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  • 201.
    Ghavanni, Farzan
    et al.
    Chalmers, Dept Microtechnol & Nanosci, BioNano Syst Lab, Micro & Nanosyst Grp, SE-41296 Gothenburg, Sweden.
    Lopez-Damian, Maria
    Chalmers, Dept Microtechnol & Nanosci, BioNano Syst Lab, Micro & Nanosyst Grp, SE-41296 Gothenburg, Sweden.
    Rafeian, Damon
    Chalmers, Dept Microtechnol & Nanosci, BioNano Syst Lab, Micro & Nanosyst Grp, SE-41296 Gothenburg, Sweden.
    Svensson, Krister
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Lundgren, Per
    Chalmers, Dept Microtechnol & Nanosci, BioNano Syst Lab, Micro & Nanosyst Grp, SE-41296 Gothenburg, Sweden.
    Enoksson, Peter
    Chalmers, Dept Microtechnol & Nanosci, BioNano Syst Lab, Micro & Nanosyst Grp, SE-41296 Gothenburg, Sweden.
    Controlling the initial phase of PECVD growth of vertically aligned carbon nanofibres on TiN2011In: Sensors and Actuators A-Physical, ISSN 0924-4247, E-ISSN 1873-3069, Vol. 172, no 1, p. 347-358Article in journal (Refereed)
    Abstract [en]

    We explore the growth of vertically aligned carbon nanofibers by plasma enhanced chemical vapor deposition, using lithographically defined Ni catalyst seeds on TiN. TiN is selected for being an electrically conducting diffusion barrier suitable for the realization of electronic devices. We show that the rate of Ni diffusion correlates to both the level of oxygen content in the TiN film and to the film resistivity. The synthesis of the nanofibers was characterized using electron microscopy with an emphasis on three growth parameters: substrate temperature, plasma power, and chamber pressure. We propose that a catalyst surface free from carbon deposits throughout the process will induce diffusion-limited growth. The growth will shift towards a supply-limited process when the balance between acetylene, as the effective carbon bearing gas, and atomic hydrogen, as the main etching agent, is skewed in favor of acetylene. This determines whether the dominating growth mode will be vertically aligned tip-type or disordered base-type, by affecting the competition between the formation of the first graphitic sheets on the catalyst surface and at the catalyst-substrate interface

  • 202. Grehk, T.M.
    et al.
    Götelidh, M.
    Karlsson, U.O.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Gray, S.M.
    Magnusson, Kjell
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    The clean and Cs-exposed Si(111)3x3:B surface studied with photoemission1995In: Phys. Rev. B52, 11165 (1995)Article in journal (Refereed)
  • 203.
    Grob, Stefan
    et al.
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Bartynski, Andrew N.
    Humboldt Univ, Inst Phys, D-12489 Berlin, Germany..
    Opitz, Andreas
    Univ So Calif, Dept Chem, Dept Chem Engn, Los Angeles, CA 90089 USA..
    Gruber, Mark
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Grassl, Florian
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Meister, Eduard
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Linderl, Theresa
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Hoermann, Ulrich
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Lorch, Christopher
    Univ Tubingen, Inst Angew Phys, D-72076 Tubingen, Germany..
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Schreiber, Frank
    Univ Tubingen, Inst Angew Phys, D-72076 Tubingen, Germany..
    Thompson, Mark E.
    Humboldt Univ, Inst Phys, D-12489 Berlin, Germany..
    Bruetting, Wolfgang
    Univ Augsburg, Inst Phys, D-86135 Augsburg, Germany..
    Solvent vapor annealing on perylene-based organic solar cells2015In: Journal of Materials Chemistry A, ISSN 2050-7488, Vol. 3, no 30, p. 15700-15709Article in journal (Refereed)
    Abstract [en]

    Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (JSC) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C-70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in J(SC), elevating the power conversion efficiency by a factor of 1.2 to more than 4%.

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  • 204. Gunnella, R.
    et al.
    Yeom, H. W.
    Bullock, E. L.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Kono, S.
    Solal, F.
    Study of Si 2p core-level shift at the As/Si(001)-2×1 surface2002In: Surface Science 499, 244 (2002)Article in journal (Refereed)
    Abstract [en]

    The atomic origin of the photoemission Si 2p core level, shifted at 0.35 eV higher binding energy, at the As/Si(0 0 1)-2×1 surface, is investigated. The study is based on the determination of the surface structure by means of multiple scattering analysis of As 3d azimuth photoelectron diffraction (PD) patterns. The obtained structure is used to assign the atomic origin of the Si 2p component by surface core level shift PD. We find that a single atom contributes to the core level, while, on the basis of final state calculations, a contribution from two different atoms would be expected

  • 205. Gurnett, Michael
    et al.
    Gustafsson, Jörgen
    Holleboom, Thijs Jan
    Karlstad University, Faculty of Economic Sciences, Communication and IT, Department of Computer Science. Karlstad University, Faculty of Economic Sciences, Communication and IT, Centre for HumanIT.
    Magnusson, Kjell
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Widstrand, Susanna
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Johansson, M.K.-J.
    Gray, S.M.
    Core-level spectroscopy study of the Li/Si(111)-3x1, Na/Si(111)-3x1 and K/Si(111)-3x1 surfaces2005In: Phys. Rev. B 71, 195408 (2005)Article in journal (Refereed)
  • 206. Gurnett, Michael
    et al.
    Gustafsson, Jörgen
    Magnusson, Kjell
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Widstrand, Susanna
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Angle-resolved photoemission study of the single-domain Si(111) 3×1/6×1 -Ag surface2002In: Phys. Rev. B 66, 161101 (R) (2002)Article in journal (Refereed)
  • 207.
    Gurnett, Michael
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Holleboom, Thijs
    Karlstad University, Faculty of Economic Sciences, Communication and IT, Department of Computer Science.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Photoemission study of the Li/Ge(111)-3×1 reconstruction2009In: Surface Science, ISSN 0039-6028, E-ISSN 1879-2758, ISSN 0039-6028, Vol. 603, no 4, p. 727-735Article in journal (Refereed)
    Abstract [en]

    In this article we report our findings on the electronic structure of the Li induced Ge(1 1 1)–3 × 1reconstruction as determined by angle-resolved ultraviolet photoelectron spectroscopy (ARUPS) and core-level spectroscopy using synchrotron radiation. The results are compared to the theoretical honeycomb-chain-channel (HCC) model for the 3 × 1reconstruction as calculated using density functional theory (DFT). ARUPS measurements were performed in both the and directions of the 1 × 1 surface Brillouin zone at photon energies of 17 and 21.2 eV. Three surface related states were observed in the direction. In the direction, at least two surface states were seen. The calculated band structure using the single-domain HCC model for Li/Ge(1 1 1)–3 × 1 was in good agreement with experiment, allowing for the determination of the origin of the experimentally observed surface states. In the Ge core-level spectra, two surface related components were identified, both at lower binding energy with respect to the Ge bulk peak. Our DFT calculations of the surface core-level shifts were found to be in fair agreement with the experimental results. Finally, in contrast to the Li/Si(1 1 1)–3 × 1 case, no double bond between Ge atoms in the top layer was found

  • 208. Gustafsson, Jörgen
    et al.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Widstrand, Susanna
    Gurnett, Michael
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Thin PTCDA Films on Si(001) II: Electronic Structure2004In: Surface Science 572, 32 (2004)Article in journal (Refereed)
    Abstract [en]

    We have studied the thin film formation and the electronic structure of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA), on clean and on hydrogen-passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), angle-resolved photoelectron spectroscopy (ARPES), near edge X-ray absorption fine structure (NEXAFS) and low energy electron diffraction (LEED). On the H passivated surface the changes in the electronic structure of the substrate and the molecules with increasing film thickness are very small. The molecular orbitals show a dispersive behavior, indicating that the PTCDA layers are ordered. On the reactive clean surface the anhydride groups of the molecule interact with the substrate as indicated by changes in the core level binding energies. This results in a much lower ordering in the film compared to PTCDA on a passivated silicon surface. There is no sign of decomposition of the molecule because of the more reactive substrate

  • 209. Gustafsson, Jörgen
    et al.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Widstrand, Susanna
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Thin PTCDA Films on Si(001) I: Growth Mode2004In: Surface Science 572, 23 (2004)Article in journal (Refereed)
    Abstract [en]

    We have studied the interface and thin film formation of the organic molecular semiconductor 3,4,9,10 perylene tetracarboxylic dianhydride (PTCDA) on clean and on hydrogen passivated Si(0 0 1) surfaces. The studies were made by means of high resolution X-ray photoelectron spectroscopy (HRXPS), near edge X-ray absorption fine structure (NEXAFS), low energy electron diffraction (LEED), and atomic force microscopy (AFM). On the passivated surface the LEED pattern is somewhat diffuse but reveals that the molecules grow in several ordered domains with equivalent orientations to the substrate. NEXAFS shows that the molecules are lying flat on the substrate. The Si 2p XPS line shape is not affected when the film is deposited so it can be concluded that the interaction at the interface between PTCDA and the substrate is weak. The evolution of the film formation appears to be homogeneous for the first monolayer with a nearly complete coverage of flat lying molecules based on the XPS attenuation. For layer thickness of 0.52 monolayers (ML) the molecules start to form islands, attracting the molecules in between, leaving the substrate partly uncovered. For thicker films there is a StranskiKrastanov growth mode with thick islands and a monolayer thick film in between. For the clean surface the ordering of the film is much lower and angle resolved photoelectron spectroscopy (ARPES) of the molecular orbitals have only a small dependence of the emission angle. NEXAFS shows that the molecules do not lie flat on the surface and also reveal a chemical interaction at the interface

  • 210. Gustafsson, Jörgen
    et al.
    Widstrand, Susanna
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Growth and characterization of thin PTCDA films on 3C-SiC(001)c(2x2)2006Article in journal (Refereed)
  • 211.
    Gustafsson, Jörgen
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    STM studies of thin PTCDA films on Ag/Si(111) )-sqrt3×sqrt32007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, p. 155414-1-155414-7Article in journal (Refereed)
  • 212. Gustafsson, Jörgen
    et al.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Electron spectroscopy studies of PTCDA on Ag/Si(111) )- √3 × √32007In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 75, no 15, p. 155413-Article in journal (Refereed)
  • 213.
    Gåård, Anders
    et al.
    Karlstad University, Faculty of Technology and Science, Materials Science. Karlstad University, Faculty of Technology and Science, Department of Mechanical and Materials Engineering.
    Hirvonen Grytzelius, Joakim
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Krakhmalev, Pavel
    Karlstad University, Faculty of Technology and Science, Materials Science. Karlstad University, Faculty of Technology and Science, Department of Mechanical and Materials Engineering.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Bergström, Jens
    Karlstad University, Faculty of Technology and Science, Department of Mechanical and Materials Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Experimental study of the relationship between temperature and adhesive forces for low-alloyed steel, stainless steel and titanium using atomic force microscopy in ultra-high vacuum2008In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, ISSN 0021-8979, Vol. 103, no 12, article id 124301Article in journal (Refereed)
  • 214.
    Hallström, Viktor
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Reläskyddsförslag till flygplatser och sjukhus: Reläskyddsförslag till flygplatser och sjukhus där driftsäkerhet är viktig2019Independent thesis Basic level (university diploma), 15 credits / 22,5 HE creditsStudent thesis
    Abstract [sv]

    Sammanfattning

    Detta examensarbete har gjorts i samarbete med Pontarius AB. Nätet som arbetet handlar om är ett redundant matande nät till sjukhus eller flygplatser där driftsäkerheten är extra viktig. Uppbyggnaden av näten till de båda ser liknande ut, det är mestadels lasten som skiljer dem åt.  Redundans behövs för att uppnå en hög driftsäkerhet vilket tas upp i rapporten. Målsättningen har varit att sammanställa en lathund på lämpliga reläskydd vid olika positioner i nätet.

    Data från matande nät har tagits från Jönköpings flygplats och utifrån det byggt ett fiktivt system i ett program som heter Easypower. I programmet kan det väljas specifika komponenter av många fabrikat. Där ingår reläskydd, brytare, transformatorer, generatorer och kablar.

    Arbetet beskriver grunderna i kortslutningsberäkningar samt reläskyddsinställningar. Kortslutningsberäkningarna är gjorda efter svensk elstandard SS-EN 60909-0. Beräkningar på största och lägsta kortslutningsström har gjorts i vissa delar av nätet, utifrån det har reläskyddsinställningarna baserats. Dessa inställningar beskrivs med hjälp av en selektivplan i resultatdelen av rapporten.

    Resultat och diskussion presenterar en lathund med rekommenderade reläskydd för systemet samt inställningar för dessa via selektivplanen. Det fanns dock vissa betydelsefulla begränsningar i Easypower som gör att selektivplan samt val av bl.a. reläskydd blir lidande.

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  • 215.
    Hamanee, Sahaphol
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Simuleringsbaserad analys av toppeffektreducering med batterisystem i lokalnät2019Independent thesis Basic level (university diploma), 15 credits / 22,5 HE creditsStudent thesis
    Abstract [en]

    In this thesis, a simulation model developed in MATLAB® in consideration of system losses based on lithium ion-battery is presented. The purpose of the simulation model is to investigate peak shaving potential in the residential distribution network. In other word to determine an optimal threshold limit and battery capacity depending on if the battery system is placed at the transformer or household level. In the report there were economic calculations executed showing that profitability of investing in a battery system depends on the threshold limit and battery capacity.

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  • 216.
    Hassan, Ahmad
    et al.
    Karlstad University, Faculty of Economic Sciences, Communication and IT. Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Hoque, Ahsanul
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Automated Microwave Antenna Alignment of Base Transceiver Station2011Independent thesis Advanced level (degree of Master (One Year)), 10 credits / 15 HE creditsStudent thesis
    Abstract [en]

    This master’s thesis is intended for the solution of Microwave (MW) link alignment which has several applications in modern communication systems. MW communication is used for point to point links with small wavelength and simultaneously compact antenna dimensions. Assuring the automated alignment of antennas provides a better communication between switching stations and allow its subscribers to communicate ceaselessly. In the first part of the work; effects of downtime on a network are discussed and at later stage a mechanism is investigated which will reduce the downtime. MW antenna used in communication systems is regularly affected by environmental factor and generally by wear and tear of mounts. These factors cause the precisely directed antennas to be perturbed from respective main lobes. Larger antennas with narrow main lobes require more sophisticated designing while smaller antennas have a broader main lobe. An automated antenna alignment that works without human intervention can decrease the down time and ensure a reliable communication between the near end and far end terminals of the Base Transceiver Station (BTS). The focus has been laid on a system that works autonomously and for that LabVIEW design and RCX Robonics Kit is used.  Results are presented to indicate antenna alignment in automatically synchronized manner.

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  • 217. Hassel, M.
    et al.
    Svensson, Krister
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Andersson, S.
    Persson, M.
    Direct infrared photodesorption of physisorbed H_{2}1998In: Physical Review Letters, Vol. 80, no 11, article id 2481Article in journal (Refereed)
  • 218. Hassel, M.
    et al.
    Svensson, Krister
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Bellman, J.
    Andersson, S.
    Persson, M.
    Photodesorption of Physisorbed hydrogen molecules2002In: Physical Review B, Vol. 65, no 20, article id 205402Article in journal (Refereed)
  • 219. Hedin, A.
    et al.
    Fenyö, D.
    Håkansson, P.
    Jonsson, Gunnar
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Sundqvist, B.
    Electronic Sputtering of Biomolecules1989Article in journal (Refereed)
  • 220.
    Heidkamp, Hannah
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Carlsson, Gunilla
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    van Stam, Jan
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Polymer film formation studied with fluorescence microscopy and AFM2010In: Molecular Processes at Solid Surfaces: 10th Annual Surface and Colloid Symposium, 2010, p. 49-Conference paper (Refereed)
    Abstract [en]

    Thin polymer films are used in many applications such as paint, paper coatings and electronic devices. For these applications, it is important to have knowledge about the film formation process, since it affect the film morphology and the morphology is important for the applications.One method for studying film formation in situ is fluorescence microscopy. By labeling a target molecule or particle with a fluorophore, the targets movements can be traced as the solvent evaporates [1-3]. If information gained from studies of particle movements during film formation and wet state behavior are combined, information about the film formation process can be obtained. Examination of the final film surfaces with regular light microscopy and AFM gives additional information about the film formation.These methods have been used for studying the formation of negatively charged latex films. It was shown that the films are greatly affected by adding positively charged surfactants [4-5]. Since latex is a water-based system it has relatively long drying times. Systems based on high-vapor pressure organic solvents have much shorter drying times and the film formation occurs under non-equilibrium conditions. This results in incomplete phase separation, which in turn gives microstructures in the film. These microstructures are of great interest since they affect the properties of the film and its function [6-7]. Our aim is to develop the methods used for latex studies in order to be able to apply them to study film formation of polymer blends used for photovoltaic applications. The goal is to get more knowledge about the film forming process and a deeper understanding about the mechanisms behind the formation of microstructures.[1] Carlsson G., Warszynski P., van Stam J., J. Colloid Interface Sci., 2003, 267, 500-508[2] Carlsson G., van Stam J., Nord. Pulp Pap. Res. J., 2005, 20, 192-199[3] Carlsson G., Järnström L., van Stam J., J. Colloid Interface Sci., 2006, 298, 162-171[4] Heidkamp H., Master thesis, Karlstad University 2009.[5] Paakkonen, J., Master thesis, Karlstad University 2010.[6] Björström C.M., Magnusson K.O., Moons E., Synth. Metals, 2005, 152, 109-112[7] Moons E., J. Phys.: Condens. Matter, 2002, 14, 12235-12260

  • 221.
    Heidkamp, Hannah
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Rogowski, Rafal
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Dzwilewski, Andrzej
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    van Stam, Jan
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Carlsson, Gunilla
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Morphology of polymer blends in films made by dip-coating2011Conference paper (Refereed)
    Abstract [en]

    Thin spincoated polymer films are used in various applications and there has been anincreasing demand to understand and get precise control over the film formation process. One of the most exciting applications is organic solar cells which have an active layer made of a polymer based blend. The film morphology has a strong effect on the efficiency of solar cells and therefore it is crucial to understand the film formation process in order to tailor thedesired morphology [1].

    In this study we are combining and comparing results from three different deposition processes: drop-casting, sphere-on-flat arrangement and dip-coating. We are using dip-coating to produce thin films of polymer blends with different morphologies under controlled conditions. The main goal is to gain a deeper insight into the processes that occur while solvent evaporates and to understand why certain structures are formed.

    Drop-casting allows for little control of the structure formation. In the sphere-on-flat arrangement a droplet of a solution is constrained between a half-sphere and the substrate, which provides more controllable conditions for the deposition process. For more precise control, dip-coating can be used, where a substrate is withdrawn from a solution at a constant speed.

    In this study we have used the polymer poly(3-hexylthiophene) (P3HT) and the fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) dissolved in toluene. These components are the model system for studies on organic solar cells [1]. The dip-coated films show a wide variety of morphologies depending on the coating speed. This dependence can be rationalized by the different mechanisms occurring at low and high speeds: At low speeds, evaporation is dominant, [2] resulting in well ordered patterns. At high speeds, viscous forces become dominant, [2] yielding optically homogeneous films.

    [1] G. Dennler, M. C. Scharber, C. J. Brabec, Adv. Mat. 21, 1323-1338 (2009)

    [2] R. Z. Rogowski and A. A. Darhuber, Langmuir 26, 11485-93 (2010)

  • 222.
    Heidkamp, Hannah
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    van Stam, Jan
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Carlsson, Gunilla
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Dzwilewski, Andrzej
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Rogowski, Rafal
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Morphology of P3HT and PCBM blends in thin films obtained with different deposition methods2011Conference paper (Refereed)
    Abstract [en]

    Patterns and structures, formed when a semiconducting polymer blend in solution is subject to controlled evaporation, have been of great interest due to their influence on the performance of organic devices. By controlling the processes of pattern formation, function properties of organic semiconductor structures can be tailored, allowing for facile manufacturing of the active layers in organic devices, e.g. solar cells.

    By analyzing the morphologies of polymer blends resulting from different deposition methods, a deeper insight into the pattern formation process can be acquired. In this study, we have analyzed the morphology of blends of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) formed upon solvent evaporation. We used the following deposition methods: dip-coating, droplet evaporation within a constrained geometry and drop-casting. Dip-coated films revealed various types of morphology depending on the coating speed. At low coating speeds, where evaporation is the dominant factor, well-ordered patterns were obtained. When increasing the coating speed, viscous forces become dominant over evaporation yielding optically homogenous films [2]. Morphologically similar structures to those observed at low coating speeds, were also obtained with spatially constrained droplets. The blend morphologies were analyzed with polarized, fluorescence and atomic force microscopy [1].

    References:

    [1] C. M. Björström Svanström, J. Rysz, A. Bernasik, A. Budkowski, F. Zhang, O. Inganäs, M. R. Andersson, K. O. Magnusson, J. J. Benson-Smith, J. Nelson, and E. Moons, Adv. Mat. 21, 4398-4403 (2009)

    [2] R. Z. Rogowski and A. A. Darhuber, Langmuir 26, 11485-93 (2010)

  • 223.
    Heidkamp, Hannah
    et al.
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
    van Stam, Jan
    Karlstad University, Faculty of Technology and Science, Department of Chemistry and Biomedical Sciences.
    Carlsson, Gunilla
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
    Moons, Ellen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Rogowski, Rafal
    Karlstad University, Faculty of Health, Science and Technology (starting 2013), Department of Engineering and Chemical Sciences.
    Morphology of PCBM and P3HT blends in films made by dip-coating on homogeneous and chemically patterned surfaces,2011Conference paper (Refereed)
  • 224. Heinzle, J. Mark
    et al.
    Uggla, Claes
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Mixmaster: Fact and Belief2009In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 26, no 075016, p. 1-34Article in journal (Refereed)
    Abstract [en]

    We consider the dynamics towards the initial singularity of Bianchi type IX vacuum and orthogonal perfect fluid models with a linear equation of state. Surprisingly few facts are known about the `Mixmaster' dynamics of these models, while at the same time most of the commonly held beliefs are rather vague. In this paper, we use Mixmaster facts as a base to build an infrastructure that makes it possible to sharpen the main Mixmaster beliefs. We formulate explicit conjectures concerning (i) the past asymptotic states of type IX solutions and (ii) the relevance of the Mixmaster/Kasner map for generic past asymptotic dynamics. The evidence for the conjectures is based on a study of the stochastic properties of this map in conjunction with dynamical systems techniques. We use a dynamical systems formulation, since this approach has so far been the only successful path to obtain theorems, but we also make comparisons with the `metric' and Hamiltonian `billiard' approaches

  • 225. Heinzle, J Mark
    et al.
    Uggla, Claes
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Monotonic functions in Bianchi models: why they exist and how to find them2010In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 27, no 1Article in journal (Refereed)
    Abstract [en]

    All rigorous and detailed dynamical results in Bianchi cosmology rest upon the existence of a hierarchical structure of conserved quantities and monotonic functions. In this paper we uncover the underlying general mechanism and derive this hierarchical structure from the scale-automorphism group for an illustrative example, vacuum and diagonal class A perfect fluid models. First, kinematically, the scale-automorphism group leads to a reduced dynamical system that consists of a hierarchy of scale-automorphism invariant sets. Second, we show that, dynamically, the scale-automorphism group results in scale-automorphism invariant monotone functions and conserved quantities that restrict the flow of the reduced dynamical system

  • 226.
    Heinzle, Mark
    et al.
    University of Vienna, Faculty of Physics, Gravitational Physics, Austria.
    Sandin, Patrik
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    The Initial Singularity of Ultrastiff Perfect Fluid Spacetimes Without Symmetries2012In: Communications in Mathematical Physics, ISSN 0010-3616, E-ISSN 1432-0916, no 313, p. 385-403Article in journal (Refereed)
    Abstract [en]

    We consider the Einstein equations coupled to an ultrastiff perfect fluid and prove the existence of a family of solutions with an initial singularity whose structure is that of explicit isotropic models. This family of solutions is ‘generic’ in the sense that it depends on as many free functions as a general solution, i.e., without imposing any symmetry assumptions, of the Einstein-Euler equations. The method we use is a that of a Fuchsian reduction.

  • 227.
    Heinzle, Mark
    et al.
    University of Vienna, Austria.
    Uggla, Claes
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Lim, Woei Chet
    University of Waikato, New Zealand.
    Spike oscillations2012In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 86, p. 104049-104075Article in journal (Refereed)
    Abstract [en]

    According to Belinskii, Khalatnikov and Lifshitz (BKL), a generic spacelike singularity is characterized by asymptotic locality: Asymptotically, toward the singularity, each spatial point evolves independently from its neighbors, in an oscillatory manner that is represented by a sequence of Bianchi type I and II vacuum models. Recent investigations support a modified conjecture: The formation of spatial structures (`spikes') breaks asymptotic locality. The complete description of a generic spacelike singularity involves spike oscillations, which are described by sequences of Bianchi type I and certain inhomogeneous vacuum models. In this paper we describe how BKL and spike oscillations arise from concatenations of exact solutions in a Hubble-normalized state space setting, suggesting the existence of hidden symmetries and showing that the results of BKL are part of a greater picture

  • 228. Heister, K.
    et al.
    Rong, H.-T.
    Buck, M.
    Zharnikov, M.
    Grunze, M.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Odd-even efffects at the S-metal interface and in the aromatic matrix in biphenyl-substituted alkanethiol self-assembled monolayers2001In: Journal of Physical Chemistry B 105, 6888 (2001)Article in journal (Refereed)
    Abstract [en]

    Synchrotron-based high-resolution X-ray photoelectron spectroscopy was applied to characterize self-assembled monolayers (SAM) of biphenyl-substituted alkanethiols CH3(C6H4)2(CH2)nSH (BPn, n = 1-4) on Au and Ag substrates. Beyond previously identified odd-even changes in the packing density and the tilt angle of the biphenyl moieties, the high-resolution spectra reveal a number of additional odd-even effects upon variation of the number of methylene groups in the aliphatic part in the BPn molecule. Their occurrence and mutual correlation suggests that a BPn SAM represents a strongly correlated, highly ordered molecular assembly. In particular, periodical changes of a shake up feature in the C 1s region are observed, which are related to the differences in the arrangement of the aromatic matrix. The width and binding energy position of the S 2p signals also exhibit odd-even changes. The width changes are associated with the occupation of either equivalent or nonequivalent adsorption sites on the polycrystalline (111) Au and Ag substrates. The comparison of the width values with those for conventional alkanethiols implies that the substrate bonding of alkanethiols on gold cannot be described by a single adsorption site

  • 229. Heister, K.
    et al.
    Zharnikov, M.
    Grunze, M.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Adsorption of alkanethiols and biphenylthiols on Au and Ag substrates: A high resolution X-ray photoelectron spectroscopy study2001In: Journal of Physical Chemistry B 105, 4058 (2001)Article in journal (Refereed)
    Abstract [en]

    Synchrotron-based high-resolution X-ray photoelectron spectroscopy was applied to monitor the formation of self-assembled monolayers (SAM) of alkanethiols (AT) and biphenylthiols on Au and Ag substrates. Pronounced chemical shifts in the adsorbate- and substrate-related photoemission lines upon SAM formation were observed. Only one sulfur species could be detected in the S 2p spectra of the investigated SAMs, consistent with a thiolate bond. From the fwhm's of the core level photoemission spectra conclusions on the heterogeneity of the adsorption sites and adsorption geometry can be made. The experimental data imply several (at least two) slightly different adsorption geometries for the AT moieties in AT/Au. Significant final state effects in the C 1s photoemission were found for both the aliphatic and aromatic SAMs

  • 230. Heister, K.
    et al.
    Zharnikov, M.
    Grunze, M.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Ulman, A.
    Characterization of X-ray induced damage in alkanethiolate monolayers by high resolution photoelectron spectroscopy,2001In: Langmuir 17, 8 (2001)Article in journal (Refereed)
    Abstract [en]

    Synchrotron-based high-resolution X-ray photoelectron spectroscopy was for the first time applied to investigate the damage in self-assembled monolayers (SAMs) of alkanethiols (AT) on Au caused by soft X-rays. The observed changes in AT SAMs and, in particular, the appearance of a new, irradiation-induced sulfur species are identical to those caused by electron bombardment, implying that most of the damage is produced by the photoelectrons and secondary electrons. The irradiation-induced sulfur species is identified as a dialkyl sulfide distributed within the AT film. Only minutes of monochromatized X-ray irradiation at an undulator beamline destroys the AT adlayer completely.

  • 231. Heister, Karin
    et al.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Grunze, Michael
    Zharnikov, Michael
    A detailed Analysis of the C 1s Photoemission of n-Alkanethiolate Films on Noble Metal Substrates2003In: Surface Science, 529, 36 (2003)Article in journal (Refereed)
    Abstract [en]

    High-resolution C 1s X-ray photoemission spectra for the series of n-alkanethiolate (AT) self-assembled monolayers (SAMs) with different lengths of the alkyl chain have been acquired and analyzed in terms of initial and final state effects and fine structure of the C 1s emission. Independent of the alkyl chain length, the binding energy position of this emission was found to be higher by ?0.35 eV for AT SAMs on Ag substrate as compared to Au, which was attributed to the differences in the final state screening mediated by the intermolecular charge transfer. It was assumed that the extent of this effect is a function of the detailed molecular arrangement in the alkyl matrix and demonstrated that the difference in the C 1s peak position disappears as soon as this arrangement is distorted by the incorporation of sulfone moiety in the alkyl chain. In addition, a change of the C 1s peak shape from a symmetric to asymmetric line with increasing alkyl chain length has been observed. This development could be described assuming the existence of a fine structure in the C 1s emission, with two low-intensity components at the higher and lower BE sides of the main emission peak. An assignment of these components has been considered

  • 232.
    Hellman, Anders
    et al.
    Chalmers Tekniska Högskola.
    Svensson, Krister
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Andersson, Stig
    Göteborgs Universitet.
    Hydrogen-Induced Reconstruction of Cu(100): Two-Dimensional and One-Dimensional Structures of Surface Hydride2014In: The Journal of Physical Chemistry C, ISSN 1932-7447, E-ISSN 1932-7455, Vol. 118, article id 15773Article in journal (Refereed)
    Abstract [en]

    Adsorption of atomic hydrogen has a remarkable influence on the structure and morphology of the Cu(100) surface. We have examined two specific situations; the well-known reconstructed p(2 × 2)-H surface and a one-dimensional (6 × 1)-H surface resembling the stripe reconstruction observed in an electrochemical environment. High resolution electron energy loss spectroscopy and density functional calculations show that the H atoms occupy only surface sites and that H bonding in induced 3-fold Cu surface hollow sites is a structural key element. The calculations support a transient scenario in which H absorption in subsurface sites is a critical intermediate step in the reconstruction of the Cu surface. We find clear evidence in terms of characteristic properties, like the H–Cu bond lengths and the dipole excited vibrational mode spectra, that these structures consist of two-dimensional and one-dimensional structures of surface hydride.

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  • 233.
    Hervik, Sigbjørn
    et al.
    Faculty of Science and Technology, University of Stavanger, Norway.
    Lim, Woei Chet
    Albert-Einstein-Institut, Am-Mûhlenberg, Germany.
    Sandin, Patrik
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Uggla, Claes
    Karlstad University, Faculty of Technology and Science.
    Future Asymptotics of Tilted Bianchi Type II Cosmologies2010In: Classical and quantum gravity, ISSN 0264-9381, E-ISSN 1361-6382, Vol. 27, p. 185006-Article in journal (Refereed)
  • 234.
    Hirvonen Grytzelius, Joakim
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Thin Mn silicide and germanide layers studied by photoemission and STM2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The research presented in this thesis concerns experimental studies of thin manganese silicide and germanide layers, grown by solid phase epitaxy on the Si(111)7×7 and the Ge(111)c(2×8) surfaces, respectively. The atomic and electronic structures, as well as growth modes of the epitaxial Mn-Si and Mn-Ge layers, were investigated by low-energy electron diffraction (LEED), angle-resolved photoelectron spectroscopy (ARPES), core-level spectroscopy (CLS), and scanning tunneling microscopy and spectroscopy (STM and STS). The magnetic properties of the Mn-Ge films were investigated by X-ray magnetic circular dichroism (XMCD).

    The Mn-Si layers, annealed at 400 °C, showed a √3×√3 LEED pattern, consistent with the formation of the stoichiometric monosilicide MnSi. Up to 4 monolayers (ML) of Mn coverage, island formation was observed. For higher Mn coverages, uniform film growth was found. Our results concerning morphology and the atomic and electronic structure of the Mn/Si(111)-√3×√3 surface, are in good agreement with a recent theoretical model for a layered MnSi structure and the √3×√3 surface structure.

    Similar to the Mn-Si case, the grown Mn-Ge films, annealed at 330 °C and 450 °C, showed a √3×√3 LEED pattern. This indicated the formation of the ordered Mn5Ge3 germanide. A strong tendency to island formation was observed for the Mn5Ge3 films, and a Mn coverage of about 32 ML was needed to obtain a continuous film. Our STM and CLS results are in good agreement with the established model for the bulk Mn5Ge3 germanide, with a surface termination of Mn atoms arranged in a honeycomb pattern. Mn-Ge films grown at a lower annealing temperature, 260 °C, showed a continuous film at lower coverages, with a film structure that is different compared to the structure of the Mn5Ge3 film. XMCD studies showed that the low-temperature films are ferromagnetic for 16 ML Mn coverage and above, with a Curie temperature of ~250 K.

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  • 235.
    Hirvonen Grytzelius, Joakim
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Core-level spectroscopy studies of Mn/Si(111)-sqrt3xsqrt32008Conference paper (Refereed)
  • 236.
    Hirvonen Grytzelius, Joakim
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Coverage dependence and surface atomic structure of Mn/Si(111)-√3×√3 studied by scanning tunneling microscopy and spectroscopy2009In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 80, p. 235324-1-235324-6Article in journal (Refereed)
    Abstract [en]

    Thin manganese silicide films of different thicknesses on Si(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and scanning tunneling spectroscopy (STM/STS). Up to a Mn coverage of 3–4 monolayers (ML), island formation is favored. For higher Mn coverages up to 12 ML uniform film growth is found. The silicide film morphology at low coverages supports a layered Mn-Si film structure. The silicide surfaces displayed a √3×√3 LEED pattern. STM images recorded from the √3×√3 surfaces mostly show a hexagonal pattern but a honeycomb pattern has also been observed. A surface atomic structure based on chained Mn triangles is proposed. Our STM results are in good agreement with a recent theoretical model. The high-quality STS spectra recorded from the different surfaces show a clear metallic character at 1.5 ML and higher coverages. The filled-state features in the STS spectra at surfaces with 3–4 ML Mn coverages are similar to earlier published angle-resolved photoelectron spectroscopy data.

  • 237.
    Hirvonen Grytzelius, Joakim
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Electronic structure of Mn/Si(111)-sqrt3xsqrt32007Conference paper (Refereed)
  • 238.
    Hirvonen Grytzelius, Joakim
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering. Karlstad University, Faculty of Technology and Science, Materials Science.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Mn5Ge3 film formation on Ge(111)c(2×8)2012In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 86, no 12, p. 125313-1-125313-7Article in journal (Refereed)
    Abstract [en]

    Thin manganese germanide films with different thicknesses on Ge(111) have been studied in detail by low-energy electron diffraction (LEED), scanning tunneling microscopy, and core-level spectroscopy (CLS). Annealing of the deposited Mn on Ge(111)c(2×8) between 330-450 C resulted in well-ordered Mn5Ge3 surfaces as seen by intense 3×3 LEED spots. Up to a coverage of 24 monolayers (ML), island formation is favored. At a coverage of 32 ML a well ordered Mn5Ge3 film was found to fully cover the surface. High-resolution Ge 3d CLS spectra were recorded with photon energies between 50 and 110 eV at normal and 60 emission angles. In contrast to earlier results, three components have been used in the line-shape analysis to achieve a consistent fit over the energy and angular range. In addition, three components have been identified for the Mn 2p CLS spectra. The two major components fit well with a layered Mn germanide structure suggested in the literature.

  • 239.
    Hirvonen Grytzelius, Joakim
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Surface atomic and electronic structure of Mn5Ge3 on Ge(111)2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, ISSN 1098-0121, Vol. 84, no 19, p. 195306-1-195306-6Article in journal (Refereed)
    Abstract [en]

    The atomic and electronic structure of the Mn5Ge3(001) surface grown on Ge(111) c(2×8) has been studied in detail by angle-resolved photoelectron spectroscopy (ARPES), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy. ARPES spectra recorded from the Γ̅ -K̅ -M̅ and Γ̅ -M̅ -Γ̅ directions of the surface Brillouin zone show six surface-related features. The STM images recorded at biases higher/lower than ±0.4 V always show a honeycomb pattern with two bright protrusions in each unit cell. At lower biases, a hexagonal, intermediate transition, and a honeycomb pattern are observed. These can be explained as arising from Mn and Ge atoms in the sublayer arranged in triangular structures and Mn atoms in the top layer arranged in a honeycomb structure, respectively. The photoemission and STM data from the germanide surface are discussed and compared to earlier published theoretical, photoelectron spectroscopy, and scanning tunneling microscopy studies.

  • 240.
    Hirvonen Grytzelius, Joakim
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Zhang, Hanmin
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Johansson, Lars
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Surface electronic structure of Mn/Si(111)-√3×√32008In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 78, no 15, p. 155406-1-155406-6Article in journal (Refereed)
    Abstract [en]

    The Mn/Si(111)-√3×√3 surface has been studied in detail by low energy electron diffraction (LEED), angle-resolved photoelectron spectroscopy (ARPES), and core-level photoelectron spectroscopy (CLS). Annealing of the deposited manganese resulted in a well-ordered surface as seen by intense √3×√3 LEED spots. ARPES spectra recorded in the Γ̅ -K̅ -M̅ direction of the √3×√3 surface Brillouin zone show five surface related features in the band gap while in the Γ̅ -M̅ -Γ̅ direction four surface features are observed. The high-resolution Si 2p CLS data were recorded at photon energies between 108–140 eV both at normal and 60° emission angle. The bulk component was identified from the bulk sensitive spectrum recorded at a photon energy of 108 eV. To achieve a consistent core-level fitting over the whole energy and angular range, five components were introduced in the line-shape analysis. The photoemission data from the √3×√3 surface have been discussed and compared with a recent theoretical model. The findings here support a layered Mn silicide film structure.

  • 241.
    Hoagg, Jesse
    et al.
    Dept. of Mech. Eng., Univ. of Kentucky, Lexington, KY, USA.
    Ali, Asad
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Mossberg, Magnus
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Bernstein, D.S
    Sliding window recursive quadratic optimization with variable regularization2011In: American Control Conference (ACC), 2011, San Francisco: IEEE conference proceedings, 2011, p. 3275-3280Conference paper (Refereed)
    Abstract [en]

    n this paper, we present a sliding-window variable-regularization recursive least squares algorithm. In contrast to standard recursive least squares, the algorithm presented in this paper operates on a finite window of data, where old data are discarded as new data become available. This property can be beneficial for estimating time-varying parameters. Furthermore, standard recursive least squares uses time-invariant regularization. More specifically, the inverse of the initial covariance matrix in standard recursive least squares can be viewed as a regularization term, which weights the difference between the next estimate and the initial estimate. This regularization is fixed for all steps of the recursion. The algorithm derived in this paper allows for time-varying regularization. In particular, the present paper allows for time varying regularization in the weighting as well as what is being weighted. Specifically, the regularization term can weight the difference between the next estimate and a time-varying vector of parameters rather than the initial estimate.

  • 242.
    Hwang, Stephen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Semi-infinite cohomology of affine Lie algebras1998In: Commun. Math.Phys. 194, 591-611 (1998)Article in journal (Refereed)
  • 243.
    Hwang, Stephen
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Unitarity of Strings and Non-compact Hermitian Symmetric spaces1998In: Physics Letters B435, 331-336 (1998)Article in journal (Refereed)
  • 244.
    Hwang, Stephen
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Fjelstad, Jens
    Equivalence of Chern-Simons gauge theory and the WZNWmodel using a BRST symmetry1999In: Physics Letters B 466 (1999) 227-233Article in journal (Refereed)
  • 245.
    Hwang, Stephen
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Liu, Yang
    Zhang, Genkai
    Hilbert spaces of tensor-valued holomorphic functions on the unit ball of C^n2004In: Pacific J. of Math 214 (2004)Article in journal (Refereed)
  • 246.
    Hwang, Stephen
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Marnelius, Robert
    Saltsidis, Panagiotis
    A general BRST approach to string theories with zeta function regularizations1999In: Journal of Mathematical Physics 40 (1999) 4639-57Article in journal (Refereed)
    Abstract [en]

    Within the operator version of BRST quantization one may proceed as follows: First define a hermitian and formally nilpotent BRST charge. Then look for possible state spaces which yield nontrivial cohomologies. Two of us have previously used this procedure for finite degrees of freedom. In the case of infinite degrees of freedom the procedure requires a precise regularization.

    We propose here a general precise regularization of the basic commutators which makes the divergent sums we encounter to be expressed in terms of Hurwitz zeta function.

    A new feature is that the formally nilpotent BRST operator need not be nilpotent on the found state space. A modification might be necessary.

    The method is applied to three string models: The ordinary bosonic string, the tensionless string and the conformal tensionless string. In the first two models we recover old results, but we believe our treatment further deepens the understanding of the BRST formulation. For the conformal tensionless string model our result is new. Here we show that there is no critical dimension

  • 247.
    Irshad, Yasir
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Some estimation concepts applied to networked control systemsManuscript (preprint) (Other academic)
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  • 248.
    Irshad, Yasir
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Some problems of modeling and parameter estimation in continous-time for control and communication2011Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    Stochastic system identification is of great interest in the areas of control and communication. In stochastic system identification, a model of a dynamic system is determined based on given inputs and received outputs from the system, where stochastic uncertainties are also involved. The scope of the report is to consider continuous-time models used within control and communication and to estimate the model parameters from sampled data with high accuracy in a computational efficient way. Continuous-time models of systems controlled in a networked environment, stochastic closed-loop systems, and wireless channels are considered. The parameters of a transfer function based model for the process in a networked control system are first estimated by a covariance function based approach, relying upon the second order statistical properties of the output signal. Some other approaches for estimating the parameters of continuous-time models for processes in networked environments are also considered. Further, the parameters of continuous-time autoregressive exogenous models are estimated from closed-loop filtered data, where the controllers in the closed-loop are of proportional and proportional integral type, and where the closed-loop also contains a time-delay. Moreover, a stochastic differential equation is derived for Jakes's wireless channel model, describing the dynamics of a scattered electric field with the moving receiver incorporating a Doppler shift.

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    FULLTEXT02
  • 249.
    Irshad, Yasir
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Mossberg, Magnus
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    A comparison of estimation concepts applied to networked control systems2012In: 19th International Conference on Systems, Signals and Image Processing (IWSSIP), 2012, Piscataway: IEEE Press, 2012, p. 114-117Conference paper (Refereed)
    Abstract [en]

    A continuous-time description of networked control systems is considered and the parameters are estimated. The discrete-time description is time-varying due to the random time-delays in the wireless links and therefore difficult to work with. Off-line as well as on-line situations are considered for parameter estimation. In the off-line situation, a linear regression is formed and then the parameters are estimated by the least squares method. In the on-line situation, the estimates of the parameters are recursively updated for each time instance. A comparative study of two different parameter estimation approaches is presented. In the first approach, the parameters are estimated by a simple linear regression. In the second approach, transformation of the differentiation operator to another casual and stable linear operator is made in linear regression to estimate the parameters. A numerical study of these approaches is also presented for comparison.

  • 250.
    Irshad, Yasir
    et al.
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Mossberg, Magnus
    Karlstad University, Faculty of Technology and Science, Department of Physics and Electrical Engineering.
    Wireless channel modeling based on stochastic differential equations2011Other (Other academic)
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