asphalt

Abstract: The analysis of the interactions between rubber and asphalt was carried out. The swelling rate and equilibrium of rubber penetration with the selected asphalt components were found to be dependent on these components and the rubber content in asphalt. At fixed conditions of stirring asphalt with rubber, the equilibrium swell value decreases with the increase in rubber content. From the gel permeation chromatography (GPC) analysis, it follows that the lighter asphalt components penetrate more readily into the internal matrix of the polymer. The gas chromatography-mass spectrometry (GC-MS) study allowed us to assess which components penetrated from asphalt into the rubber and which moved from rubber to asphalt. It has been found that of the nonpolar components, the n-alkanes and n-alkylbenzenes, possess the highest propensity to penetrate into rubber particles. Preferential absorption of the compounds with linear aliphatic chains into the rubber suggests that these components have a good compatibility with ...

Intrinsic ultracontractivity of the Feynman-Kac semigroup for relativistic stable processes

Abstract: Let X t be the relativistic α-stable process in R d , a ∈ (0, 2), d > α, with infinitesimal generator H (α) 0 = -((-A + m 2/α ) α/2 - m). We study intrinsic ultracontractivity (IU) for the Feynman-Kac semigroup T t for this process with generator H (α) 0 - V, V > 0, V locally bounded. We prove that if lim |x|→∞ V(x) = ∞, then for every t > 0 the operator T t is compact. We consider the class V of potentials V such that V > 0, lim |x|→∞ V(x) = ∞ and V is comparable to the function which is radial, radially nondecreasing and comparable on unit balls. For V in the class V we show that the semigroup T t is IU if and only if lim |x|→∞ V(x)/|x| = ∞. If this condition is satisfied we also obtain sharp estimates of the first eigenfunction Φ 1 for T t . In particular, when V(x) = |x| β , β > 0, then the semigroup T t is IU if and only if β > 1. For β > 1 the first eigenfunction Φ 1 (x) is comparable to.

Design of Compensated Coupled-Stripline 3-dB Directional Couplers, Phase Shifters, and Magic-T's—Part I: Single-Section Coupled-Line Circuits

Abstract: The problem of discontinuities in coupled-stripline 3-dB directional couplers, phase shifters, and magic-T's, regarding the connections of coupled and terminating signal lines, has been comprehensively investigated for the first time. The proposed equivalent circuit of these discontinuities, of which parameters can be computed in a process of fitting curves of the circuit and electromagnetic analyses, has been used for accurate modeling of coupled-stripline circuits. It has been shown that parasitic reactances, which result from connections of signal and coupled lines, severely deteriorate the return losses and the isolation of such circuits, and that these discontinuity effects can be substantially reduced by connecting compensating shunt capacitances to both coupled and signal lines. Results of measurements carried out for various designed and manufactured coupled-line circuits are most promising and prove the efficiency of the proposed compensation technique. This paper describes the technique of capacitive compensation of discontinuity effects in single-section coupled-line circuits

Properties of the halogen-hydride interaction: an ab initio and "atoms in molecules" analysis.

Abstract: X−Cl···H−Y interactions are analyzed by applying ab initio methods as well as the Bader theory. All calculations were performed using Pople's basis sets (6-311++G(2df,2pd) and 6-311++G(3df,3pd)) as well as the Dunning-type bases (aug-cc-pVDZ and aug-cc-pVTZ) within the MP2 method. For the complexes analyzed here, X−Cl and H−Y may be treated as a Lewis acid and a Lewis base, respectively. The Cl···H interactions are rather weak or at most moderate since, for the strongest interaction of the F3CCl···HLi complex, the binding energy calculated at the MP2/6-311++G(3df,3pd) level of approximation amounts to −3.4 kcal/mol, and the H···Cl distance is equal to 2.65 A, less than the corresponding sum of van der Waals radii. These interactions may be classified as halogen−hydride interactions. However, some of the complexes analyzed, especially F3SiCl···HBeF and F3SiCl···HBeF, are very weakly bound, probably by typical van der Waals interactions.

The possible covalent nature of N-H...O hydrogen bonds in formamide dimer and related systems: an ab initio study.

Abstract: The N−H···O hydrogen bonds are analyzed for formamide dimer and its simple fluorine derivatives representing a wide spectrum of more or less covalent interactions. The calculations were performed at the MP2/6-311++G(d,p) level of approximation. To explain the nature of such interactions, the Bader theory was also applied, and the characteristics of the bond critical points (BCPs) were analyzed: the electron density at BCP and its Laplacian, the electron energy density at BCP and its components, the potential electron energy density, and the kinetic electron energy density. These parameters are used to justify the statement that some of the interactions analyzed are partly covalent in nature. An analysis of the interaction energy components for the systems considered indicates that the covalent character of the hydrogen bond is manifested by a markedly increased contribution of the delocalization term relative to the electrostatic interaction energy. Moreover, the ratio of stabilizing the delocalization/e...

LTCC Microfluidic System

Abstract: A simple and inexpensive low-temperature cofired ceramic (LTCC) microfluidic device with integrated optical fibers is designed, manufactured, and tested with positive results. Fluidic channels, mixer, detector, optical fiber, light source, light detector, heater, and temperature sensor are integrated in one LTCC module. The optical system in the LTCC microsystem permits measurements of light transmittance and fluorescence. The design, technology, and results of the module's evaluation are presented.

On Kelvin Transformation

Abstract: We prove that in the Euclidean space of arbitrary dimension the inversion of the isotropic stable Levy process killed at the origin is, after an appropriate change of time, the same stable process conditioned in the sense of Doob by the Riesz kernel. Using this identification we derive and explain transformation rules for the Kelvin transform acting on the Green function and the Poisson kernel of the stable process and on solutions of Schrodinger equation based on the fractional Laplacian. The Brownian motion and the classical Laplacian are included as a special case.

Convenience Yields for CO2 Emission Allowance Futures Contracts

Abstract: In January 2005 the EU-wide CO2 emissions trading system (EU-ETS) has formally entered into operation.Within the new trading system, the right to emit a particular amount of CO2 becomes a tradable commodity - called EU Allowances (EUAs) - and affected companies, traders and investors will face new strategic challenges. In this paper we investigate the nature of convenience yields for CO2 emission allowance futures. We conduct an empirical study on price behavior, volatility term structure and correlations in different CO2 EUA contracts. Our findings are that the market has changed from initial backwardation to contango with significant convenience yields in future contracts for the Kyoto commitment period starting in 2008. A high fraction of the yields can be explained by the price level and volatility of the spot prices. We conclude that the yields can be interpreted as market expectation on the price risk of CO2 emissions allowance prices and the uncertainty of EU allocation plans for the Kyoto period.