Showing papers by "Johannes Kepler University of Linz published in 2001"

Plastic Solar Cells

Abstract: Recent developments in conjugated-polymer-based photovoltaic elements are reviewed. The photophysics of such photoactive devices is based on the photo-induced charge transfer from donor-type semiconducting conjugated polymers to acceptor-type conjugated polymers or acceptor molecules such as Buckminsterfullerene, C60. This photo-induced charge transfer is reversible, ultrafast (within 100 fs) with a quantum efficiency approaching unity, and the charge-separated state is metastable (up to milliseconds at 80 K). Being similar to the first steps in natural photosynthesis, this photo-induced electron transfer leads to a number of potentially interesting applications, which include sensitization of the photoconductivity and photovoltaic phenomena. Examples of photovoltaic architectures are presented and their potential in terrestrial solar energy conversion discussed. Recent progress in the realization of improved photovoltaic elements with 3 % power conversion efficiency is reported.

Origin of the Open Circuit Voltage of Plastic Solar Cells

Abstract: A series of highly soluble fullerene derivatives with varying acceptor strengths (i.e., first reduction potentials) was synthesized and used as electron acceptors in plastic solar cells. These fullerene derivatives, methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM), a new azafulleroid, and a ketolactam quasifullerene, show a variation of almost 200 mV in their first reduction potential. The open circuit voltage of the corresponding devices was found to correlate directly with the acceptor strength of the fullerenes, whereas it was rather insensitive to variations of the work function of the negative electrode. These observations are discussed within the concept of Fermi level pinning between fullerenes and metals via surface charges.

A Riemannian interpolation inequality a la Borell, Brascamp and Lieb

Abstract: A concavity estimate is derived for interpolations between L 1(M) mass densities on a Riemannian manifold. The inequality sheds new light on the theorems of Prekopa, Leindler, Borell, Brascamp and Lieb that it generalizes from Euclidean space. Due to the curvature of the manifold, the new Riemannian versions of these theorems incorporate a volume distortion factor which can, however, be controlled via lower bounds on Ricci curvature. The method uses optimal mappings from mass transportation theory. Along the way, several new properties are established for optimal mass transport and interpolating maps on a Riemannian manifold.

Developing groupware for requirements negotiation: lessons learned

Abstract: Defining requirements is a complex and difficult process, and defects in the process often lead to costly project failures. There is no complete and well-defined set of requirements waiting to be discovered in system development. Different stakeholders: users, customers, managers, domain experts, and developers, come to the project with diverse expectations and interests. Requirements emerge in a highly collaborative, interactive, and interdisciplinary negotiation process that involves heterogeneous stakeholders. At the University of Southern California's Center for Software Engineering, we have developed a series of groupware implementations for the WinWin requirements negotiation approach. The WinWin approach involves having a system's success-critical stakeholders participate in a negotiation process so they can converge on a mutually satisfactory or win-win set of requirements. The WinWin groupware system, which has evolved over four generations, enables and facilitates heterogeneous stakeholder participation and collaboration. Each generation reflects an increase in our understanding of what is needed for successful WinWin groupware operations and technology support. The authors present the major lessons they learned during WinWin's development.

A level set method for inverse problems

Abstract: This paper is devoted to the solution of shape reconstruction problems by a level set method. The basic motivation for the setup of this level set algorithm is the well-studied method of asymptotic regularization, which has been developed for ill-posed problems in Hilbert spaces. Using analogies to this method, the convergence analysis of the proposed level set method is established and it is shown that the evolving level set converges to a solution in the symmetric difference metric as the artificial time evolves to infinity. Furthermore, the regularizing properties of the level set method are shown, if the discrepancy principle is used as a stopping rule. The numerical implementation of the level set method is discussed and applied to some examples in order to compare the numerical results with theoretical statements. The numerical results demonstrate the power of the level set method, in particular for examples where the number of connected components the solution consists of is not known a priori.

Local field enhancement effects for nanostructuring of surfaces.

Abstract: We report on a method that allows the nanostructuring of surfaces with intense laser pulses. For this purpose isolated polystyrene spheres with diameters in the order of the laser wavelength were deposited on a silicon or glass surface. Illumination with short and ultrashort laser pulses produced holes underneath these particles. Calculations of the field near the particles make clear that geometrical optics, that is, focusing by a spherical lens, as well as near-field effects, contribute to the size and shape of these holes. This technique can be utilized for the parallel structuring of large surface areas with a single laser shot.

Determination of strain fields and composition of self-organized quantum dots using x-ray diffraction

Abstract: We give a detailed account of an x-ray diffraction technique which allows us to determine shape, strain fields, and interdiffusion in semiconductor quantum dots grown in the Stranski\char21{}Krastanov mode. A scattering theory for grazing incidence diffraction is derived for the case of highly strained, uncapped nanostructures. It is shown that strain resolution can be achieved by ``decomposing'' the dots in their iso-strain areas. For a selected iso-strain area, it is explained how lateral extent, height above the substrate and radius of curvature can be determined from the intensity distribution around a surface Bragg reflection. The comparison of intensities from strong and weak reflections reveals the mean material composition for each strain state. The combination of all these strain resolved functional dependences yields tomographic images of the dots showing strain field and material composition.

An empirical approach to the stopping power of solids and gases for ions from 3Li to 18Ar – Part II

Abstract: A large collection of stopping power data for projectiles from 3Li to 18Ar is investigated as a possible basis for producing a table of stopping powers. We divide the experimental stopping powers for a particular projectile (nuclear charge Z1) by those for alpha particles in the same element, as given in ICRU Report 49. With proper normalization, we then obtain experimental stopping power ratios Srel that lie approximately on a single curve, provided we treat solid and gaseous targets separately, and provided we exclude H2 and He targets. For every projectile, this curve is then fitted by a 3-parameter sigmoid function Srel=Srel(a,b,c). We find that the three parameters a, b and c depend smoothly on Z1 and can themselves be fitted by suitable functions af,bf and cf of Z1, separately for solid and gaseous targets. The low energy limit (coefficient a) for solids agrees approximately with the prediction by Lindhard and Scharff. We find that agas

Reconciling software requirements and architectures: the CBSP approach

Abstract: Little guidance and few methods are available to refine a set of software requirements into an architecture satisfying those requirements. Part of the challenge stems from the fact that requirements and architectures leverage different terms and concepts to capture the artifacts relevant to each. We present CBSP (Component-Bus-System- Property), a lightweight approach intended to provide a systematic way of reconciling requirements and architectures. CBSP leverages a simple set of architectural concepts (components, connectors, overall systems, and their properties) to recast the requirements in a way that facilitates their straightforward mapping to architectures. Furthermore, the approach allows us to capture and maintain arbitrarily complex relationships between requirements and architectural artifacts, as well as across different CBSP artifacts. We have extensively applied CBSP within the context of particular requirements and architecture definition techniques, EasyWinWin and C2. We leverage that experience to demonstrate the CBSP method and tool support using a large-scale example that highlights the transition from an EasyWinWin requirements negotiation into a C2-style architectural model.

Surfacing tacit knowledge in requirements negotiation: experiences using EasyWinWin

Abstract: Defects in the requirements definition process often lead to costly project failures. One eminent problem is that it can be difficult to take deliberate advantage of important tacit knowledge of success-critical stakeholders. People know more that they can ever tell. Implicit stakeholder goals, hidden assumptions, unshared expectations often result in severe problems in the later stages of software development. We present a set of collaborative techniques that support a team of success-critical stakeholders in surfacing tacit knowledge during systems development projects. We discuss these techniques in the context of the EasyWinWin requirements negotiation methodology and illustrate our approach with examples from real-world negotiations.

Hermite interpolation by pythagorean hodograph curves of degree seven

Abstract: Polynomial Pythagorean hodograph (PH) curves form a remarkable subclass of polynomial parametric curves; they are distinguished by having a polynomial arc length function and rational offsets (parallel curves). Many related references can be found in the article by Farouki and Neff on C 1 Hermite interpolation with PH quintics. We extend the C 1 Hermite interpolation scheme by taking additional curvature information at the segment boundaries into account. As a result we obtain a new construction of curvature continuous polynomial PH spline curves. We discuss Hermite interpolation of G 2 [C 1 ] boundary data (points, first derivatives, and curvatures) with PH curves of degree 7. It is shown that up to eight possible solutions can be found by computing the roots of two quartic polynomials, With the help of the canonical Taylor expansion of planar curves, we analyze the existence and shape of the solutions. More precisely, for Hermite data which are taken from an analytical curve, we study the behaviour of the solutions for decreasing stepsize Δ. It is shown that a regular solution is guaranteed to exist for sufficiently small stepsize Δ, provided that certain technical assumptions are satisfied. Moreover, this solution matches the shape of the original curve; the approximation order is 6. As a consequence, any given curve, which is assumed to be G 2 (curvature continuous) and to consist of analytical segments can approximately be converted into polynomial PH form. The latter assumption is automatically satisfied by the standard curve representations of Computer Aided Geometric Design, such as Bezier or B-spline curves. The conversion procedure acts locally, without any need for solving a global system of equations, It produces G 2 polynomial PH spline curves of degree 7.

Crouzeix-Raviart type finite elements on anisotropic meshes

Abstract: The paper deals with a non-conforming finite element method on a class of anisotropic meshes. The Crouzeix-Raviart element is used on triangles and tetrahedra. For rectangles and prismatic (pentahedral) elements a novel set of trial functions is proposed. Anisotropic local interpolation error estimates are derived for all these types of element and for functions from classical and weighted Sobolev spaces. The consistency error is estimated for a general differential equation under weak regularity assumptions. As a particular application, an example is investigated where anisotropic finite element meshes are appropriate, namely the Poisson problem in domains with edges. A numerical test is described.

Comparison theorems for exit times

Abstract: We study bounds on the exit time of Brownian motion from a set in terms of its size and shape, and the relation of such bounds with isoperimetric inequalities. The first result is an upper bound for the distribution function of the exit time from a subset of a sphere or hyperbolic space of constant curvature in terms of the exit time from a disc of the same volume. This amounts to a rearrangement inequality for the Dirichlet heat kernel. To connect this inequality with the classical isoperimetric inequality, we derive a formula for the perimeter of a set in terms of the heat flow over the boundary. An auxiliary result generalizes Riesz' rearrangement inequality to multiple integrals.

Nanoscale dislocation patterning in PbTe/PbSe(001) lattice-mismatched heteroepitaxy.

Abstract: Dislocation patterning in PbTe on PbSe (001) heteroepitaxy is studied using scanning tunneling microscopy. It is shown that exceedingly regular square arrays of misfit dislocations are formed during strain relaxation. This is based on the existence of a homogeneous dislocation nucleation process, a high dislocation mobility within the interface, and an effective repulsive interaction between neighboring dislocations. Similar results are expected also for other highly mismatched heteroepitaxial systems.

Asymptotics of orthonormal polynomials in the presence of a denumerable set of mass points

Abstract: Let a be a positive measure whose support is an interval E plus a denumerable set of mass points which accumulate at the boundary points of E only. Under the assumptions that the mass points satisfy Blaschke's condition and that the absolutely continuous part of a satisfies Szego's condition, asymptotics for the orthonormal polynomials on and off the support are given. So far asymptotics were only available if the set of mass points is finite.

Computation of some examples of Brown's spectral measure in free probability

Abstract: We use free probability techniques for computing spectra and Brown measures of some non hermitian operators in finite von Neumann algebras. Examples include u_n+u_oo where u_n and u_oo are the generators of Z_n and Z respectively, in the free product group Z_n*Z, or elliptic elements, of the form S_a+iS_b where S_a and S_b are free semi-circular elements of variance a and b. We give some pictorial evidence for connections with spectra of random matrices.

A mann iterative regularization method for elliptic cauchy problems

Abstract: We investigate the Cauchy problem for linear elliptic operators with C ∞–coefficients at a regular set Ω ⊂ R 2, which is a classical example of an ill-posed problem. The Cauchy data are given at the manifold Γ ⊂ ∂Ω and our goal is to reconstruct the trace of the H 1(Ω) solution of an elliptic equation at ∂Ω/Γ. The method proposed here composes the segmenting Mann iteration with a fixed point equation associated with the elliptic Cauchy problem. Our algorithm generalizes the iterative method developed by Maz'ya et al., who proposed a method based on solving successive well-posed mixed boundary value problems. We analyze the regularizing and convergence properties both theoretically and numerically. †Supported by CNPq under grant 200049-94.1 and FWF project F–1308 within Spezial-forschungsbereich 13.

Low-energy ion neutralization at surfaces: Resonant and Auger processes

Abstract: The interaction of ${\mathrm{He}}^{+}$ with a typical metal surface (Al or Pd) is described, analyzing in detail the different mechanisms that contribute to the neutralization of the projectile when backscattered from the surface Auger and resonant neutralization processes are considered and analyzed including a detailed quantum-mechanical description of the He-metal interaction, for projectile energies between 100 eV and 3 keV We show that the promotion of the $\mathrm{H}\mathrm{e}\ensuremath{-}1s$ level, due to its interaction with the metal-atom-core orbitals, is the crucial mechanism making resonant processes operative We find, however, that resonant processes are much more important for Al than for Pd In Al, both Auger and resonant processes are equally important for neutralization of the ion, while for Pd we find that Auger is the dominant mechanism, making the He/Pd system the ideal case for which Hagstrum's exponential law appears to be practically valid for all velocities We also find qualitative agreement with experimental data, which we consider a satisfactory result in view of the fact that our theory is a complex ab initio calculation free of adjustable parameters

Personality influences on ultimatum bargaining decisions

Abstract: We investigate individual differences in behaviour within an ultimatum game with advance production. It is a simple bargaining game, in which the size of the ‘pie’ to be distributed between two parties depends on both players' monetary investments. One player, the ‘proposer’, has to state a demand, which can be accepted as it is or rejected (with the consequence of zero return for both players) by the second player (‘responder’). We find that global personality measures contribute significantly to the explanation of both demands and rejection decisions. Proposers who score high on independence and tough-mindedness, two global personality dimensions with affinity to selfishness, demand higher return shares than proposers who score low on these dimensions. This is particularly true when the proposer's cost share is low, i.e. when a high demanded return share cannot be justified by a concern for equity. In this situation there is a strong conflict between economic rationality and equity. Regarding rejection decisions we find that reciprocity oriented responders (i.e. persons who are either emotionally unstable and extraverted or emotionally stable and introverted) reject a proposal more often than others. Rejection of an unsatisfying offer is interpreted as an act of angry retaliation (negative reciprocity) against an interaction partner who violates the social norm of equity. Copyright © 2001 John Wiley & Sons, Ltd.

Absorption Wavelengths and Bandwidths for Interstellar Searches of C 60 in the 2400-4100 Å Region

Abstract: Low-temp. gas-phase wavelengths and bandwidths for astronomical searches for C60 in the 2400-4100 .ANG. region are detd. from the absorption spectra of C60 in rare gas matrixes at 4 K, coupled with information on the dependence of the vibronic transition frequencies of this fullerene on the Lorentz-Lorenz polarizability parameter of matrixes and solvents. These bandwidths increase from 25 +- 5 cm-1 for the 1 1T1u-1 1Ag band at 4024.0 +- 0.5 .ANG. to 2300 +- 200 cm-1 for the 6 1T1u-1 1Ag band at 2488 +- 8 .ANG.. Spectral observations of astrophys. objects considered to be likely sites of C60 were examd. in the context of the wavelengths and bandwidths of bands of the allowed transitions of C60 which we recommend for searches of this species. No certain pos. assignment of a C60 band was made in the 2400-4100 .ANG. spectral region of these objects. The phys. origin of the obsd. C60 absorption bandwidths is analyzed and discussed in an appendix. [on SciFinder (R)]

A wireless spread-spectrum communication system using SAW chirped delay lines

Abstract: We report on the use of broad-band chirp signals for spread-spectrum communications in indoor and industrial environments. The well-known pulse compression technique associated with chirp signals is exploited to achieve a highly robust communication system. For the generation and compression of the chirp signals, surface acoustic wave delay lines fabricated from an LiTaO/sub 3/-X112rotY substrate are used. Center frequency, bandwidth, chirp duration, and chirp rate are 348.8 MHz, 80 MHz, 500 ns, and /spl plusmn/40 MHz//spl mu/s, respectively. Different modulation schemes for chirp signals are introduced, the effects of nonlinearities, frequency drift, and temperature drift are addressed, and simulations and measurement results from a hardware demonstrator are presented for the use of /spl pi//4-differential quadrature phase-shift keying (DQPSK) modulation. A data rate of up to 40 Mb/s has been achieved experimentally and shows that the proposed system is highly robust against multipath effects.