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  • 1.
    Abbas, Ghazanfar
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology. COMSATS Institute of Information Technology, Pakistan.
    Raza, Rizwan
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology. COMSATS Institute of Information Technology, Pakistan.
    Ahmad, M. Ashfaq
    Khan, M. Ajmal
    Hussain, M. Jafar
    Ahmad, Mukhtar
    Aziz, Hammad
    Ahmad, Imran
    Batool, Rida
    Altaf, Faizah
    Zhu, Bin
    KTH, School of Industrial Engineering and Management (ITM), Energy Technology.
    Electrochemical investigation of mixed metal oxide nanocomposite electrode for low temperature solid oxide fuel cell2017In: International Journal of Modern Physics B, ISSN 0217-9792, Vol. 31, no 27, 1750193Article in journal (Refereed)
    Abstract [en]

    Zinc-based nanostructured nickel (Ni) free metal oxide electrode material Zn-0.60/CU0.20Mn0.20 oxide (CMZO) was synthesized by solid state reaction and investigated for low temperature solid oxide fuel cell (LTSOFC) applications. The crystal structure and surface morphology of the synthesized electrode material were examined by XRD and SEM techniques respectively. The particle size of ZnO phase estimated by Scherer's equation was 31.50 nm. The maximum electrical conductivity was found to be 12.567 S/cm and 5.846 S/cm in hydrogen and air atmosphere, respectively at 600 degrees C. The activation energy of the CMZO material was also calculated from the DC conductivity data using Arrhenius plots and it was found to be 0.060 and 0.075 eV in hydrogen and air atmosphere, respectively. The CMZO electrode-based fuel cell was tested using carbonated samarium doped ceria composite (NSDC) electrolyte. The three layers 13 mm in diameter and 1 mm thickness of the symmetric fuel cell were fabricated by dry pressing. The maximum power density of 728.86 mW/cm(2) was measured at 550 degrees C.

  • 2. Aberg, D
    et al.
    Hallberg, T
    Svensson, B G
    Lindstrom, J L
    Linnarsson, M K
    KTH, School of Information and Communication Technology (ICT).
    Ultra-shallow thermal donor formation in oxygen-containing ambient1998In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 210, 527-532 p.Article in journal (Refereed)
    Abstract [en]

    Czochralski-grown phosphorus-doped (approximate to 2 x 10(14) cm(-3)) silicon wafers have been annealed in nitrogen, wet nitrogen, argon, oxygen, and vacuum ambients at 470 degrees C for times up to 500 h. Sample characterization was made with capacitance-voltage, four-point probe, DLTS, thermally stimulated capacitance, admittance spectroscopy, secondary ion-mass spectrometry, and Fourier transform infrared spectroscopy. This study finds a strong relation between the previously reported ultra-shallow thermal donors (USTDs) and shallow thermal donors (STDs), and it is shown that the net concentration of thermally formed donors is independent on annealing ambient within the experimental accuracy. It was found that the majority of formed donors for long anneals consisted of either STDs or USTDs, however, it was found that oxygen-containing ambient is indispensable for forming USTDs.

  • 3. Aberg, D
    et al.
    Linnarsson, M K
    KTH, School of Information and Communication Technology (ICT).
    Svensson, B G
    Hallberg, T
    Lindstrom, J L
    Ultrashallow thermal donor formation in silicon by annealing in ambient oxygen1999In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 85, 8054-8059 p.Article in journal (Refereed)
    Abstract [en]

    Czochralski-grown silicon wafers doped with phosphorus (similar to 10(14) cm(-3)) have been annealed in nitrogen, wet nitrogen, oxygen, argon, and vacuum ambients at 470 degrees C for times up to 500 h. Sample characterization was made using predominantly electrical techniques such as admittance spectroscopy and thermally stimulated capacitance measurements but also secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy were employed. In all samples, an increasing concentration of free carrier electrons is observed with increasing annealing time, reaching a maximum of similar to 10(16) cm(-3) at 100 h. For durations in excess of 100 h gradual decrease of the free electron concentration takes place except for the samples treated in wet nitrogen and oxygen atmospheres, which display donors stable even after 200 h. These stable centers are found to have shallower donor level positions in the energy band gap (similar to 25 meV below the conduction band edge E-c) than those of the centers formed in vacuum, argon, and nitrogen atmospheres (similar to 35 meV below E-c). The latter centers are associated with the well-established shallow thermal donors (STDs) while the origin of the former ones, which are labeled ultrashallow thermal donors (USTDs) is less known. However, on the basis of a wealth of experimental results we show that the USTDs are most likely perturbated STDs modified through interaction with fast-in diffusing oxygen species, possibly oxygen dimers. Further, comparison between the electrical data and the SIMS measurements reveals unambiguously that neither the STD nor the USTD centers involve nitrogen, in contrast to recent suggestions in the literature. (C) 1999 American Institute of Physics. [S0021-8979(99)06512-3].

  • 4.
    Abergel, David
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Mucha-Kruczynski, Marcin
    Infrared absorption of closely aligned heterostructures of monolayer and bilayer graphene with hexagonal boron nitride2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 92, no 11, 115430Article in journal (Refereed)
    Abstract [en]

    We model optical absorption of monolayer and bilayer graphene on hexagonal boron nitride for the case of closely aligned crystal lattices. We show that perturbations with different spatial symmetry can lead to similar absorption spectra. We suggest that a study of the absorption spectra as a function of the doping for an almost completely full first miniband is necessary to extract meaningful information about the moire characteristics from optical absorption measurements and to distinguish between various theoretical proposals for the physically realistic interaction. Also, for bilayer graphene, the ability to compare spectra for the opposite signs of electric-field-induced interlayer asymmetry might provide additional information about the moire parameters.

  • 5. Abrikosov, I. A.
    et al.
    Olsson, Pär
    Department of Materials and Mechanics of Components, EDF RandD, les Renardières, Moret-sur-Loing, France.
    Ponomareva, A. V.
    Correlation between electronic structure, magnetism and physical properties of Fe-Cr alloys: Ab initio modeling2008In: MATERIALS ISSUES FOR GENERATION IV SYSTEMS: STATUS, OPEN QUESTIONS AND CHALLENGES, 2008, 153-168 p.Conference paper (Refereed)
    Abstract [en]

    We review recent developments in the field of ab initio electronic structure theory and its application for studies of phase stability of alloy systems. Basic ideas behind state-of-the-art techniques for first-principles theoretical simulations of the phase stabilities and properties of intermetallic compounds and alloys based on the density functional theory are outlined. We concentrate on methods that allow for an efficient treatment of disorder effects, and illustrate their predictive power for the case of Fe-Cr system. We show that in the ferromagnetic alloys there are peculiarities of the mixing enthalpy in the low-Cr region in the bee phase. Thus the stability of the Cr containing steels stems from the negative mixing enthalpy at low concentrations of chromium. We explain the effect by the strong concentration dependence of the interatomic interactions in Fe-Cr system.

  • 6. Achtziger, N
    et al.
    Grillenberger, J
    Witthuhn, W
    Linnarsson, M K
    KTH, School of Information and Communication Technology (ICT).
    Janson, M S
    Svensson, B G
    Hydrogen passivation of silicon carbide by low-energy ion implantation1998In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 73, 945-947 p.Article in journal (Refereed)
    Abstract [en]

    implantation of deuterium is performed to investigate the mobility and passivating effect of hydrogen in epitaxial alpha-SiC (polytypes 4H and 6H). To avoid excessive damage and the resulting trapping of hydrogen, the implantation is performed with low energy (600 eV H-2(2)+). The H-2 depth profile is analyzed by secondary ion mass spectrometry. Electrical properties are measured by capacitance-voltage profiling and admittance spectroscopy. In p-type SIG, hydrogen diffuses on a mu m scale even at room temperature and effectively passivates accepters. In n-type SiC, the incorporation of H is suppressed and no passivation is detected. (C) 1998 American Institute of Physics.

  • 7. Achtziger, N
    et al.
    Hulsen, C
    Janson, M
    Linnarsson, M K
    KTH, School of Information and Communication Technology (ICT).
    Svensson, B G
    Witthuhn, W
    Formation of passivated layers in p-type SiC by low energy ion implantation of hydrogen2000In: SILICON CARBIDE AND RELATED MATERIALS - 1999 PTS, 1 & 2, 2000, Vol. 338-3, 933-936 p.Conference paper (Refereed)
    Abstract [en]

    The mobility of hydrogen and its passivating effect on accepters in p-type SiC is investigated. Hydrogen (isotope H-1 or H-2 alternatively) is implanted at temperatures between 300 K and 680 K with low energy (300 eV per atom) in order to minimize implantation damage. The depth profiles of 2H and of passivated accepters correspond closely. Up to 500 K, a fully passivated layer with a well defined thickness is formed. Its depth ton the order of 1 micrometer) is investigated as a function of doping level and hydrogen fluence. At higher temperatures, the incorporation drastically increases, but the electrical passivation is partial only. Qualitative explanations are given.

  • 8. Achtziger, N
    et al.
    Hulsen, C
    Witthuhn, W
    Linnarsson, M K
    KTH, School of Information and Communication Technology (ICT).
    Janson, M
    Svensson, B G
    Mobility passivating effect and thermal stability of hydrogen in silicon carbide1998In: Physica status solidi. B, Basic research, ISSN 0370-1972, E-ISSN 1521-3951, Vol. 210, 395-399 p.Article in journal (Refereed)
    Abstract [en]

    The diffusion and passivating effect of hydrogen (isotope H-2) in epitaxial p-type SiC is studied by secondary ion mass spectrometry and capacitance-voltage profiling on Schottky diodes. The incorporation of hydrogen is achieved by low-energy ion implantation. The influence of implantation energy, temperature and subsequent annealing is presented. Annealing experiments with an electric field applied reveal a reactivation of passivated accepters and a H+ ion drift at a surprisingly low temperature of 530 K.

  • 9.
    Agnarsson, Björn
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Applied Physics, MAP.
    Surface studies on α–sapphire for potential use in GaN epitaxial growth2009Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This Licentiate thesis summarizes the work carried out by the author the years 2004 to 2008 at the University of Iceland and the Royal Institute of Technology (KTH) in Sweden. The aim of the project was to investigate the structure of sapphire (alpha-Al2O3) surfaces, both for pure scientific reasons and also for potential use as substrate for GaN-growth by molecular beam epitaxy.

    More generally the thesis describes some surface science methods used for investigating the substrates; the general physical back ground, the experi- mental implementation and what information they can give. The described techniques are used for surface analysis on sapphire substrates which have been treated variously in order to optimize them for use as templates for epi- taxial growth of GaN or related III-V compounds.

    The thesis is based on three published papers.

    The first paper focuses on the formation a thin AlN layer on sapphire, which may act as a buffer layer for potential epitaxial growth of GaN or any related III-V materials. Two types of sapphire substrates (reconstructed and non- reconstructed) were exposed to ammonia resulting in the formation of AlN on the surface. The efficiency of the AlN formation (nitridation efficiency) for the two surfaces was then compared as a function of substrate temperature through photoelectron spectroscopy and low electron energy diffraction. The reconstructed surface showed a much higher nitridation efficiency than the non-reconstructed surface.

    In the second paper, the affect of different annealing processes on the sapphire morphology, and thus its capability to act as a template for GaN growth, was studied. Atomic force microscopy, X-ray diffraction analysis together with ellipsometry measurements showed that annealing in H2 ambient and subse- quent annealing at 1300 °C in O2 for 11 hours resulted in high quality and atomically flat sapphire surface suitable for III-V epitaxial growth.

    The third paper describes the effect of argon sputtering on cleaning GaN surfaces and the possibility of using indium as surfactant for establishing a clean and stoichiometric GaN surface, after such sputtering. Soft sputtering, followed by deposition of 2 ML of indium and subsequent annealing at around 500 °C resulted in a well ordered and clean GaN surface while hard sputtering introduced defects and incorporated both metallic gallium and indium in the surface.

  • 10.
    Agnarsson, Björn
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Göthelid, Mats
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Olafsson, S.
    Gislason, H. P.
    Karlsson, Ulf O.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Influence of initial surface reconstruction on nitridation of Al2O3 (0001) using low pressure ammonia2007In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 101, no 1, 013519- p.Article in journal (Refereed)
    Abstract [en]

    The purpose of this study is to investigate the effect of initial surface reconstruction on the nitridation process of Al2O3 (0001). This was done by exposing differently reconstructed sapphire substrates at different temperatures to low pressure ammonia (NH3). Structural and chemical analysis were carried out using low-electron energy diffraction and x-ray photoelectron spectroscopy. The experiments revealed that using low pressure ammonia (P-NH3 < 1 X 10(-5) Torr), no nitridation takes place on (1x1) unreconstructed surfaces. However, when the unreconstructed surface starts to change to a (root 31 x root 31) R +/- 9 degrees reconstructed surface, with increasing substrate temperature, the nitridation becomes successful. When using the initially reconstructed surface, the nitridation is successful even from the lowest temperature used. These results suggest that the initial surface reconstruction has a major effect on the nitridation process. This kinetic behavior has not been reported before, with most nitridation studies mainly focusing on the effect of surface temperature on the resulting surface morphology, rather than the actual kinetics of the process itself.

  • 11. Agterberg, Daniel F.
    et al.
    Garaud, Julien
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Condensed Matter Theory.
    Checkerboard order in vortex cores from pair-density-wave superconductivity2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 10, 104512Article in journal (Refereed)
    Abstract [en]

    We consider competing pair-density-wave (PDW) and d-wave superconducting states in a magnetic field. We show that PDW order appears in the cores of d-wave vortices, driving checkerboard charge-density-wave (CDW) order in the vortex cores, which is consistent with experimental observations. Furthermore, we find an additional CDW order that appears on a ring outside the vortex cores. This CDW order varies with a period that is twice that of the checkerboard CDW and it only appears where both PDW and d-wave order coexist. The observation of this additional CDW order would provide strong evidence for PDW order in the pseudogap phase of the cuprates. We further argue that the CDW seen by nuclear magnetic resonance at high fields is due to a PDW state that emerges when a magnetic field is applied.

  • 12.
    Ahmadi, Sareh
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF.
    Structure-dependent charge transfer at the interafce between organic thin films, and metals and metal oxides2013Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The purpose of the research work, presented in this thesis is to offer a detailed atomic level study of interfaces created by adsorption of organic molecules on metals and metal oxides to point out significant impact of substrate, dye structure as well as different mediators on the charge transfer at these interfaces, which is proven to influence the device performance to a great extent.

    Adsorption of organic photosensitive molecules on metals and metal-oxides is the main focus of this thesis. Phthalocyanines which are organic semiconductors offer a broad range of properties, such as thermal and chemical stability, high charge mobility and strong absorption coefficient in the visible and near-IR regions, which make them very attractive to be applied in various systems and devices. Fuel cells, organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs) and solar cells are examples of phthalocyanine’s applications. The main focus of this work is to characterize the interfaces of Dye Sensitized Solar Cells (DSSCs).

    DSSC was invented by Michael Grätzel and Brian O’Regan in 1988. At the heart of this cell there is an oxide which is coated by a photosensitive dye. Under illumination, an electron is excited from HOMO to LUMO of the molecule, which can be further transferred to the conduction band of the oxide by a proper energy level alignment. The original state of the dye is regenerated by electron donation via the electrolyte, which usually is an organic solvent containing a redox couple e.g., iodide/triiodide. The iodide is regenerated by reduction of triiodide at the counter electrode. To improve the functionality of the cell, different additives can be added to the electrolyte.

    To mimic the interfaces of this cell, molecular layers of MPc (M: Fe, Zn, Mg) are adsorbed on both metallic surfaces, Au(111) and Pt(111), and rutile TiO2(110). Layers of iodine were inserted between metallic substrates and dyes to investigate the electronic properties and charge transfer at these multi-interface systems. 4-tert-butyl pyridine is a significant additive to the electrolyte and has proven to enhance the cell’s performance. This molecule was also adsorbed on Pt(111) and TiO2(110). Phthalocyanines were deposited by organic molecular beam deposition and 4TBP was evaporated at room temperature. Surface structures and reconstructions were confirmed by LEED measurements. Surface sensitive synchrotron radiation based spectroscopy methods, XPS and NEXAFS were applied to characterize these surfaces and interfaces. STM images directly give a topographical and electronic map over the surface. All measurements were carried out in UHV condition.

    When MPc was adsorbed on Au(111) and TiO2(110), charge transfer from molecule to substrate is suggested, while the opposite holds for MPc adsorbed on Pt(111). Moreover, stronger interaction between MPc and Pt(111) and TiO2(110) compared to Au(111) also demonstrates the effect of substrate on the charge transfer at the interface. The stronger interaction observed for these two substrates disturbed the smooth growth of a monolayer; it also resulted in bending of the molecular plane. Interaction of MPc with metallic surfaces was modified by inserting iodine at the interface. Another substrate-related effect was observed when MgPc was adsorbed on TiO2(110);  and -cross linked surfaces, where the surface reconstruction directly affect the molecular configuration as well as electronic structure at the interface. Besides, it is shown that the d-orbital filling of the central metal atom in MPc plays an important role for the properties of the molecular layer as well as charge transfer at the interface.

    Upon adsorption of 4TBP on Pt(111), C-H bond is dissociatively broken and molecules is adsorbed with N atoms down. Modification of surface by iodine, prevent this dissociation. In the low coverage of iodine, there is a competition between 4TBP and iodine to directly bind to Pt(111). Investigation on the adsorption of 4TBP on TiO2(110) illustrated that these molecules in low coverage regime, prefer the oxygen vacancy sites and their adsorption on these sites, results in a downward band bending at the substrate’s surface. 

  • 13. Ahola-Tuomi, M.
    et al.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Laukkanen, P.
    Kuzmin, M.
    Lang, J.
    Schulte, K.
    Pietzsch, A.
    Perala, R. E.
    Rasanen, N.
    Vayrynen, I. J.
    Properties of self-assembled Bi nanolines on InAs(100) studied by core-level and valence-band photoemission, and first-principles calculations2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 24, 245401- p.Article in journal (Refereed)
    Abstract [en]

    We have studied self-assembled bismuth (Bi) nanolines on the Bi-terminated InAs(100) surface by core-level and valence-band photoelectron spectroscopy, and ab initio first-principles calculations. A structural model for this intriguing surface is suggested based on the comparison of the measured and calculated core-level shifts. Also, the atomic origins for the core-level shifts are proposed based on the calculations. A clear peak related to this surface was observed in the valence band 0.34 eV below the Fermi level, which can be used as a "fingerprint" of a well-ordered Bi/InAs(100) nanoline surface.

  • 14. Airiskallio, E.
    et al.
    Nurmi, E.
    Heinonen, M. H.
    Vayrynen, I. J.
    Kokko, K.
    Ropo, M.
    Punkkinen, M. P. J.
    Pitkanen, H.
    Alatalo, M.
    Kollar, J.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Third element effect in the surface zone of Fe-Cr-Al alloys2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 3Article in journal (Refereed)
    Abstract [en]

    The third element effect to improve the high temperature corrosion resistance of the low-Al Fe-Cr-Al alloys is suggested to involve a mechanism that boosts the recovering of the Al concentration to the required level in the Al-depleted zone beneath the oxide layer. We propose that the key factor in this mechanism is the coexistent Cr depletion that helps to maintain a sufficient Al content in the depleted zone. Several previous experiments related to our study support that conditions for such a mechanism to be functional prevail in real oxidation processes of Fe-Cr-Al alloys.

  • 15.
    Albrecht, Steffen
    KTH, School of Information and Communication Technology (ICT), Microelectronics and Information Technology, IMIT.
    Sigma-delta based techniques for future multi-standard wireless radios2005Doctoral thesis, monograph (Other scientific)
    Abstract [en]

    Improvements in process technology and design innovations have resulted in compact and cost effective digital baseband solutions. The radio part, however, has remained a bottleneck in terms of chip area and power consumption as the feature size of analog devices does not directly benefit from scaling. With the addition of yet more standards into emerging products, the requirements of future radios will extend over characteristic performance features into demands of programmable and reconfigurable hardware for radios covering multiple frequency bands. Hence, a guideline in the design of such radios is a large degree of hardware sharing.

    The thesis investigates the application of sigma-delta modulation to arising multistandard wireless radios. First, it reviews principles in wireless radios, such as selected modulation and access techniques. It also examines several communication standards of personal wireless radios as well as common receiver architectures for their implementation. This is followed by general considerations and background information about sigma-delta modulators. In the third and fourth chapter, implementations to two blocks of a wireless radio receiver system are suggested: An architecture for a frequency synthesizer and an implementation of an analog-to-digital converter.

    In the first contribution, the thesis develops a novel concept for frequency synthesis that is more suitable for multi-band, multi-standard radio architectures, achieving a large amount of hardware sharing among different wireless standards. As a second pillar, the thesis contributes with the design of a dual-standard sigma-delta modulator for data conversion within a radio receiver. Parts of the work concerning the dual-standard modulator are embodied in a granted swedish patent.

  • 16.
    Al-Zoubi, Noura
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Describing Interstitials in Close-packed Lattices: First-principles Study2011Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Describing interstitial atoms in intermetallics or simple mono-atomic close-packed metals is a straightforward procedure in common full-potential calculations. One establishes a sufficiently large supercell, introduces the interstitial impurity and performs the electronic structure and total energy calculation. Real systems, however, are rarely mono-atomic or ordered metals. In most of the cases, the matrix is a random or quasirandom mixture of several chemically and/or magnetically distinct components. Because of that a proper computational tool should incorporate advanced alloy theory and at the same time have sufficiently high accuracy to describe interstitial positions in close-packed solids. The purpose of the present thesis is to make a step towards solving this fundamental problem in computational materials science. To this end, in the first part of the thesis a prestudy on some selected metals and compounds was presented, and in the second part tools were applied to investigate the effect of interstitial carbon on the structural properties of steels.

    For the prestudy, the equation of state for the selected Al, Cu and Rh was investigated in two equivalent phases: in conventional face-centered-cubic lattice (fcc, str-I) and in a face-centered-cubic lattice with one atomic and three interstitial empty potentialwells per primitive cell (str-II). A proper basis set of the exact muffin-tin orbitals as well as a proper potential sphere radius were established by calculating the equilibrium Wigner-Seitz radius and bulk modulus of the above elements in str-I and str-II using the exact muffin-tin orbitals (EMTO) first-principle density functional method. It was found that for Al spd orbitals are sufficient to describe the equilibrium bulk properties in both structures, while for str-II Rh and Cu at least five orbitals (spdfg) are needed to get accurate equilibrium volume and bulk modulus. Furthermore, it was shown that in general, for the str-II type of structure (close-packed structure with interstitials) the optimized overlapping muffin-tin potential in combination with spdfg orbitals ensures well converged bulk properties.

    As an application of the above work in alloys, (i) the chemical reaction between hydrogen H2 molecule and ScAl1−xMgx (0≤x≤0.3) random alloys, (ii) the phase stability of the hydrogenated alloys in different structures and (iii) the hydrogen absorption/desorption temperatures were studied by calculating the Gibbs energy for the components of the reaction. Experimental and theoretical studies by Sahlberg et al . showed that the ScAl0.8Mg0.2 compound with CsCl structure absorbs hydrogen by decomposing into ScH2 with CaF2 structure and fcc Al0.8Mg0.2. This reaction was found to be very fast, even without adding catalyst, and fully reversible. The theoretical hydrogen absorption/desorption temperatures agree well with the experimental values. On the other hand, the stability field of the hydrogenated alloys was found to be strongly depends on Mg content and on the microstructure of the hydrogenated alloys. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with the Mg concentration.

    The second part of the thesis focused on steel materials with special emphases on the effect of interstitial carbon. Steels are considered to be one of the most important engineering materials. They are mainly composed of iron and carbon. Other alloying elements in steel are introduced to get specific properties like microstructure, corrosion resistance, hardness, brittleness, etc. In order to describe the effect of carbon interstitial in iron alloys, it is important to know how the substitutional alloying elements affect the softness and some other properties of iron alloys. For that reason, the alloying effects on the energetic and magnetic structure of paramagnetic Fe0.85Cr0.1M0.05 (M = Cr, Mn, Fe, Co and Ni) alloys along the tetragonal distortion path connecting the body centered cubic (bcc) and the face centered cubic (fcc) phases were investigated. It was shown that Cr stabilizes bcc phase and increases the energy barrier (relative to bcc phase) between fcc and bcc phases. Cobalt and Ni stabilize fcc structure. Cobalt increases whereas Ni slightly decreases the energy barrier relative to fcc structure. Manganese and iron have negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. The local magnetic moments on Fe atoms have maximum values at bcc phase and minimum values at fcc phase. Cobalt atoms possess local magnetic moments only for tetragonal lattices with c/a < 1.30, and the Mn magnetic moments have almost constant value along the Bain path.

    The tetragonality of Fe-C martensite was discovered in 1928. Early experimental works showed that the tetragonality of Fe-C is linearly depends on C content. However, Later many observations indicated that the tetragonality of martensite is influenced also by alloying and interstitial carbon distributions. Very few ab initio studies focus on investigating the tetragonality of Fe-C based alloys. In this thesis the interstitial carbon in ferromagnetic Fe-based alloys and it is impact on the tetragonal lattice ratio of Fe matrix as well as the alloying effect on the tetragonality of Fe-C system were investigated. It was found that the ferromagnetic Fe-C system with C content ∼ 1.3 wt. % has a body-centered tetragonal (bct) structure with c/a ∼ 1.07. Alloying has an impact on the tetragonality; adding 5% Al, Co or Ni enhances while 5% Cr addition decreases the tetragonal lattice ratio.

    The electronic structure and total energy calculations from this thesis are based on firstprinciples exact muffin-tin orbitals method. The chemical and magnetic disorder was treated using coherent-potential approximation and the paramagnetic phase was modeled by the disordered local magnetic moments approach. Some test calculations involved also full-potential tools as implemented in Vienna ab-initio simulation package (VASP).

  • 17.
    Al-Zoubi, Noura
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Describing interstitials in close-packed lattices: first-principles study2010Licentiate thesis, comprehensive summary (Other academic)
  • 18.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Nilson, G.
    Uddeholms AB, Hagfors, Sweden.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    The bain path of paramagnetic Fe-Cr based alloys2011In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 110, no 1, 013708- p.Article in journal (Refereed)
    Abstract [en]

    Employing the first-principles exact muffin-tin orbital method in combination with the coherent potential approximation, we calculated the total energy and local magnetic moments of paramagnetic Fe-Cr-M (M = Cr, Mn, Fe, Co, Ni) alloys along the tetragonal distortion (Bain) path connecting the body centered cubic (bcc) and the face centered cubic (fcc) structures. The paramagnetic phase is modeled by the disordered local magnetic moment scheme. For all alloys, the local magnetic moments on Fe atoms decrease from the maximum value corresponding to the bcc phase toward the minimum value realized for the fcc phase. Cobalt atoms have non-vanishing local magnetic moments only for tetragonal lattices with c/a < 1.30, whereas the local magnetic moments of Mn show weak crystal structure dependence. We find that Cr stabilizes the bcc lattice and increases the energy barrier as going from the bcc toward the fcc phase. Both Co and Ni favor the fcc lattice and decrease the energy barrier relative to the bcc phase. On the other hand, the tetragonal distortion around the fcc phase is facilitated by Cr and to a somewhat lesser extent also by Ni, but strongly impeded by Co. Manganese has negligible effect on the structural energy difference as well as on the energy barrier along the Bain path. Our findings on the alloying induced softening or hardening of Fe-Cr based alloys against tetragonal distortions are important for understanding the interstitial driven martensitic transformations in alloy steels.

  • 19.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Completeness of the exact muffin-tin orbitals: application to hydrogenated alloys2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 81, no 4Article in journal (Refereed)
    Abstract [en]

    We investigate the basis set convergence of the exact muffin-tin orbitals by monitoring the equation of state for Al, Cu, and Rh calculated in the conventional face-centered-cubic lattice (str-I) and in a face-centered-cubic lattice with one atomic and three empty sites per primitive cell (str-II). We demonstrate that three (spd) muffin-tin orbitals are sufficient to describe Al in both structures, but for str-II Cu and Rh at least five (spdfg) orbitals are needed to get converged equilibrium Wigner-Seitz radius (within <= 0.8%) and bulk modulus (<= 3.3%). We ascribe this slow convergence to the nearly spherical densities localized around the Cu and Rh atoms, which create strongly asymmetric charge distributions within the nearest cells around the empty sites. The potential sphere radius dependence of the theoretical results for structure str-II is discussed. It is shown that a properly optimized overlapping muffin-tin potential in combination with the spdfg basis yields acceptable errors in the equilibrium bulk properties. The basis set convergence is also shown on hydrogenated Sc and Sc-based alloys.

  • 20.
    Al-Zoubi, Noura
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Punkkinen, Marko Patrick John
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Vitos, Levente
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Influence of Magnesium on hydrogenated ScAl1-xMgx alloys: a theoretical study2011In: Computational materials science, ISSN 0927-0256, E-ISSN 1879-0801, Vol. 50, no 10, 2848-2853 p.Article in journal (Refereed)
    Abstract [en]

    Ab initio total energy calculations, based on the projector augmented wave method and the exact mu±n-tin orbitals method in combination with the coherent-potential approximation, are used to examine the effect of magnesium on hydrogen absorption/desorption temperature and phase stability of hydrogenated ScAl1-xMgx (0 ≤ x ≤ 0:3) alloys. According to the  experiments, ScAl1-xMgx adopts the CsCl structure, and upon hydrogen absorption it decomposes into ScH2 with CaF2 structure and Al-Mg with face centered cubic structure. Here we demonstrate that the stability field of the hydrogenated alloys depends sensitively on Mg content and on the microstructure of the decomposed system. For a given microstructure, the critical temperature for hydrogen absorption/desorption increases with Mg concentration.

  • 21.
    Andersson, Andreas
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Phase fluctuation phenomena in superconductors2012Doctoral thesis, comprehensive summary (Other academic)
  • 22.
    Andersson, Andreas
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Simulations of thermoelectric transport in granularsuperconductors2010Licentiate thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis presents results from numerical simulations of the Nernst effect dueto phase fluctuations in models of two-dimensional granular superconductors. Inaddition other transport properties, such as thermal conductivity and electrical re-sistivity are calculated. The models are based on a phase only description withLangevin or resistively and capacitively shunted Josephson junction (RCSJ) dy-namics, generalized to be valid for any type of two-dimensional lattice structure.All transport coefficients are evaluated from equilibrium correlation functions usingKubo formulas.

    In Paper I, anomalous sign reversals of the Nernst signal eN , corresponding tovortex motion from colder to hotter regions, are observed. These are attributedto geometric frustration effects close to magnetic fields commensurate with theunderlying lattice structure. The effect is seen also in systems with moderategeometric disorder, and should thus be possible to observe in real two-dimensionalgranular superconductors or Josephson junction arrays.

    Paper II presents two different derivations of an expression for the heat current inLangevin and RCSJ dynamics. The resulting expression is through our simulationsseen to obey the required Onsager relation, as well as giving consistent resultswhen calculating κ and eN via Kubo formulas and through the responses to anapplied temperature gradient. In zero magnetic field and at low-temperatures, thecontribution to the thermal conductivity κ in RCSJ dynamics is calculated usinga spin-wave approximation, and is shown to be independent of temperature anddiverge logarithmically with system size. At higher temperatures, κ shows a non-monotonic temperature dependence. In zero magnetic field κ has a anomalouslogarithmic size dependence also in this regime. The off-diagonal component ofthe thermoelectric tensor αxy is calculated and displays the very same ∼1/T dependence at low temperatures predicted from calculations based on Gaussiansuperconducting fluctuations.

  • 23.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Influence of vortices and phase fluctuations on thermoelectric transport properties of superconductors in a magnetic field2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 83, no 17, 174502- p.Article in journal (Refereed)
    Abstract [en]

    We study heat transport and thermoelectric effects in two-dimensional superconductors in a magnetic field. These are modeled as granular Josephson-junction arrays, forming either regular or random lattices. We employ two different models for the dynamics: relaxational model-A dynamics or resistively and capacitively shunted Josephson junction dynamics. We derive expressions for the heat current in these models, which are then used in numerical simulations to calculate the heat conductivity and Nernst coefficient for different temperatures and magnetic fields. At low temperatures and zero magnetic field the heat conductivity in the RCSJ model is calculated analytically from a spin wave approximation, and is seen to have an anomalous logarithmic dependence on the system size, and also to diverge in the completely overdamped limit C -> 0. From our simulations we find at low magnetic fields that the Nernst signal displays a characteristic "tilted hill" profile similar to experiments and a nonmonotonic temperature dependence of the heat conductivity. We also investigate the effects of granularity and randomness, which become important for higher magnetic fields. In this regime geometric frustration strongly influences the results in both regular and random systems and leads to highly nontrivial magnetic field dependencies of the studied transport coefficients.

  • 24.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Modeling and simulations of quantum phase slips in ultrathin superconducting wiresManuscript (preprint) (Other academic)
  • 25.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics.
    Modeling and simulations of quantum phase slips in ultrathin superconducting wires2015In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 91, no 13, 134504Article in journal (Refereed)
    Abstract [en]

    We study quantum phase slips (QPS) in ultrathin superconducting wires. Starting from an effective one-dimensional microscopic model, which includes electromagnetic fluctuations, we map the problem to a (1+1)-dimensional gas of interacting instantons. We introduce a method to calculate the tunneling amplitude of quantum phase slips directly from Monte Carlo simulations. This allows us to go beyond the dilute instanton gas approximation and study the problem without any limitations of the density of QPS. We find that the tunneling amplitude shows a characteristic scaling behavior near the superconductor-insulator transition. We also calculate the voltage-charge relation of the insulating state, which is the dual of the Josephson current-phase relation in ordinary superconducting weak links. This evolves from a sinusoidal form in the regime of dilute QPS to more exotic shapes for higher QPS densities, where interactions are important.

  • 26.
    Andersson, Andreas
    et al.
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Lidmar, Jack
    KTH, School of Engineering Sciences (SCI), Theoretical Physics, Statistical Physics.
    Scaling, Finite Size Effects, and Crossovers of the Resistivity and Current-Voltage Characteristics in Two-Dimensional SuperconductorsManuscript (preprint) (Other academic)
    Abstract [en]

    We revisit the scaling properties of the resistivity and the current-voltage characteristics at and below the Berezinskii-Kosterlitz-Thouless transition, both in zero and nonzero magnetic field. The scaling properties are derived by integrating the renormalization group flow equations up to a scale where they can be reliably matched to simple analytic expressions. The vortex fugacity turns out to be dangerously irrelevant for these quantities below $T_c$, thereby altering the scaling behavior. We derive the possible crossover effects as the current, magnetic field or system size is varied, and find a strong multiplicative logarithmic correction near $T_c$, all which is necessary to account for when interpreting experiments and simulation data. Our analysis clarifies a longstanding discrepancy between the finite size dependence found in many simulations and the current-voltage characteristics of experiments. We further show that the logarithmic correction can be avoided by approaching the transition in a magnetic field, thereby simplifying the scaling analysis. We confirm our results by large scale numerical simulations, and calculate the dynamic critical exponent $z$, for relaxational Langevin dynamics and for resistively and capacitively shunted Josephson junction dynamics.

  • 27.
    Andersson, David A.
    et al.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Korzhavyi, Pavel A.
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Johansson, Börje
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    First-principles based calculation of binary and multicomponent phase diagrams for titanium carbonitride2008In: Calphad, ISSN 0364-5916, E-ISSN 1873-2984, Vol. 32, no 3, 543-565 p.Article in journal (Refereed)
    Abstract [en]

    In this paper we have used a combined first principles and Calphad approach to calculate phase diagrams in the titanium-carbon-nitrogen system, with particular focus on the vacancy-induced ordering of the substoichiometric carbonitride phase, TiCxNy (x + y <= 1). Results from earlier Monte Carlo simulations of the low-temperature binary phase diagrams are used in order to formulate sublattice models for TiCxNy within the compound energy formalism (CEF) that are capable of describing both the low temperature ordered and the high-temperature disordered state. We parameterize these models using first-principles calculations and then we demonstrate how they can be merged with thermodynamic descriptions of the remaining Ti-C-N phases that are derived within the Calphad method by fitting model parameters to experimental data. We also discuss structural and electronic properties of the ordered end-member compounds, as well as short range order effects in the TiCxNy phase.

  • 28.
    Andersson, Magnus
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Comment on “Superconducting state of Ca-VII below a critical temperature of 29 K at a pressure of 216 GPa”2011In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 84, no 21Article in journal (Refereed)
    Abstract [en]

    In a recent paper, Sakata et al. [ Phys. Rev. B 83 220512 (2011)] report on superconductivity at 29 K in Ca under high pressure. Here, I argue that their method to define the onset of the superconducting transition temperature from resistivity measurements is misleading and that the correct value for the onset of the transition based on resistivity data shown in their paper should be 21 K. The discrepancy between these two temperatures is explained by superconducting fluctuations occurring at temperatures above the superconducting transition temperature.

  • 29.
    Andersson, Magnus
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Helt orimliga förhoppningar2012In: Forskning och framsteg, ISSN 0015-7937, no 2, 72-72 p.Article in journal (Other (popular science, discussion, etc.))
  • 30.
    Andersson, Magnus
    KTH, Superseded Departments, Physics.
    Studies of magnetoresistance and Y substitutions in YBa2Cu3O7-delta high temperature superconductors1992Doctoral thesis, comprehensive summary (Other academic)
  • 31.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Börjesson, Lars
    Physics Department, Chalmers University of Technology.
    Jarlborg, T
    Departement de Physique de la Matière Condensée, Université de Genève.
    Phuong, HV
    Physics Department, Chalmers University of Technology.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    RELATION BETWEEN THE CU(1)-O(4) DISTANCE AND T(C) IN Y1-2XCAXTHXBA2CU3O7-DELTA - RAMAN-SCATTERING AND CALCULATION OF THE ELECTRON-PHONON INTERACTION1992In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 46, no 10, 6501-6504 p.Article in journal (Refereed)
    Abstract [en]

    The strong depression of T(c) with Ca-Th substitution in Y1-2xCaxThxBa2Cu3O7-delta has been investigated in a model calculation of the electronic and phononic contributions from the apex oxygen, O(4), on the electron-phonon interaction, lambda. The shift with Ca-Th substitution of the characteristic O(4) vibrational mode was measured by Raman scattering. The electronic part eta of lambda has been calculated at several distances between chain Cu(1) and O(4) in the region of the observed variation with x. Eta was found to depend strongly on this distance. The results show a small increase with x of the O(4) frequency and a strong decrease of eta. Both these factors thus contribute to a decrease of lambda. We therefore find evidence that a decreasing electron-phonon interaction is associated with the depression of T(c) in Ca-Th substituted YBa2Cu3O7-delta.

  • 32.
    Andersson, Magnus
    et al.
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    de Boissieu, M.
    Bruehne, S.
    Drescher, C.
    Assmus, W.
    Ohahshi, S.
    Tsai, A. P.
    Mihalkovic, M.
    Krajci, M.
    Rapp, Östen
    KTH, School of Information and Communication Technology (ICT), Material Physics, Material Physics, MF.
    Electronic and structural properties of Laves-phase MgZn(2) of varying chemical disorder2010In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 82, no 2, 024202- p.Article in journal (Refereed)
    Abstract [en]

    The C14 Laves-phase MgZn(2) has been investigated from 30 to 36 at. % Mg. In this way chemical disorder can be monitored over a limited concentration range and the influence on electron properties can be investigated. Our studies include thermodynamic calculations of atomic configurations of Mg and Zn at off-stoichiometric compositions, electronic-transport measurements, and electronic band-structure calculations of MgZn(2). The disorder introduced by alloying was found to be substitutional for all C14 alloys, and to have a markedly stronger effect on resistivity and magnetoresistance, Delta rho(B)/rho(0), on the Mg-rich side due to strain introduced when Mg substitutes for Zn. rho(T) and Hall constant were characteristic for weakly disordered binary alloys. Delta rho/rho of MgZn(2) was large, reached 6 at 4.2 K and 8 T, and decreased strongly at off-stoichiometric compositions. The results are discussed in view of the band-structure results and in terms of relations between atomic order and electronic properties. Several properties were found to resemble pure Zn. An empirical correlation over more than six orders of magnitude in Delta rho/rho was found for Zn and Zn-based alloys.

  • 33.
    Andersson, Magnus
    et al.
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fivat, Patrice
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fabrega, Lourdes
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Obara, Haruhiko
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Decroux, Michel
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Triscone, Jean-Marc
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fischer, Oystein
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Vortex solid-to-liquid transition in DyBa2Cu3O7-delta/(Y0.45Pr0.55)Ba2Cu3O7-delta multilayers1996In: Physical Review B. Condensed Matter and Materials Physics, ISSN 1098-0121, E-ISSN 1550-235X, Vol. 54, no 1, 675-681 p.Article in journal (Refereed)
    Abstract [en]

    Current-voltage (I-V) characteristics of a coupled multilayer system consisting of thin (24 Angstrom) superconducting layers of DyBa2Cu3O7-delta each separated by 96 Angstrom of (Y0.45Pr0.55)Ba2Cu3O7-delta have been studied in the mixed state. Analyses of vortex glass scaling and signs of the clean system melting transition in the dynamics of our multilayers both suggest that the vortex system is close to three-dimensional for magnetic fields up to several T. Therefore, the theoretically predicted crossover when in-plane vortex correlations become more important than out-of-plane vortex correlations (occurring at a few tenths of a T in our multilayers) does not correspond to a complete decoupling of the superconducting layers in these materials.

  • 34.
    Andersson, Magnus
    et al.
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fivat, Patrice
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Obara, Haruhiko
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fabrega, Lourdes
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Triscone, Jean-Marc
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Decroux, Michel
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    Fischer, Oystein
    Dept. de la Matière Condensée, University of Geneva, Geneva.
    H-T PHASE-DIAGRAM OF DYBA2CU3O7-DELTA(Y0.45PR0.55)BA2CU3O7-DELTA MULTILAYERS IN THE MIXED-STATE1994In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 235, 2617-2618 p.Article in journal (Refereed)
    Abstract [en]

    From measurements of I-V characteristics of DyBa2Cu3O7-delta/(Y0.45Pr0.55)Ba2Cu3O7.delta multilayers, we have experimentally determined the characteristic temperature separating the vortex liquid from the vortex solid. We suggest that the cross-over temperature is determined by the anisotropy given by the artificially layered structure of these multilayers and that the studied system can be used as a test system for vortex dynamics.

  • 35.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Hegedüs, Zsolt
    Arrheniulaboratoriet, Stockholms Universitet.
    Hörlin, Tom
    Arrheniuslaboratoriet, Stockholms Universitet.
    Nygren, Mats
    Arrhemiuslaboratoriet, Stockholms Universitet.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Wen, T. L.
    Arrheniuslaboratoriet, Stockholms Universitet.
    (Ca,Pr) substitutions in Y- and Sm-based 1:2:3 superconductors1992In: / [ed] Per-Anker Lindgård, Odense, Denmark: Risø National Laboratory , 1992, 236-236 p.Conference paper (Other academic)
  • 36.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Hegedüs, Zsolt
    Arrheniuslaboratoriet, Stockholms Universitet.
    Nygren, Mats
    Arrheniuslaboratoriet, Stockholms Universitet.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    STRUCTURAL, THERMAL AND ELECTRICAL STUDIES OF THE Y1-2XCAXTHXBA2CU3O7-DELTA SYSTEM WITH 0 LESS-THAN-OR-EQUAL-TO 0.201989In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 160, no 1, 65-68 p.Article in journal (Refereed)
  • 37.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    MAGNETORESISTANCE MEASUREMENTS ON (CA,TH)-DOPED YBA2CU3O7-DELTA1991In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 185, 1901-1902 p.Article in journal (Refereed)
    Abstract [en]

    Magnetoresistance measurements on sintered Y1-2xCaxThxBa2Cu3O7-delta samples with x < = 0.1 are reported. By fitting the measured data to the Hikami-Larkin theory, we obtain coherence lengths, xi-ab and xi-c’ and a phase breaking time tau-phi All these parameters are found to decrease with (Ca,Th) doping. We also conclude that magnetoresistance measurements are highly sensitive to inhomogeneities in the sample.

  • 38.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    MAGNETORESISTANCE MEASUREMENTS ON POLYCRYSTALLINE YBA2CU3O7-DELTA1991In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 44, no 14, 7722-7725 p.Article in journal (Refereed)
    Abstract [en]

    The magnetoresistance of a polycrystalline sample of YBa2Cu3O7-delta has been measured by an ordinary resistive method at temperatures from about 1 to 15 K above T(c) and in magnetic fields up to 12 T. From our experimental results and the theory of Hikami and Larkin, we obtain values of the coherence lengths xi-ab = 15.6 angstrom and xi-c = 3.6 angstrom and of the phase-breaking time tau-phi = 1.8 x 10(-13) s.

  • 39.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    MAGNETORESISTANCE MEASUREMENTS ON SINTERED YBA2CU3O7-DELTA1990In: Physica. B, Condensed matter, ISSN 0921-4526, E-ISSN 1873-2135, Vol. 165, 1365-1366 p.Article in journal (Refereed)
  • 40.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    UPPER CRITICAL MAGNETIC-FIELD OF Y1-2XCAXTHXBA2CU3O7-DELTA1989In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 162, 725-726 p.Article in journal (Refereed)
  • 41.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Hegedüs, Zsolt
    Arrheniuslaboratoriet, Stockholms Universitet.
    Wen, TL
    Arrheniuslaboratoriet, Stockholms Universitet.
    Nygren, Mats
    Arrheniuslaboratoriet, Stockholms Universitet.
    STRUCTURAL AND SUPERCONDUCTING PROPERTIES OF Y1-2XCAXPRXBA2CU3O7-GAMMA1992In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 190, no 3, 255-260 p.Article in journal (Refereed)
    Abstract [en]

    Samples of the nominal composition Y1-2xCaxPrxBa2Cu3O7-delta with 0 less-than-or-equal-to x less-than-or-equal-to 0.4 have been studied by X-ray diffraction techniques, chemical analyses, and measurements of the super conducting transition temperature and the upper critical magnetic field. For x less-than-or-equal-to 0.25, the results indicate that Ca and Pr can both replace Y in agreement with the nominal composition, while for x > 0.25 only Pr enters the orthorhombic unit cell. Compared to Pr substitution in 1:2:3 we observe small changes in the depression of T(c) and strong changes in the critical field slopes. These results suggest that if Pr has a magnetic pair-breaking effect, it is close to being temperature independent.

  • 42.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Tellgren, Roland
    Institutet för kemi, Uppsala Universitet.
    NEUTRON-DIFFRACTION STUDIES OF CA-PR DOPED YBA2CU3O7-DELTA1993In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 205, no 1-2, 105-110 p.Article in journal (Refereed)
    Abstract [en]

    We have performed neutron powder diffraction on samples in a series Y1-2xCaxPrxBa2Cu3O7-delta, with x=0.0, 0.1 and 0.2, and analyzed the data with the Rietveld refinement method. Small changes were observed in atomic bond distances when the amount of Ca-Pr was increased. In general, these changes could qualitatively be viewed as the sum of effects from the Ca and Pr substitutions in YBa2Cu3O7-delta samples. Analyses of the distance between the superconducting CuO2 Planes suggest that the Pr ions in the Ca-Pr substituted samples have an effective valence close to + 4.

  • 43.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Tellgren, Roland
    Institutet för kemi, Uppsala Universitet.
    NEUTRON-DIFFRACTION STUDIES OF Y1-2XCAXTHXBA2CU3O7-DELTA WITH X-LESS-THAN=0.11991In: Physica. C, Superconductivity, ISSN 0921-4534, E-ISSN 1873-2143, Vol. 185, 915-916 p.Article in journal (Refereed)
    Abstract [en]

    We have made Rietveld refinements on neutron powder diffraction data from Y1-2xCaxThxBa2Cu3O7-delta samples with x < = 0.1. Only small changes in lattice distances are detected. Most noticeable are a flattening of the CuO-planes and a possible small increase in the distance between Cu(1) and the edge oxygen O(4). A comparison with the Y1-xCaxBa2Cu3O7-delta system is made.

  • 44.
    Andersson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Wen, TL
    Arrheniuslaboratoriet, Stockholms Universitet.
    Hegedüs, Zsolt
    Arrheniuslaboratoriet, Stockholms Universitet.
    Nygren, Mats
    Arrheniuslaboratoriet, Stockholms Universitet.
    CA AND PR SUBSTITUTION IN Y-BASED AND SM-BASED 1-2-3 COMPOUNDS1993In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795, Vol. 48, no 10, 7590-7597 p.Article in journal (Refereed)
    Abstract [en]

    Substitution with equal amounts of Ca and Pr has been studied mainly in Y- and Sm-based 1:2:3 samples and with some results also for Nd-based samples. Structural and chemical analysis and measurements of the superconducting T(c) and the upper critical magnetic field were performed. It was found that Ca and Pr can be dissolved in the orthorhombic structure up to about 25 at. % each in both Y- and Sm-based samples while for larger concentrations, only Pr continued to enter into the 1:2:3 structure. The depression of T(c) with Ca-Pr doping was found to be linear in concentration in contrast to the accelerated decrease in samples doped with only Pr. The rate of depression of T(c) increased in the sequence Y-, Sm-, and Nd-based hosts. The results suggest that Ca-Pr doping isolates a characteristic Pr-impurity effect in 1:2:3 samples. The critical magnetic-field slopes of Y- and Sm-based samples were almost independent of Ca-Pr content in the cosolubility region while a strong decrease was observed for larger Pr content, similar to 1:2:3 samples doped with Pr only. These results suggest an almost-temperature-independent magnetic pair-breaking effect by Pr.

  • 45.
    Andersson, Sebastian
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Spin-diode effect and thermally controlled switching in magnetic spin-valves2012Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis demonstrates two new device concepts that are based on the tunneling and giant magnetoresistance effects. The first is a semiconductor-free asymmetric magnetic double tunnel junction that is shown to work as a diode, while at the same time exhibiting a record high magnetoresistance. It is experimentally verified that a diode effect, with a rectification ratio of at least 100, can be obtained in this type of system, and that a negative magnetoresistance of nearly 4000% can be measured at low temperature. The large magnetoresistance is attributed to spin resonant tunneling, where the parallel and antiparallel orientation of the magnetic moments shifts the energy levels in the middle electrode, thereby changing their alignment with the conduction band in the outer electrodes. This resonant tunneling can be useful when scaling down magnetic random access memory; eliminating the need to use external diodes or transistors in series with each bit.

    The second device concept is a thermally controlled spin-switch; a novel way to control the free-layer switching and magnetoresistance in spin-valves. By exchange coupling two ferromagnetic films through a weakly ferromagnetic Ni-Cu alloy, the coupling is controlled by changes in temperature. At room temperature, the alloy is weakly ferromagnetic and the two films are exchange coupled through the alloy. At a temperature higher than the Curie point, the alloy is paramagnetic and the two strongly ferromagnetic films decouple. Using this technique, the read out signal from a giant magnetoresistance element is controlled using both external heating and internal Joule heating. No degradation of device performance upon thermal cycling is observed. The change in temperature for a full free-layer reversal is shown to be 35 degrees Celsius for the present Ni-Cu alloy. It is predicted that this type of switching theoretically can lead to high frequency oscillations in current, voltage, and temperature, where the frequency is controlled by an external inductor or capacitor. This can prove to be useful for applications such as voltage controlled oscillators in, for example, frequency synthesizers and function generators. Several ways to optimize the thermally controlled spin switch are discussed and conceptually demonstrated with experiments.

  • 46.
    Andersson, Sebastian
    et al.
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Korenivski, Vladislav
    KTH, School of Engineering Sciences (SCI), Applied Physics, Nanostructure Physics.
    Thermoelectrically Controlled Spin-Switch2010In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 46, no 6, 2140-2143 p.Article in journal (Refereed)
    Abstract [en]

    The search for novel spintronic devices brings about new ways to control switching in magnetic thin-films. In this work we experimentally demonstrate a device based on thermoelectrically controlled exchange coupling. The read out signal from a giant magnetoresistance element is controlled by exchange coupling through a weakly ferromagnetic Ni-Cu alloy. This exchange coupling is shown to vary strongly with changes in temperature, and both internal Joule heating and external heating is used to demonstrate magnetic switching. The device shows no degradation upon thermal cycling. Ways to further optimize the device performance are discussed. Our experimental results show a new way to thermoelectrically control magnetic switching in multilayers.

  • 47.
    Anderssson, Magnus
    et al.
    KTH, Superseded Departments, Physics.
    Rapp, Östen
    KTH, Superseded Departments, Physics.
    Tellgren, Roland
    Institutet för kemi, Uppsala Universitet.
    NEUTRON-DIFFRACTION STUDIES OF INTERATOMIC DISTANCES IN THE Y1-2XCAXTHXBA2CU3O7-DELTA SYSTEM1992In: Solid State Communications, ISSN 0038-1098, E-ISSN 1879-2766, Vol. 81, no 5, 425-429 p.Article in journal (Refereed)
    Abstract [en]

    Neutron diffraction studies have been performed for three samples of Y(1-2x)Ca(x)Th(x)Ba2Cu3O7-delta with 0 less-than-or-equal-to x less-than-or-equal-to 0.1. With increasing x, a decreased puckering of the Cu-O planes is found. Changes in copper oxygen distances are generally small. Evidence is found for a small increase in the distance between Cu(1) and O(4) (on the cell edge) with increased CaTh concentration. An increased oxygen disorder is observed at x = 0.1.

  • 48. Aparicio, Francisco J.
    et al.
    Holgado, Miguel
    Borras, Ana
    Blaszczyk-Lezak, Iwona
    Griol, Amadeu
    Barrios, Carlos A.
    Casquel, Rafael
    Sanza, Francisco J.
    Sohlström, Hans
    KTH, School of Electrical Engineering (EES), Microsystem Technology.
    Antelius, Mikael
    KTH, School of Electrical Engineering (EES), Microsystem Technology.
    Gonzalez-Elipe, Agustin R.
    Barranco, Angel
    Transparent Nanometric Organic Luminescent Films as UV-Active Components in Photonic Structures2011In: Advanced Materials, ISSN 0935-9648, E-ISSN 1521-4095, Vol. 23, no 6, 761-765 p.Article in journal (Refereed)
    Abstract [en]

    A new kind of visible-blind organic thin-film material, consisting of a polymeric matrix with a high concentration of embedded 3-hydroxyflavone (3HF) dye molecules, that absorbs UV light and emits green light is presented. The thin films can be grown on sensitive substrates, including flexible polymers and paper. Their suitability as photonic active components in photonic devices is demonstrated.

  • 49. Araujo, C. Moyses
    et al.
    Nagar, Sandeep
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Ramzan, Muhammad
    Shukla, R.
    Jayakumar, O. D.
    Tyagi, A. K.
    Liu, Yi-Sheng
    Chen, Jeng-Lung
    Glans, Per-Anders
    Chang, Chinglin
    Blomqvist, Andreas
    Lizarraga, Raquel
    Holmström, Erik
    Belova, Lyubov
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Engineering Material Physics.
    Guo, Jinghua
    Ahuja, Rajeev
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering, Applied Material Physics.
    Rao, K. Venkat
    KTH, School of Industrial Engineering and Management (ITM), Materials Science and Engineering.
    Disorder-induced Room Temperature Ferromagnetism in Glassy Chromites2014In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 4, 4686- p.Article in journal (Refereed)
    Abstract [en]

    We report an unusual robust ferromagnetic order above room temperature upon amorphization of perovskite [YCrO3] in pulsed laser deposited thin films. This is contrary to the usual expected formation of a spin glass magnetic state in the resulting disordered structure. To understand the underlying physics of this phenomenon, we combine advanced spectroscopic techniques and first-principles calculations. We find that the observed order-disorder transformation is accompanied by an insulator-metal transition arising from a wide distribution of Cr-O-Cr bond angles and the consequent metallization through free carriers. Similar results also found in YbCrO3-films suggest that the observed phenomenon is more general and should, in principle, apply to a wider range of oxide systems. The ability to tailor ferromagnetic order above room temperature in oxide materials opens up many possibilities for novel technological applications of this counter intuitive effect.

  • 50. Armbrecht, L.
    et al.
    Dincer, C.
    Kling, A.
    Horak, Josef
    KTH, School of Biotechnology (BIO), Protein Technology. Department of Microsystems Engineering - IMTEK, University of Freiburg, Freiburg, Germany.
    Kieninger, J.
    Urban, G.
    Signal amplification using magnetic bead chains in microfluidic electrochemical biosensors2015In: 2015 Transducers - 2015 18th International Conference on Solid-State Sensors, Actuators and Microsystems, IEEE , 2015, 1601-1604 p.Conference paper (Refereed)
    Abstract [en]

    We present a novel approach to increase the sensitivity of microfluidic biosensor platforms using magnetic micro-bead chains. An almost 2-fold sensitivity enhancement is achieved by introducing a magnetic field gradient along a microfluidic channel by means of a soft-magnetic lattice with lattice spacings down to 100 μm. The magnetic field gradient induces self-assembly of the magnetic beads in chains or clusters and thus improves the active contact between analyte and beads. This facile strategy significantly increases the active bead surface while allowing for complete independence of traditional biosensor materials and channel geometries, chip-reusability and shortened measurement times. Bead chain properties were validated with optical microscopy in a glass capillary and with electrochemical measurements via glucose oxidase (GOx) labels on an integrated microfluidic chip fabricated in dry-film photo resist technology (DFR).

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