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  • 1. Adey, J
    et al.
    Jones, R
    Palmer, D W
    Briddon, P R
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Erratum: Degradation of Boron-Doped Czochralski-Grown Silicon Solar Cells [Phys. Rev. Lett., 93, 055504 (2004)]2004In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, no 16, p. 169904-Article in journal (Other academic)
  • 2.
    Adey, J.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Palmer, D.W.
    School of Physics, University of Exeter.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Degradation of boron-doped Czochralski-grown silicon solar cells2004In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 93, no 5, p. 055504-1Article in journal (Refereed)
    Abstract [en]

    The formation mechanism and properties of the boron-oxygen center responsible for the degradation of Czochralski-grown Si(B) solar cells during operation is investigated using density functional calculations. We find that boron traps an oxygen dimer to form a bistable defect with a donor level in the upper half of the band gap. The activation energy for its dissociation is found to be 1.2 eV. The formation of the defect from mobile oxygen dimers, which are shown to migrate by a Bourgoin mechanism under minority carrier injection, has a calculated activation energy of 0.3 eV. These energies and the dependence of the generation rate of the recombination center on boron concentration are in good agreement with observations.

  • 3.
    Adey, J.
    et al.
    School of Physics, University of Exeter.
    Palmer, D.W,
    School of Physics, University of Exeter.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Theory of boron-vacancy complexes in silicon2005In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 71, no 16, p. 165211-Article in journal (Refereed)
    Abstract [en]

    The substitutional boron-vacancy BsV complex in silicon is investigated using the local density functional theory. These theoretical results give an explanation of the experimentally reported, well established metastability of the boron-related defect observed in p-type silicon irradiated at low temperature and of the two hole transitions that are observed to be associated with one of the configurations of the metastable defect. BsV is found to have several stable configurations, depending on charge state. In the positive charge state the second nearest neighbor configuration with C1 symmetry is almost degenerate with the second nearest neighbor configuration that has C1h symmetry since the bond reconstruction is weakened by the removal of electrons from the center. A third nearest neighbor configuration of BsV has the lowest energy in the negative charge state. An assignment of the three energy levels associated with BsV is made. The experimentally observed Ev+0.31 eV and Ev+0.37 eV levels are related to the donor levels of second nearest neighbor BsV with C1 and C1h symmetry respectively. The observed Ev+0.11 eV level is assigned to the vertical donor level of the third nearest neighbor configuration. The boron-divacancy complex BsV2 is also studied and is found to be stable with a binding energy between V2 and Bs of around 0.2 eV. Its energy levels lie close to those of the V2. However, the defect is likely to be an important defect only in heavily doped material.

  • 4.
    Adjizian, J.J.
    et al.
    Department of Chemistry, University of Sussex, Falmer, Brighton.
    Latham, Chris
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
    Briddon, P.R.
    Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne.
    Heggie, M.I.
    Department of Chemistry, University of Sussex, Falmer, Brighton.
    DFT study of the chemistry of sulfur in graphite, including interactions with defects, edges and folds2013In: Carbon, ISSN 0008-6223, E-ISSN 1873-3891, Vol. 62, p. 256-262Article in journal (Refereed)
    Abstract [en]

    Sulfur has several roles, desirable and undesirable, in graphitization. We perform density functional theory calculations within the local density approximation to define the structures and energetics of sulphur in graphite, including its interactions with point defects and edges, in order to understand its role in the later stages of graphitization. We find sulphur does not cross-link layers, except where there are defects. It reacts very strongly with vacancies in neighbouring layers to form a six coordinate split vacancy structure, analogous to that found in diamond. It is also highly stable at basal edge sites, where, as might be expected, the size and valency of sulfur can be easily accommodated. This suggests a role for sulphur in stabilizing graphene edges, and following from this, we show that sulfur dimers can open, i.e. unzip, folds in graphite rapidly and exothermically.

  • 5.
    Al-Hamadandy, Raied
    et al.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Goss, J.P.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Briddon, P.R.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Mojarad, Shahin A.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Al-Hadidi, Meaad
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    O'Neill, A.G.
    School of Electrical and Electronic Engineering, Newcastle University, Newcastle upon Tyne.
    Rayson, Mark
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Oxygen vacancy migration in compressively strained SrTiO32013In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 2Article in journal (Refereed)
    Abstract [en]

    The intrinsic properties of strontium titanate render it promising in applications such as gate dielectrics and capacitors. However, there is growing evidence that oxygen vacancies significantly impact upon its use, with the diffusion and deep donor level of the oxygen vacancy leading to electrical leakage. Where grown epitaxially on a lattice mismatched substrate, SrTiO 3 undergoes bi-axial strain, altering its crystal structure and electronic properties. In this paper, we present the results of first-principles simulations to evaluate the impact of strain in a (001) plane upon the migration of oxygen vacancies. We show that in the range of strains consistent with common substrate materials, diffusion energies in different directions are significantly affected, and for high values of strain may be altered by as much as a factor of two. The resulting diffusion anisotropy is expected to impact upon the rate at which oxygen vacancies are injected into the films under bias, a critical factor in the leakage and resistive switching seen in this material

  • 6.
    Al-Hamadany, R.
    et al.
    Newcastle University.
    Goss, J.P.
    Newcastle University.
    Briddon, P.R.
    Newcastle University.
    Mojarad, S.A.
    Newcastle University.
    O'Neill, A.G.
    Newcastle University.
    Rayson, Mark
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Impact of tensile strain on the oxygen vacancy migration in SrTiO3: Density functional theory calculations2013In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 113, no 22Article in journal (Refereed)
    Abstract [en]

    Strontium titanate is a promising dielectric material for device applications including capacitors and gate dielectrics. However, oxygen vacancies, which are inevitable donor defects mobile under bias at room temperature, lead to undesirable leakage current in SrTiO3 thin films. Epitaxially grown SrTiO3 on lattice mismatched substrates leads to strained SrTiO3, inducing structural phase transitions from a cubosymmetric non-ferroelectric geometry to tetragonal and orthorhombic structures, depending upon the sign of the strain. In this study, density functional calculations have been performed to determine the impact of isotropic biaxial tensile strain in a (001) plane upon the phase of SrTiO3 and the activation energy for the migration of oxygen vacancies in such strained SrTiO3. The phase transition of the host material yields anisotropy in oxygen vacancy diffusion for diffusion within and between planes parallel to the strain. We found a general reduction in the barrier for diffusion within and normal to the plane of tensile strain. The inter-plane diffusion barrier reduces up to 25% at high values of strain. The variation in the barrier corresponding to in-plane diffusion is smaller in comparison to inter-plane diffusion. Finally, we reflect upon how the interplay between lattice strain with native defects plays a crucial role in the conduction mechanism of thin film, strained SrTiO3

  • 7.
    Andersen, O.
    et al.
    Centre for Electronic Materials, University of Manchester.
    Dobaczewski, L.
    Centre for Electronic Materials, University of Manchester.
    Peaker, A.R.
    Centre for Electronic Materials, University of Manchester.
    Nielsen, K. Bonde
    Institute of Physics and Astronomy, University of Aarhus.
    Hourahine, B.
    Department of Physics, University of Exeter.
    Jones, R.
    Department of Physics, University of Exeter.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Piezospectroscopic analysis of the hydrogen-carbon complexes in silicon2001In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 308, p. 139-142Article in journal (Refereed)
    Abstract [en]

    We have observed the donor (Ec-0.22 eV) and acceptor (Ec-0.16 eV) levels related to hydrogen-carbon complexes in silicon. The donor level is only detected at low temperatures after proton implantation. This hydrogen-carbon complex irreversibly reconfigures at temperatures above 225 K to a configuration characterized by the acceptor level, which is stable up to room temperature. The same acceptor level is also observed after atomic hydrogen diffusion. We have used Laplace transform deep level transient spectroscopy (DLTS) to show the influence of uniaxial stress on the electron emission process and the effect of the stress-induced alignment for the acceptor state. The pattern of the Laplace DLTS peak splittings indicate a trigonal symmetry of the defect. First principles calculations were carried out on the hydrogen-carbon defects with a view of determining their electrical levels and stress response for comparison with the experimental results.

  • 8.
    Andersen, O.
    et al.
    Centre for Electronic Materials, University of Manchester.
    Peaker, A.R.
    Centre for Electronic Materials, University of Manchester.
    Dobaczewski, L.
    Centre for Electronic Materials, University of Manchester.
    Nielsen, K. Bonde
    Institute of Physics and Astronomy, University of Aarhus.
    Hourahine, B.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Electrical activity of carbon-hydrogen centers in Si2002In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 66, no 23, p. 235205-1Article in journal (Refereed)
    Abstract [en]

    The electrical activity of Cs-H defects in Si has been investigated in a combined modeling and experimental study. High-resolution Laplace capacitance spectroscopy with the uniaxial stress technique has been used to measure the stress-energy tensor and the results are compared with theoretical modeling. At low temperatures, implanted H is trapped as a negative-U center with a donor level in the upper half of the gap. However, at higher temperatures, H migrates closer to the carbon impurity and the donor level falls, crossing the gap. At the same time, an acceptor level is introduced into the upper gap making the defect a positive-U center.

  • 9.
    Andersson, Lennart
    et al.
    Luleå tekniska universitet.
    Grennberg, Anders
    Hedberg, Torbjörn
    Luleå tekniska universitet.
    Näslund, Reinhold
    Persson, Lars-Erik
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    von Sydow, Björn
    Luleå tekniska universitet.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Linjär algebra med geometri1999Book (Other (popular science, discussion, etc.))
  • 10.
    Antzutkin, Oleg
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Kota, Hanumantha Rao
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Ikumapayi, Fatai
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Holmgren, Allan
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Gunneriusson, Lars
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Johansson, Björn
    New Boliden AB.
    Berggren, Andreas
    New Boliden AB.
    Larsson, Anna-Carin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Mouzon, Johanne
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Bhuiyan, Iftekhar Uddin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Forsmo, Seija
    LKAB.
    Interactions in multi-component mineral systems2011Conference paper (Other academic)
  • 11.
    Antzutkin, Oleg
    et al.
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Kota, Hanumantha Rao
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Ikumapayi, Fatai
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Johansson, Björn
    New Boliden AB.
    Berggren, Andreas
    New Boliden AB.
    Larsson, Anna-Carin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Holmgren, Allan
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Hedlund, Jonas
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Mouzon, Johanne
    Bhuiyan, Iftekhar Uddin
    Luleå University of Technology, Department of Civil, Environmental and Natural Resources Engineering, Sustainable Process Engineering.
    Forsmo, Seija
    LKAB.
    Interactions in multi-component mineral systems2010Conference paper (Other academic)
  • 12.
    Arslan, O.
    et al.
    Department of Mathematics, Cukurova University.
    Edlund, Ove
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Ekblom, Håkan
    Algorithms to compute CM - and S-estimates for regression2003In: International Conference on Robust Statistics: ICORS / [ed] Rudolf Dutter; P. Filzmoser; U. Gather; P. J. Pousseeuw, Physica-Verlag Rudolf Liebig GmbH , 2003, p. 62-76Conference paper (Refereed)
    Abstract [en]

    Constrained M-estimators for regression were introduced by Mendes and Tyler in 1995 as an alternative class of robust regression estimators with high breakdown point and high asymptotic efficiency. To compute the CM-estimate, the global minimum of an objective function with an inequality constraint has to be localized. To find the S-estimate for the same problem, we instead restrict ourselves to the boundary of the feasible region. The algorithm presented for computing CM-estimates can easily be modified to compute S-estimates as well. Testing is carried out with a comparison to the algorithm SURREAL by Ruppert

  • 13.
    Ashwin, M.J.
    et al.
    Interdisciplinary Research Centre for Semiconductor Materials, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London.
    Pritchard, R.E.
    Interdisciplinary Research Centre for Semiconductor Materials, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London.
    Newman, R.C.
    Interdisciplinary Research Centre for Semiconductor Materials, The Blackett Laboratory, Imperial College of Science, Technology and Medicine, London.
    Joyce, T.B.
    Department of Materials Science and Engineering, Liverpool University.
    Bullough, T.J.
    Department of Materials Science and Engineering, Liverpool University.
    Wagner, J.
    Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastrasse 72, D-79108 Freiburg.
    Jeynes, C.
    Department of Electronic and Electrical Engineering, University of Surrey, Guildford.
    Breuer, S.J.
    Department of Physics, University of Exeter.
    Jones, R.
    Department of Physics, University of Exeter.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    The bonding of CAs acceptors in InxGa1-xAs grown by chemical beam epitaxy using carbon tetrabromide as the source of carbon1996In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 80, no 12, p. 6754-6760Article in journal (Refereed)
    Abstract [en]

    InxGa1-xAs layers (0≤x≤0.37) doped with carbon (>1020 cm-3) were grown on semi-insulating GaAs substrates by chemical beam epitaxy using carbon tetrabromide (CBr4) as the dopant source. Hall measurements imply that all of the carbon was present as CAs for values x up to 0.15. The C acceptors were passivated by exposing samples to a radio frequency hydrogen plasma for periods of up to 6 h. The nearest-neighbor bonding configurations of CAs were investigated by studying the nondegenerate antisymmetric hydrogen stretch mode (A-1 symmetry) and the symmetric XH mode (A+1 symmetry) of the H-CAs pairs using IR absorption and Raman scattering, respectively. Observed modes at 2635 and 450 cm-1 had been assigned to passivated Ga4CAs clusters. New modes at 2550 and 430 cm-1 increased in strength with increasing values of x and are assigned to passivated InGa3CAs clusters. These results were compared with ab initio local density functional theory. Modes due to AlInGaCAs clusters were detected in samples containing grown in Al and In. These results demonstrate that for InGaAs, CBr4 is an efficient C doping source since both In-CAs bonds as well as Ga-CAs bonds are formed, whereas there is no evidence for the formation of In-CAs bonds in samples doped with C derived from trimethylgallium or solid sources

  • 14.
    Atumi, M.K.
    et al.
    Newcastle University.
    Goss, J.P.
    Newcastle University.
    Briddon, P.R.
    Newcastle University.
    Shirf, F.E.
    Newcastle University.
    Rayson, Mark
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Hyperfine interactions at nitrogen interstitial defects in diamond2013In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 25, no 6Article in journal (Refereed)
    Abstract [en]

    Diamond has many extreme physical properties and it can be used in a wide range of applications. In particular it is a highly effective particle detection material, where radiation damage is an important consideration. The WAR9 and WAR10 are electron paramagnetic resonance centres seen in irradiated, nitrogen-containing diamond. These S = 1/2 defects have C2v and C1h symmetry, respectively, and the experimental spectra have been interpreted as arising from nitrogen split-interstitial centres. Based upon the experimental and theoretical understanding of interstitial nitrogen defect structures, the AIMPRO density functional code has been used to assess the assignments for the structures of WAR9 and WAR10. Although the calculated hyperfine interaction tensors are consistent with the measured values for WAR9, the thermal stability renders the assignment problematic. The model for the WAR10 centre yields principal directions of the hyperfine tensor at variance with observation. Alternative models for both centres are discussed in this paper, but no convincing structures have been found.

  • 15.
    Bangert, U.
    et al.
    School of Physics, University of Exeter.
    Barnes, R.
    Lunar & Planetary Lab., Arizona University, Tucson.
    Hounsome, L.S.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Blumenau, A.T.
    Max-Planck-Institut für Eisenforschung GmbH.
    Briddon, P.R.
    School of Natural Science, University of Newcastle upon Tyne.
    Shaw, M.J.
    School of Natural Science, University of Newcastle upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Electron energy loss spectroscopic studies of brown diamonds2006In: Philosophical Magazine, ISSN 1478-6435, E-ISSN 1478-6443, Vol. 86, no 29-31, p. 4757-4779Article in journal (Refereed)
    Abstract [en]

    We investigate both experimentally and theoretically, low-loss electron energy losses in brown type IIa monocrystalline diamonds both before and after high-temperature, high-pressure anneals which remove the brown colouration. We find additional losses within and near the band edge for brown diamond which are significantly reduced after treatment. The additional losses are not associated with dislocations. Graphitic inclusions are detected by EELS as well as TEM studies for some brown diamonds before treatment. These lead to pronounced subgap absorption. However, all brown diamonds exhibit additional losses which are due to point defects lying in the regions between dislocations. First principles theoretical modelling shows that common dislocations are not responsible for the brown colouration but a -bonded vacancy disk lying on {111} planes gives broad bands lying in the diamond band gap, possesses an optical absorption spectrum similar to that of brown diamond, and leads to additional electron energy losses in the band edge region. These and similar defects are suggested to be responsible for the brown colouration. Mechanisms are proposed for their formation and removal.

  • 16.
    Barestrand, Henrik
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Forslund, Tobias
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Numerical & physical modelling of fluid flow in a continuous casting mould: Flow dynamics studies for flexible operation of continuous casters2016Independent thesis Advanced level (professional degree), 20 credits / 30 HE creditsStudent thesis
    Abstract [en]

    The current demands on Swedish steel industry to produce low quantity batches of specialized products requires research on steel casting processes. There are several physical processes that need be taken into account for this problem to be viewed in full light such as thermal-processes, solidification and fluid dynamics. This work focuses on the fluid-dynamics part; more specifically, the dependence of flow quality within the caster on nozzle and mould geometry.

    The simulations are carried out using a scale-resolving method, in specific LES (Large Eddy Simulation) which is coupled with a DPM (Discrete Phase Model) to model Argon behaviour. The results of these simulations are presented and validated against physical experiment and data from industrial trials. Conclusions are drawn regarding optimal nozzle types in respect to different mould geometries.

    The mould eigenfrequencies are shown to exhibit a connection with the casting velocity. This results in so called sweet spots in casting velocity where flow irregularities due to sloshing is minimal. It is shown that the mountain type nozzle is preferable for smaller geometries whilst comparatively larger geometries benefit from a cup type.

  • 17.
    Berezovsky, Vladimir
    et al.
    Pomorskii State University, Arkhangelsk.
    Men'shikov, Leonid
    Russian Research Centre, Kurchatov Institute, Moscow.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Latham, Chris
    Effect of dipole-dipole interactions between atoms in an active medium2008In: Journal of the Optical Society of America. B, Optical physics, ISSN 0740-3224, E-ISSN 1520-8540, Vol. 25, no 3, p. 458-462Article in journal (Refereed)
    Abstract [en]

    On the basis of the results of numerical modeling, it is shown that dipole-dipole interactions among atoms in the active medium strongly influences the character of the associated superradiation. The main effect is to make the nuclear subsystem behave chaotically. Its strength increases with the atom density and leads to the suppression of distant collective correlations and superradiation. Near correlations between the atoms are established, causing a confinement effect: a shielding of radiation in the active medium.

  • 18.
    Berezovsky, V.V.
    et al.
    Lomonosov Pomor State University, Arkhangelsk.
    Men'shikov, L.I.
    Russian Research Centre, Kurchatov Institute, Moscow.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Latham, Chris
    Effect of the dipole-dipole interaction of particles in an active medium on the character of superradiation2008In: Plasma physics reports (Print), ISSN 1063-780X, E-ISSN 1562-6938, Vol. 34, no 7, p. 555-561Article in journal (Refereed)
    Abstract [en]

    The motion of a system of interacting nonlinear charged oscillators is investigated numerically. Because of nonlinearity, the total collective electric field gives rise to a phasing effect-correlations in the phases of the oscillators. The consequence is superradiation-the enhanced spontaneous short-term emission of the energy stored in the oscillators. It is shown that the oscillations of the oscillators become stochastic because of the dipole-dipole interaction between them and their nearest neighbors. As a result, as the density of the oscillators increases, distant collective correlations are suppressed, superradiation ceases to be generated, and radiation is shielded in the medium. The phenomena considered in the present paper can play an important role in cyclotron emission from a plasma and thus should be taken into account in emission calculations. The process whereby the energy of the transverse electron motion in electron cooling devices decreases is analyzed as an example. This process occurs as a result of the development of cyclotron maser instability and has the nature of superradiation. The onset of correlations between individual electrons moving in their Larmor circles is the initial, linear stage of instability developing in the plasma. Superradiation is the final, nonlinear instability stage.

  • 19.
    Berglund, Tomas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Brodnik, Andrej
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science.
    Jonsson, Håkan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science.
    Mrozek, Kent
    Staffansson, Mats
    Luleå tekniska universitet.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Minimum curvature variation B-splines: validation of a path-planning model2004Report (Other academic)
  • 20.
    Berglund, Tomas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Brodnik, Andrej
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science. University of Primorska, Slovenia. .
    Jonsson, Håkan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science.
    Staffansson, Mats
    Luleå tekniska universitet.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Planning smooth and obstacle-avoiding b-spline paths for autonomous mining vehicles2010In: IEEE Transactions on Automation Science and Engineering, ISSN 1545-5955, E-ISSN 1558-3783, Vol. 7, no 1, p. 167-172Article in journal (Refereed)
    Abstract [en]

    We study the problem of automatic generation of smooth and obstacle-avoiding planar paths for efficient guidance of autonomous mining vehicles. Fast traversal of a path is of special interest. We consider four-wheel four-gear articulated vehicles and assume that we have an a priori knowledge of the mine wall environment in the form of polygonal chains. Computing quartic uniform B-spline curves, minimizing curvature variation, staying at least at a proposed safety margin distance from the mine walls, we plan high speed paths.

  • 21.
    Berglund, Tomas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Erikson, Ulf
    Luleå tekniska universitet.
    Jonsson, Håkan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science.
    Mrozek, Kent
    Navigator AB.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Automatic generation of smooth paths bounded by polygonal chains2001In: CIMCA 2001: 2001 international conference on computational intelligence for modelling, control & automation : 9-11 July 2001, Las Vegas, Nevada, USA : proceedings / [ed] M. Mohammadian, CIMCA , 2001, p. 528-535Conference paper (Refereed)
  • 22. Berglund, Tomas
    et al.
    Jonsson, Håkan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    An obstacle-avoiding minimum variation B-spline problem2003In: Proceedings: 2003 International Conference on Geometric Modeling and Graphics, GMAG 2003 ; 16 - 18 July 2003, London, England, Los Alamitos, Calif: IEEE Communications Society, 2003, p. 156-161Conference paper (Refereed)
    Abstract [en]

    We study the problem of computing a planar curve, restricted to lie between two given polygonal chains, such that the integral of the square of arc-length derivative of curvature along the curve is minimized. We introduce the minimum variation B-spline problem, which is a linearly constrained optimization problem over curves, defined by B-spline functions only. An empirical investigation indicates that this problem has one unique solution among all uniform quartic B-spline functions. Furthermore, we prove that, for any B-spline function, the convexity properties of the problem are preserved subject to a scaling and translation of the knot sequence defining the B-spline.

  • 23.
    Berglund, Tomas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Jonsson, Håkan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    The problem of computing an obstacle-avoiding minimum variation B-spline2003Report (Other academic)
    Abstract [en]

    We study the problem of computing a planar curve restricted to lie between two given polygonal chains such that the integral of the square of arc- length derivative of curvature along the curve is minimized. We introduce the Minimum Variation B-spline problem which is a linearly constrained optimization problem over curves defined by B-spline functions only. An empirical investigation indicates that this problem has one unique solution among all uniform quartic B-spline functions. Furthermore, we prove that, for any B-spline function, the convexity properties of the problem are preserved subject to a scaling and translation of the knot sequence defining the B-spline.

  • 24.
    Berglund, Tomas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Strömberg, Thomas
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Jonsson, Håkan
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Computer Science.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Epi-convergence of minimum curvature variation B-splines2003Report (Other academic)
    Abstract [en]

    We study the curvature variation functional, i.e., the integral over the square of arc-length derivative of curvature, along a planar curve. With no other constraints than prescribed position, slope angle, and curvature at the endpoints of the curve, the minimizer of this functional is known as a cubic spiral. It remains a challenge to effectively compute minimizers or approximations to minimizers of this functional subject to additional constraints such as, for example, for the curve to avoid obstacles such as other curves. In this paper, we consider the set of smooth curves that can be written as graphs of three times continuously differentiable functions on an interval, and, in particular, we consider approximations using quartic uniform B- spline functions. We show that if quartic uniform B-spline minimizers of the curvature variation functional converge to a curve, as the number of B-spline basis functions tends to infinity, then this curve is in fact a minimizer of the curvature variation functional. In order to illustrate this result, we present an example of sequences of B-spline minimizers that converge to a cubic spiral.

  • 25.
    Berglund, Tomas
    et al.
    Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
    Synnergren, Per
    Luleå tekniska universitet.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Estimation of curl in paper: an industrial application combining shape measurement and least squares modeling2000In: Proceedings / International Conference on Trends in Optical Nondestructive Testing: Lugano, Switzerland, May 3-6, 2000 / [ed] Pramod K. Rastogi; D. Inaudi, Lugano: Ecole Polytechnique Fédérale de Lausanne , 2000, p. 23-33Conference paper (Refereed)
    Abstract [en]

    If a sheet of paper is subjected to humidity changes and have structural variations through its thickness such as gradients of fibre orientation, density and filler content, the sheet will curl and hence assume a cylindrical shape. Curl is a quality problem that makes the paper less suitable for printing. We propose a method to measure curl that can be used for automated analysis of the paper quality. The shape of the curled paper is measured from the perspective difference in a stereoscopic camera system, which is viewing an irregular pattern that is projected onto the specimen. The perspective difference is calculated by a correlation algorithm, a technique often referred to as digital speckle photography. The most interesting quality parameters are the magnitude of curl, which is defined as the inverse of the radius of curvature and also the orientation of the curled paper. These parameters are estimated by performing a least squares fit of a cylindrical shape to the three-dimension...

  • 26.
    Bergström, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Computational methods for on-line shape inspection2009Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This licentiate thesis describes computational methods that solve problems occurring in industrial on-line shape quality inspection of produced items. These items are measured and compared with their corresponding CAD object. The meaning of on-line is that the inspection is done on-line in the production line, i.e. the items are not removed from the line. In practice this means that the inspection must be done very fast, both the measurement and the data analysis. The measurement is done using an optical non-contact method based on projection of fringes.The presented methods are mainly based on finding a transformation, a rotation and a translation, of the measurement values which consists of a point cloud representing the measured surface. This transformation is calculated using the iterative closest point (ICP) method such that the point cloud fits the corresponding surface of the CAD object properly. The method for finding this transformation is adapted for reiterated use, i.e. it makes use of the fact that the same CAD object is used several times for different measurements. A search tree making it possible to do this fast is proposed.When dealing with real measurements obtained from optical methods undesired measurement errors will occur, caused by reflections, dirt on lenses or other likely matters in the industrial environment. The iteratively re-weighted least squares (IRLS) method for different robust functions are used in combination with ICP for handling these errors, in order to do a correct surface matching. This result in much higher matching accuracy and almost no additional computations are needed.

  • 27.
    Bergström, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Computational methods for shape verification of free-form surfaces2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [sv]

    Beräkningsmetoder för formverifiering av friformsytor utgör huvudinnehållet i denna doktorsavhandling. En gemensam egenskap för dessa metoder är att de möjliggör formverifiering online direkt i produktionslinan. Av den anledningen måste metoderna vara snabba och robusta. Ett av problemen som uppkommer i formverifieringen av friformsytor är registrering. Det är problemet med att matcha datapunkter i 3D-rymden, som representerar den uppmätta ytan, med ett CAD-objekt genom att ansätta en stelkropps transformation. En metod för att utföra registreringen snabbt och robust är utvecklad. Metoden är en utveckling av ”the iterative closest point method, ICP”. Vi förprocessar CAD-objektet genom att skapa en datastruktur för att möjliggöra snabb närmsta-punkt sökning. Initialt läggs mycket tid på att skapa datastrukturen för att de enskilda registreringarna skall gå snabbt. Den robusta registreringen baserar sig på teorier från robust statistik genom att tillämpa ”iteratively re-weighted least squares” i kombination med ICP metoden. Detta resulterar i en snabb registreringsmetod som är okänslig för avvikande data. Metoden med registreringen används i en tillämpning för att hitta avvikelser mellan formen för ett objekt och dess ideala form. Den ideala formen är känd och ges av ett CAD-objekt. En optisk formmätningsmetod, projicerade fransar med en enda mönsterdetektering, används för att skapa datapunkter av den uppmätta ytan. Denna metod är snabb och okänslig för vibrationer men datapunkterna kan innehålla fel i vissa regioner, vilket hanteras av registreringen. Ett inversproblem som uppkommer i många optiska formmätningsmetoder är fasuppvikning. Vi introducerar en uppvikningsmetod med regularisering genom att använda information från ett CAD-objekt. Formmätningsmetoden som vi använder oss av här baserar sig på två-våglängds holografi. Vår fasuppvikningsmetod funkar oberoende av diskoninuiteter men mätobjektet får inte avvika alltför mycket i form jämfört med CAD-objektet. En metod för att snabbt få fram den behövda forminformationen från CAD-objektet är också utvecklad. För att få fram lämplig forminformation från datapunkter kan en parametrisk kurva eller yta, t.ex. NURBS, anpassas till dessa punkter. Ett delproblem som uppstår vid NURBS-anpassning vid användandet av Gauss-Newton metoden är studerad. Beräkningsaspekter för att få fram en sökriktning är diskuterade. Vi behandlar också metoder för NURBS anpassning som baserar sig på en teknik för separabla icke-linjära minstakvadratproblem. Denna teknik använder sig av variabelprojektioner för att separera beräkningarna av de linjära parametrarna från beräkningarna av de icke-linjära parametrarna.

  • 28.
    Bergström, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Evaluation of NURBS Surfaces for Regular Structured Parameter Values2015In: Journal of Computing and Information Science in Engineering, ISSN 1530-9827, E-ISSN 1944-7078, Vol. 15, no 1, article id 11005Article in journal (Refereed)
    Abstract [en]

    The evaluation of surface points and derivatives of NURBS surfaces for parameter values that are regularly distributed in a rectangular structure is considered. Because of the regularity, parts of the computations can be done on just a small portion of all parameter values and computed data is stored and reused for many other parameter values. Hence, the evaluation of NURBS surfaces can be performed faster when the regularity is used. We are making a complexity analysis of the number of floating point operations, which is required for the evaluations. To get knowledge about how the evaluations perform in practice, we are doing a numerical experiment where we are measuring the runtime to obtain the output both by using ordinary evaluation of the NURBS surface and by making use of the regular structure. Making use of the regularity gives significantly faster output.

  • 29.
    Bergström, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Perspective depth extraction of points on a surface under an instantaneous rigid body transformation2012In: Journal of Computing and Information Science in Engineering, ISSN 1530-9827, E-ISSN 1944-7078, Vol. 12, no 2Article in journal (Refereed)
  • 30.
    Bergström, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Reliable updates of the transformation in the iterative closest point algorithm2016In: Computational optimization and applications, ISSN 0926-6003, E-ISSN 1573-2894, Vol. 63, no 2, p. 543-557Article in journal (Refereed)
    Abstract [en]

    The update of the rigid body transformation in the iterative closest point (ICP) algorithm is considered. The ICP algorithm is used to solve surface registration problems where a rigid body transformation is to be found for fitting a set of data points to a given surface. Two regions for constraining the update of the rigid body transformation in its parameter space to make it reliable are introduced. One of these regions gives a monotone convergence with respect to the value of the mean square error and the other region gives an upper bound for this value. Point-to-plane distance minimization is then used to obtain the update of the transformation such that it satisfies the used constraint.

  • 31.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Edlund, Ove
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Robust registration of point sets using iteratively reweighted least squares2014In: Computational optimization and applications, ISSN 0926-6003, E-ISSN 1573-2894, Vol. 58, no 3, p. 543-561Article in journal (Refereed)
    Abstract [en]

    Registration of point sets is done by finding a rotation and translation that produces a best fit between a set of data points and a set of model points. We use robust M-estimation techniques to limit the influence of outliers, more specifically a modified version of the iterative closest point algorithm where we use iteratively re-weighed least squares to incorporate the robustness. We prove convergence with respect to the value of the objective function for this algorithm. A comparison is also done of different criterion functions to figure out their abilities to do appropriate point set fits, when the sets of data points contains outliers. The robust methods prove to be superior to least squares minimization in this setting.

  • 32.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Edlund, Ove
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Efficient computation of the Gauss-Newton direction when fitting NURBS using ODR2012In: BIT Numerical Mathematics, ISSN 0006-3835, E-ISSN 1572-9125, Vol. 52, no 3, p. 571-588Article in journal (Refereed)
    Abstract [en]

    We consider a subproblem in parameter estimation using the Gauss-Newton algorithm with regularization for NURBS curve fitting. The NURBS curve is fitted to a set of data points in least-squares sense, where the sum of squared orthogonal distances is minimized. Control-points and weights are estimated. The knot-vector and the degree of the NURBS curve are kept constant. In the Gauss-Newton algorithm, a search direction is obtained from a linear overdetermined system with a Jacobian and a residual vector. Because of the properties of our problem, the Jacobian has a particular sparse structure which is suitable for performing a splitting of variables. We are handling the computational problems and report the obtained accuracy using different methods, and the elapsed real computational time. The splitting of variables is a two times faster method than using plain normal equations.

  • 33.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Edlund, Ove
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics.
    Repeated surface registration for on-line use2011In: The International Journal of Advanced Manufacturing Technology, ISSN 0268-3768, E-ISSN 1433-3015, Vol. 54, no 5-8, p. 677-689Article in journal (Refereed)
    Abstract [en]

    We consider the problem of matching sets of 3D points from a measured surface to the surface of a corresponding computer-aided design (CAD) object. The problem arises in the production line where the shape of the produced items is to be compared on-line with its pre-described shape. The involved registration problem is solved using the iterative closest point (ICP) method. In order to make it suitable for on-line use, i.e., make it fast, we pre-process the surface representation of the CAD object. A data structure for this purpose is proposed and named Distance Varying Grid tree. It is based on a regular grid that encloses points sampled from the CAD surfaces. Additional finer grids are added to the vertices in the grid that are close to the sampled points. The structure is efficient since it utilizes that the sampled points are distributed on surfaces, and it provides fast identification of the sampled point that is closest to a measured point. A local linear approximation of the surface is used for improving the accuracy. Experiments are done on items produced for the body of a car. The experiments show that it is possible to reach good accuracy in the registration and decreasing the computational time by a factor 700 compared with using the common kd-tree structure.

  • 34.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Khodadad, Davood
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Hällstig, Emil
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Dual-wavelength digital holography: single-shot shape evaluation using speckle displacements and regularization2014In: Applied Optics, ISSN 1559-128X, E-ISSN 2155-3165, Vol. 53, no 1, p. 123-131Article in journal (Refereed)
  • 35.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Khodadad, Davood
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Hällstig, Emil
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Single shot shape evaluation using dual-wavelength holographic reconstructions and regularization2014In: Fringe 2013: 7th International Workshop on Advanced Optical Imaging and Metrology / [ed] Wolfgang Osten, Berlin: Encyclopedia of Global Archaeology/Springer Verlag, 2014, p. 103-108Conference paper (Refereed)
    Abstract [en]

    The aim of this work is to evaluate the shape of a free form object using single shot digital holography. The digital holography results in a gradient field and wrapped phase maps representing the shape of the object. The task is then to find a surface representation from this data which is an inverse problem. To solve this inverse problem we are using regularization with additional shape information from the CAD-model of the measured object.

  • 36.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Rosendahl, Sara
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Gren, Per
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Shape verification using dual-wavelength holographic interferometry2011In: Optical Engineering: The Journal of SPIE, ISSN 0091-3286, E-ISSN 1560-2303, Vol. 50, no 10, article id 101503Article in journal (Refereed)
    Abstract [en]

    In automotive industry there is an interest of controlling the shape of a large number of identical components on-line in the manufacturing process. We propose a method to do this by capturing a digital hologram of the object and then using information from its computer aided design (CAD) model to calculate the shape and determine the agreement between the manufactured object and the CAD-model. The holographic recording of the object is done using dual wavelengths with a synthetic wavelength of approximately 400 μm. The optical measurement results in a wrapped phase map with the phase values in the interval [−π, π]. Each phase interval represents a depth distance on the object of about 0.2 mm. The phase unwrapping is done iteratively using information from the CADmodel. This implies that it is possible to measure large discontinuities on the surface of the measured object. The method also gives a point-to-point correspondence between the measurement and the CAD-model which is vital for tolerance control.

  • 37.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Rosendahl, Sara
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Sjödahl, Mikael
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Fluid and Experimental Mechanics.
    Shape verification aimed for manufacturing process control2011In: Optics and lasers in engineering, ISSN 0143-8166, E-ISSN 1873-0302, Vol. 49, no 3, p. 403-409Article in journal (Refereed)
    Abstract [en]

    We describe a method to verify the shape of manufactured objects by using their design model. A non-contact measuring method that consists of a stereo-camera system and a single projected fringe pattern is used. The method acquires one image from each camera. Additional shape information from the design model is also used. This surface-measurement method gives an accuracy of about 45 µm. Deviations from the design model within ±1.6 mm can be correctly detected. The measured surface representation is matched to the design model using the ICP-method. Fast performance has been considered adapting the method for on-line use.

  • 38.
    Bergström, Per
    et al.
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Söderkvist, Inge
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Fitting NURBS using separable least squares techniques2012In: International Journal of Mathematical Modelling and Numerical Optimisation, ISSN 2040-3607, E-ISSN 2040-3615, Vol. 3, no 4, p. 319-334Article in journal (Refereed)
    Abstract [en]

    We consider the problem of fitting a non-uniform rational B-spline (NURBS) curve to a set of data points by determining the control points and the weights using techniques aimed for solving separable least squares problems. The main technique under consideration is the variable projection method which utilises that the NURBS model depends linearly on its control points but non-linearly on the weights. The variable projection method can be used with the Gauss-Newton algorithm but also with Newton's algorithm. We investigate the efficiency of the different algorithms when fitting NURBS and observe that the variable projection methods do not perform as well as reported for its use on, e.g., exponential fitting problems.

  • 39.
    Blumenau, A.T.
    et al.
    Max-Planck-Institut für Eisenforschung GmbH.
    Eberlein, T.A.G.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Frauenheim, T.
    Theoretische Physik, Universität Paderborn.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    The effect of charge on kink migration at 90° partial dislocations in SiC2005In: Physica status solidi. A, Applied research, ISSN 0031-8965, E-ISSN 1521-396X, Vol. 202, no 5, p. 877-882Article in journal (Refereed)
    Abstract [en]

    SiC bipolar devices show a degradation under forward-biased operation which has been linked with a recombination enhanced motion of one of the two glide dislocations having either Si or C core atoms. We have carried out calculations of the core structures and dynamics of partial dislocations in 3C and 2H-SiC using the density functional based codes DFTB and AIMPRO. After in earlier theoretical work we reported on the structure, energetics and electronic activity of both of the Shockley partials, and on the formation and migration barriers of kinks, in this work we present first results on the effect of charge on the disloction kinks. The calculations give insights into the device degradation mechanism.

  • 40.
    Blumenau, A.T.
    et al.
    University of Exeter.
    Elsner, J.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Heggie, M.I.
    University of Sussex.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Frauenheim, T.
    Universität Paderborn.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Dislocations in hexagonal and cubic GaN2000In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, p. 10223-10233Article in journal (Refereed)
    Abstract [en]

    The structure and electronic activity of several types of dislocations in both hexagonal and cubic GaN are calculated using first-principles methods. Most of the stoichiometric dislocations investigated in hexagonal GaN do not induce deep acceptor states and thus cannot be responsible for the yellow luminescence. However, it is shown that electrically active point defects, in particular gallium vacancies and oxygen-related defect complexes, can be trapped at the stress field of the dislocations and may be responsible for this luminescence. For cubic GaN, we find the ideal stoichiometric 60° dislocation to be electrically active and the glide set to be more stable than the shuffle. The dissociation of the latter is considered

  • 41.
    Blumenau, A.T.
    et al.
    Theoretische Physik, Universität Paderborn.
    Fall, C.J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Heggie, M.I.
    CPES, University of Sussex, Falmer, Brighton.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Frauenheim, T.
    Theoretische Physik, Universität Paderborn.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Straight and kinked 90° partial dislocations in diamond and 3C-SiC2002In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 14, no 48, p. 12741-12747Article in journal (Refereed)
    Abstract [en]

    Density-functional based calculations are used to investigate low energy core structures of 90° partial dislocations in diamond and 3C-SiC. In both materials dislocation glide is analysed in terms of kink formation and migration and the fundamental steps to kink migration are investigated. We find the C terminated core structure in SiC to be more mobile than the Si core. However, the Si partial is electrically active and this opens the possibility of recombination-enhanced glide under ionizing conditions or an enhanced mobility in doped material.

  • 42.
    Blumenau, A.T.
    et al.
    School of Physics, University of Exeter.
    Fall, C.J.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Frauenheim, T.
    Department of Physics, Faculty of Science, Universität Paderborn.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Structure and motion of basal dislocations in silicon carbide2003In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 68, no 17, p. 174108-1Article in journal (Refereed)
    Abstract [en]

    30° and 90° Shockley partial dislocations lying in {111} and basal planes of cubic and hexagonal silicon carbide, respectively, are investigated theoretically. Density-functional-based tight-binding total-energy calculations are used to determine the core structure and energetics of the dislocations. In a second step their electronic structure is investigated using a pseudopotential method with a Gaussian basis set. Finally, the thermal activation barriers to glide motion of 30° and 90° Shockley partials are calculated in terms of a process involving the formation and migration of kinks along the dislocation line. The mechanism for enhanced dislocation movement observed under current injection conditions in bipolar silicon carbide devices is discussed.

  • 43.
    Blumenau, A.T.
    et al.
    Universität Paderborn.
    Frauenheim, T.
    Universität Paderborn.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Willems, B.
    University of Antwerp.
    Tendeloo, G. van
    University of Antwerp.
    Dislocation structures in diamond: density-functional based modelling and high-resolution electron microscopy2004In: Diffusion and defect data, solid state data. Part A, Defect and diffusion forum, ISSN 1012-0386, E-ISSN 1662-9507, Vol. 226-228, p. 11-30Article in journal (Refereed)
    Abstract [en]

    The core structures of perfect 60° and edge dislocations in diamond are investigated atomistically in a density-functional based tight-binding approach, and their dissociation is discussed both in terms of structure and energy. Furthermore, high resolution electron microscopy is performed on dislocation cores in high-temperature, high-pressure annealed natural brown diamond, and HRTEM image simulation allows a comparison of theoretically predicted and experimentally observed structures.

  • 44.
    Blumenau, A.T.
    et al.
    Department of Physics, Universität Paderborn.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Frauenheim, T.
    Department of Physics, Universität Paderborn.
    Basal plane partial dislocations in silicon carbide2003In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 340, p. 160-164Article in journal (Refereed)
    Abstract [en]

    Under operating conditions (forward bias) bipolar 4H- and 6H-SiC devices are known to degrade rapidly through stacking fault formation and expansion in the basal plane. It has been suggested that a recombination-enhanced dislocation glide (REDG) mechanism allows the bordering Shockley partial dislocations to overcome their barrier to glide motion and thus results in the observed stacking fault growth. In this work, we investigate the structure and properties of the participating Shockley partials by means of density functional-based atomistic calculations. Their glide motion is modelled in a process involving the formation and subsequent migration of kinks. This in combination with an analysis of the electronic structure of the partials allows an identification of those types which will be affected by the REDG mechanism.

  • 45.
    Blumenau, A.T.
    et al.
    Department of Physics, Faculty of Science, University of Paderborn.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Frauenheim, T.
    Department of Physics, University of Newcastle.
    Density functional based modelling of 30° partial dislocations in SiC2004In: Proceedings of the 10th International Conference on Silicon Carbide and Related Materials, ICSCRM 2003, Stafa-Zuerich: Trans Tech Publications Inc., 2004, Vol. 1, p. 453-456Conference paper (Refereed)
    Abstract [en]

    Experiment has shown that 4H- and 6H-SiC pin diodes degrade rapidly during forward biased operation. This degradation is accompanied by the formation and expansion of stacking faults in the basal plane. It is believed that the observed rapid stacking fault growth is due to a recombination-enhanced dislocation glide (REDG) mechanism at the bordering partial dislocations. In our work we investigate the structure and properties of basal plane 30° Shockley partials in SiC by means of density functional based calculations. Barriers to their glide motion, and thus the expansion of the accompanying stacking fault is modelled in a process involving the formation and subsequent migration of kinks in the dislocation. In combination with an analysis of the electronic structure of the partials and stacking faults, this allows an identification of those types of partials which will be affected by the REDG mechanism in this model.

  • 46.
    Blumenau, A.T.
    et al.
    School of Physics, University of Exeter.
    Jones, R.
    School of Physics, University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    Department of Physics, University of Newcastle.
    Frauenheim, T.
    Theoretische Physik, Universität Paderborn.
    Dislocation related photoluminescence in silicon2001In: Physical Review Letters, ISSN 0031-9007, E-ISSN 1079-7114, Vol. 87, no 18, p. 187404-Article in journal (Refereed)
    Abstract [en]

    Dislocation related photoluminescence in Si and SiGe is attributed to stable interstitial clusters bound to 60° dislocations. Density functional based total energy calculations in Si give binding energies between 1.5 and 3.6 eV for I3 and I4 clusters with 90° and 30° partials. They possess donor levels around Ev+0.4 eV which are consistent with deep level transient spectroscopic studies on p-Si. It is further suggested that the clusters would act as the obstacles to the movement of dislocations which may have been observed in recent transmission electron microscopy studies.

  • 47.
    Blumenau, A.T.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Frauenheim, T.
    Universität Paderborn.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Optical bands related to dislocations in Si2000In: Journal of Physics: Condensed Matter, ISSN 0953-8984, E-ISSN 1361-648X, Vol. 12, p. 10123-10129Article in journal (Refereed)
    Abstract [en]

    First-principles calculations are used to investigate the interaction of self-interstitial aggregates with the 90° partial dislocation in Si. We find that I4 is bound to the line with an energy of around 3 eV. The defect causes deep levels to appear in the band gap and optical transitions between these levels may account for the luminescent bands relating to plastically deformed Si.

  • 48.
    Breuer, S J
    et al.
    University of Exeter.
    Jones, R
    University of Exeter.
    Briddon, P R
    University of Newcastle Upon Tyne.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Interstitial hydrogen and enhanced dissociation of C-H complexes in GaAs1996In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 53, no 24, p. 16289-16296Article in journal (Refereed)
    Abstract [en]

    Local-density-functional-based ab initio calculations are used to investigate hydrogen and carbon-hydrogen defects in GaAs. The equilibrium structure for both the C-H and C-H- complexes are shown to be similar, with the hydrogen located at a C-Ga bond-centered site. The dissociation of these complexes is investigated and it is found that the energy barrier of 1.84 eV for the process C-H → C- + H+ is substantially lowered to 0.88 eV in the presence of an electron resonantly bound to the defect. This is in good quantitative agreement with recent experiments. Isolated interstitial hydrogen is found to lie at a Ga-As bond-centered site for both H+ and H0 and at an antibonding site relative to a Ga atom for H-. It is also found that the stable form of the hydrogen dimer is a H2 molecule, the dissociation energy of which is 1.64 eV, and that interstitial hydrogen is a negative-U defect. Finally, a mechanism for minority-carrier-induced device degradation is proposed.

  • 49.
    Breuer, S.J.
    et al.
    University of Exeter.
    Jones, R.
    University of Exeter.
    Öberg, Sven
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Briddon, P.R.
    University of Newcastle Upon Tyne.
    Interstitial hydrogen and the dissociation of C-H defects in GaAs1995In: Proceedings of the 18th International Conference on Defects in Semiconductors: ICDS-18; Sendai, Japan, July 23 - 28, 1995 / [ed] Masashi Suezawa, Trans Tech Publications Inc., 1995, p. 951-956Conference paper (Refereed)
  • 50.
    Briddon, Patrick
    et al.
    School of Electrical, Electronic and Computer Engineering, University of Newcastle upon Tyne.
    Rayson, Mark
    Luleå University of Technology, Department of Engineering Sciences and Mathematics, Mathematical Science.
    Accurate Kohn–Sham DFT with the speed of tight binding: Current techniques and future directions in materials modelling2011In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 248, no 6, p. 1309-1318Article in journal (Refereed)
    Abstract [en]

    We present a review of methodological and implementation details of the AIMPRO Kohn–Sham density functional code. It is demonstrated that full Kohn–Sham density functional theory calculations can be performed in a time only marginally greater than tight binding implementations and a route is opened to achieve full and demonstrable convergence with respect to basis size. Topics covered will include both the kernel and functionality of the current code, a discussion of recent developments as well as future research directions and perspectives. Also, a broad discussion regarding the application of these methods is made that, it is hoped, will serve as a useful guide to application specialists.

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