<p>Thin metal films have found their use in many magnetic devices. They form pseudo two-dimensional systems, where the mechanisms for the magnetic interactions between the layers are not completely understood. Layered crystal structures have an advantage over such artificial systems, since the layers can be strictly mono-atomic without any unwanted admixture. In this study, some model systems of layered magnetic crystal structures and their solid solutions have been investigated by x-ray and neutron diffraction, Mössbauer and electron spectroscopy, heat-capacity and magnetic measurements, and first-principle electronic structure calculations, with the goal of deepening our understanding through controlled chemical synthesis.</p><p>The compounds TlCo₂S₂, TlCo₂Se₂ and their solid solution TlCo₂Se_2-xS_x, all containing well separated cobalt atom sheets, order with the moments ferromagnetically aligned within the sheets. In TlCo₂S₂, the net result is ferromagnetism, while TlCo₂Se₂ exhibits antiferromagnetism. The inter-layer distance is crucial for the long-range coupling, and it was varied systematically through Se-S substitution. The incommensurate helical magnetic structure found for TlCo₂Se₂ (x = 0) prevails in the composition range 0 ≤ x ≤ 1.5 but the pitch of the helix changes. The accompanying reduction in inter-layer distance on sulphur substitution varies almost linearly with the coupling angle of the helix. An additional competing commensurate helix (90°) appears in the medium composition range (found for x = 0.5 and 1.0).</p><p>The systems TlCo_2-xMe_xSe₂ show helical magnetic ordering for Me = Fe or Cu, while a collinear antiferromagnetic structure occurs for Me = Ni. Magnetic order is created by iron substitution for copper in the Pauli paramagnetic TlCu₂Se₂, but now with the moments perpendicular to the metal sheets.</p><p>TlCrTe₂ forms a quite different crystal structure, with intra-layer ferromagnetic alignment and net collinear antiferromagnetism. In contrast to the other phases, the values of the moments conform well to a localised model for Cr³⁺.</p>

In this thesis point defects in semiconductors are studied by electronic structure calculations. Results are presented for the stability and equilibrium concentrations of native defects in GaP, InP, InAs, and InSb, for the entire range of doping conditions and stoichiometry. The native defects are also studied on the (110) surfaces of InP, InAs, and InSb. Comparing the relative stability at the surface and in the bulk, it is concluded that the defects have a tendency to migrate to the surface. It is found that the cation vacancy is not stable, but decomposes into an anion antisite-anion vacancy complex. The surface charge accumulation in InAs is explained by complementary intrinsic doping by native defects and extrinsic doping by residual hydrogen. A technical investigation of the supercell treatment of defects is performed, testing existing correction schemes and suggesting a more reliable alternative. It is shown that the defect level of [2VCu-IIICu] in the solarcell-material CuIn1-xGaxSe2 leads to a smaller band gap of the ordered defect γ-phase, which possibly explains why the maximal efficiency for CuIn1-xGaxSe2 has been found for x=0.3 and not for x=0.6, as expected from the band gap of the α-phase. It is found that Zn diffuses via the kick-out mechanism in InP and GaP with activation energies of 1.60 eV and 2.49 eV, respectively. Explanations are found for the tendency of Zn to accumulate at pn-junctions in InP and to why a relatively low fraction of Zn is found on substitutional sites in InP. Finally, it is shown that the equilibrium solubility of dopants in semiconductors can be increased significantly by strategic alloying. This is shown to be due to the local stress in the material, and the solubility in an alloy can in fact be much higher than in either of the constituting elements. The equilibrium solubility of Zn in Ga0.9In0.1P is for example five orders of magnitude larger than in GaP or InP.

In the last decades NMR spectroscopy has become an invaluable tool both in academic research and in the pharmaceutical industry. This thesis describes applications of NMR spectroscopy in biomedicinal research for structure elucidation of biologically active peptides and peptidomimetics as well as in studies of ligand-protein interactions. The first part of this thesis describes the theory and methodology of structure calculations of peptides using experimental restraints derived from NMR spectroscopy. This methodology has been applied to novel mimetics of the peptide hormones desmopressin and Leu-enkephalin. The results of these studies highlight the complicating issue of conformational exchange often encountered in structural determination of peptides and how careful analysis of experimental data as well as optimization of experimental conditions can enable structure determinations in such instances. Although the mimetics of both desmopressin and Leu-enkephalin were found to adopt the wanted conformations, they exhibited no or very poor biological activity. These results demonstrate the difficulties in designing peptidomimetics without detailed structural information of the receptors. A stereoselective synthetic route towards XxxΨ[CH2O]Ala pseudodipeptides is also presented. Such pseudodipeptides can be used as isosteric amide bond replacements in peptides in order to increase their resistance towards proteolytic degradation. The second part of this thesis describes the study of the interaction between compounds that inhibit pilius assembly, pilicides, and periplasmic chaperones from uropathogenic Escherichia coli. Periplasmic chaperones are key components in assembly of pili, i.e. hair-like protein complexes located on the surface of Escherichia coli that cause urinary tract infections. Detailed knowledge about this interaction is important in understanding how pilicides can inhibit pilus assembly by binding to chaperones. Relaxation-edited NMR experiments were used to confirm the affinity of the pilicides for the chaperones and chemical shift mapping was used to study the pilicide-chaperone interaction surface. These studies show that at least two interaction sites are present on the chaperone surface and consequently that two different mechanisms resulting in inhibition of pilus assembly may exist.

Thin metal films have found their use in many magnetic devices. They form pseudo two-dimensional systems, where the mechanisms for the magnetic interactions between the layers are not completely understood. Layered crystal structures have an advantage over such artificial systems, since the layers can be strictly mono-atomic without any unwanted admixture. In this study, some model systems of layered magnetic crystal structures and their solid solutions have been investigated by x-ray and neutron diffraction, Mössbauer and electron spectroscopy, heat-capacity and magnetic measurements, and first-principle electronic structure calculations, with the goal of deepening our understanding through controlled chemical synthesis. The compounds TlCo2S2, TlCo2Se2 and their solid solution TlCo2Se2-xSx, all containing well separated cobalt atom sheets, order with the moments ferromagnetically aligned within the sheets. In TlCo2S2, the net result is ferromagnetism, while TlCo2Se2 exhibits antiferromagnetism. The inter-layer distance is crucial for the long-range coupling, and it was varied systematically through Se-S substitution. The incommensurate helical magnetic structure found for TlCo2Se2 (x = 0) prevails in the composition range 0 ≤ x ≤ 1.5 but the pitch of the helix changes. The accompanying reduction in inter-layer distance on sulphur substitution varies almost linearly with the coupling angle of the helix. An additional competing commensurate helix (90°) appears in the medium composition range (found for x = 0.5 and 1.0). The systems TlCo2-xMexSe2 show helical magnetic ordering for Me = Fe or Cu, while a collinear antiferromagnetic structure occurs for Me = Ni. Magnetic order is created by iron substitution for copper in the Pauli paramagnetic TlCu2Se2, but now with the moments perpendicular to the metal sheets. TlCrTe2 forms a quite different crystal structure, with intra-layer ferromagnetic alignment and net collinear antiferromagnetism. In contrast to the other phases, the values of the moments conform well to a localised model for Cr3+.

Computational approaches play an important role in today's materials science owing to the remarkable advances in modern supercomputing architecture and algorithms. Ab initio simulations solely based on a quantum description of matter are now very able to tackle materials problems in which the system contains up to a few thousands atoms. This dissertation aims to address the modern electronic structure calculation methods applied to a range of various materials such as liquid and amorphous phase materials, nanostructures, and small organic molecules. Our simulations were performed within the density functional theory framework, emphasizing the use of real-space ab initio pseudopotentials. On the first part of our study, we performed liquid and amorphous phase simulations by employing a molecular dynamics technique accelerated by a Chebyshev-subspace filtering algorithm. We applied this technique to find l- and a- SiO₂ structural properties that were in a good agreement with experiments. On the second part, we studied nanostructured semiconducting oxide materials, i.e., SnO₂ and TiO₂, focusing on the electronic structures and optical properties. Lastly, we developed an efficient simulation method for non-contact atomic force microscopy. This fast and simple method was found to be a very powerful tool for predicting AFM images for many surface and molecular systems. / text

Electronic structure calculations constitute a valuable tool to predict the properties of materials. In this study we propose an efficient scheme to study correlated electron systems with essentially only one free parameter, the screening length of the Coulomb potential. A general reformulation of the exchange energy of the correlated electron shell is combined with this method in order to analyze the calculations. The results are interpreted in terms of different polarization channels, due to different multipoles. The method is applied to various actinide compounds, in order to increase the understanding of the complicate behaviour of 5f electrons in these systems. We studied the non-magnetic phase of δ-Pu, where the spin polarization is taken over by a spin-orbit-like term that does not break the time reversal symmetry. We also find that a non-trivial high multipole of the magnetization density, the triakontadipole, constitutes the ordering parameter in the mysterious hidden order phase of the heavy-fermion superconductor URu2Si2. This type of multipolar ordering is also found to play an essential role in the hexagonal-based superconductors UPd2Al3,  UNi2Al3 and UPt3 and in the dioxide insulators UO2, NpO2 and PuO2. The triakontadipole moments are also present in all magnetic actinides we considered, except for Cm. These results led us to formulate a new set of rules for the ground state of a system, that are valid in presence of strong spin-orbit coupling interaction instead of those of Hund; the Katt's rules. Finally, we applied our method to a new class of high-Tc superconductors, the Fe-pnictides, where the Fe 3d electrons are moderately correlated. In these materials we obtain the stabilization of a low spin moment solution, in agreement with experiment, over a large moment solution, due to the gain in exchange energy in the formation of large multipoles of the spin magnetization density. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 705

A termodinâmica computacional é uma ferramenta capaz de fornecer informações básicas sobre soluções e concentração de seus constituintes, em um dado sistema complexo a uma certa temperatura e pressão. Nos últimos anos, a união entre a teoria do funcional da densidade e a termodinâmica computacional tem renovado o estudo dos materiais intermetálicos ordenados, uma vez que os cálculos de estrutura eletrônica de primeiros princípios são hoje capazes de proporcionar resultados extremamente precisos para as energias de formação de compostos estequiométricos. Dentro deste contexto, investigamos os diagramas de fases dos sistemas Fe-Al, Mo- Fe, Cr-Al, Fe-Cr, Fe-Cr-Al na estrutura cúbica de corpo centrado utilizando o método Full-Potential Linear Augmented Plane Waves(FP-LAPW) aliado ao Método Variacional de Clusters(CVM) na aproximação do tetraedro irregular. Através do método FP-LAPW determinamos a energia total de configurações cristalinas dos sistemas cúbicos de corpo centrado. Esses valores são utilizados como parâmetros de entrada do CVM para a determinação do potencial termodinâmico do sistema em suas diferentes fases e os correspondentes equilíbrios entre essas fases em função da composição e da temperatura (diagrama de fases). Embora o Fe-Al tenha grande interesse tecnológico, o seu comportamento magnético é bastante complicado. A inclusão de Cr na liga de Fe-Al tem um alto interesse industrial na utilização desta ligas em altas temperaturas, mas o banco de dados referentes as caracteristicas estruturais e eletrônicas são escassos. Neste contexto de aplicações procuramos investigar as características estruturais e eletrônicas buscando correlacionar as informações experimentais com as obtidas via cálculo de estrutura eletrônica e o CVM. / Thermodynamics is a computational tool capable of providing information basics solutions and concentrations of constituents in a given complex system at a given temperature and pressure. In recent years, the union between the density functional theory and computational thermodynamics has renewed study of ordered intermetallic materials, since the electronic structure calculations from first principles are now able to provide extremely accurate results for the energies of formation of stoichiometric compounds. in this context, we investigate the phase diagrams of the systems Fe-Al, Fe- Mo, Cr-Al, Fe-Cr, Fe-Cr-Al in the body-centered cubic structure using the method Full-Potential Linear Augmented Plane Waves (FP-LAPW) ally the clusters variational method(CVM) in approximation of the tetrahedron irregular. Through the FP-LAPW method determines the total energy of the crystal with configurations body centered cubic systems. These values are used as input parameters of the CVM for the determination of the thermodynamic potential of the system in its different phases and the corresponding equilibrium between these phases as a function of composition and temperature (phase diagram). Although the Fe-Al has a great interest technology, its magnetic behavior is quite complicated. The inclusion of Cr in Fe-Al alloy has a high industrial interest in using this alloy in high temperatures, but the database regarding the structural characteristics and electronics are scarce. In this context we seek to investigate the application structural and electronic characteristics correlate the information seeking experiments with those obtained via the electronic structure calculation and the CVM.

Neste trabalho realizamos um estudo do comportamento do momento magnético e do deslocamento isomérico de uma impureza intersticial de Fe em hospedeiros metálicos de valências 4 (Ti, Zr), 3 (Sc, Y). Investigamos também impurezas intersticiais e substitucionais de Mo e Fe em hospedeiros divalentes. Para realizar este estudo fizemos cálculos de estrutura eletrônica utilizando o RS-LMTO-ASA (\"Real Space - Linear Muffin-Tin Orbital - Atomic Spherical Approximation\"), um método de primeiros princípios dentro da aproximação do funcional densidade local, implementado no espaço real. Calculamos o momento magnético no sítio da impureza nos sistemas acima e constatamos que a impureza intersticial de Fe é não magnética nos hospedeiros de valências 4 e 3, e que tanto as impurezas intersticiais como as substitucionais podem apresentar momento magnético nos hospedeiros divalentes. Mostramos que para os sistemas divalentes o momento magnético depende fortemente da relaxação. Os nossos resultados são explicados através de um modelo simples, baseado no modelo de Wolff. Investigamos também o comportamento do deslocamento isomérico no sítio da impureza de Fe nesses vários sistemas. Constatamos que os nossos resultados concordam razoavelmente bem com os dados experimentais e explicam o comportamento das tendências observadas. / In the present work, we have studied the magnetic moments and the behavior of the isomer shift at the interstitial Fe impurity site in Ti, Sc, Zr and Y hosts. We have also investigated interstitial and substitutional Fe and Mo impurities in Ca, Sr and Yb hosts. To perform the calculations, we have used the RS-LMTO-ASA scheme, a first principles method, within the local spin density approximation, implemented in real space. We calculated the magnetic moments at the impurity site in the above systems and all the substitucional impurities are found to be magnetic. The results show that interstitial Fe is non-magnetic in the tri- and tetravalent hosts, but interstitial Fe and Mo impurities could develop local magnetic moment in divalent hosts. \'We show that the magnetic moment at the impurity site in these divalent hosts is strongly dependent on lattice relaxation. The results can be explained using simple arguments based on Wolff model. We have investigated in a systematic way the behavior of the isomer shift of Fe impurities in these systems. We observed that our results are in generally good agreement with experiment and lead to better understanding of the observed trends in terms of the volume occupied by the Fe in each host.

Durant l'irradiation en réacteur la fission des atomes d'actinides entraine la création de grandes quantités de défauts, qui affecte les propriétés physiques et chimiques des matériaux dans le réacteur, en particulier les matériaux combustibles ou de structure. Une des méthodes non destructives pouvant être utilisées pour caractériser les défauts induits par irradiation, vides ou contenant les produits de fission, est la spectroscopie d'annihilation de positons (SAP). Cette technique expérimentale consiste à détecter le rayonnement généré lors de l'annihilation du paire électron-positon dans un échantillon et en déduire les propriétés de la matière étudiée. Les positons peuvent être piégés dans les défauts de type lacunaire dans les solides, et en mesurant leur temps de vie et les distribution de moment du rayonnement d'annihilation, on peut obtenir des informations sur les volumes libres et les environnements chimiques des défauts. Dans ce travail, des calculs de structure électronique des caractéristiques d'annihilation de positons ont été effectués en utilisant la théorie de la fonctionnelle de la densité à deux composants (TCDFT). Pour calculer les distributions de moment rayonnement d'annihilation, nous avons implémenté les méthodes nécessaires dans le code de calcul libre ABINIT. Les résultats théoriques ont été utilités pour contribuer à l'identification des défauts d'irradiation dans deux céramiques nucléaires, le carbure de silicium (SiC) et le dioxyde d'uranium (UO2). / During in-pile irradiation the fission of actinide nuclei causes the creation of large amounts of defects, which affect the physical and chemical properties of materials inside the reactor, in particular the fuel and structural materials. Positron annihilation spectroscopy (PAS) can be used to characterize irradiation induced defects, empty or containing fission products. This non-destructive experimental technique involves detecting the radiation generated during electron-positron annihilation in a sample and deducing the properties of the material studied. As positrons get trapped in open volume defects in solids, by measuring their lifetime and momentum distributions of the annihilation radiation, one can obtain information on the open and the chemical environments of the defects. In this work electronic structure calculations of positron annihilation characteristics were performed using two-component density functional theory (TCDFT). To calculate the momentum distributions of the annihilation radiation, we implemented the necessary methods in the open-source ABINIT program. The theoretical results have been used to contribute to the identification of the vacancy defects in two nuclear ceramics, silicon carbide (SiC) and uranium dioxide (UO2).

The goal of solid state physics and chemistry is to gain deeper understanding of the basic principles of condensed matter. This ongoing process is achieved by the combination of experimental methods and theoretical models. One theoretical approach are the so-called first-principles calculations, which are based on the concept of density functional theory (DFT). In order to test the reliability of a band structure calculation, its results have to be compared with experiments. Since the electron density, the main constituent of DFT codes, cannot be directly determined experimentally with sufficient accuracy (e.g., by X-ray diffraction), other experimentally available properties are needed for the comparison with the calculation. A quantity that can be measured with high accuracy and that provides indirect information about the electron density is the electric field gradient (EFG). The EFG reflects local structural symmetry properties of the charge distribution surrounding a nucleus: the EFG is nonzero if the density deviates from cubic symmetry and therefore generates an inhomogeneous electric field at the nucleus. Since the EFG is highly sensitive to structural parameters and to disorder, it is a valuable tool to extract structural information. Furthermore, the evaluation of the EFG can provide valuable insight into the chemical bonding. Whereas the experimental determination of the quadrupole frequency and the closely related EFG has been possible for more than 70 years, reliable values for calculated EFGs could not be obtained before 1985, when an EFG module was implemented in the full-potential, linearised-augmented-plane-wave code WIEN. Since the full-potential local-orbital minimum-basis scheme FPLO is numerically very efficient and its local-orbital scheme allows an easy analysis of the different contributions to the EFG, one goal of this work was the implementation of an EFG module within the FPLO code. The newly implemented EFG module was applied to different systems: starting from simple metals, then approaching more complex systems and finally tackling strongly correlated oxides. Simultaneously, the EFGs for the studied compounds were determined experimentally by NMR spectroscopists. This close collaboration enables the comparison of the calculated EFGs with the experimental observations, which makes it possible to extract more physical and chemical information from the measured values regarding structural relaxation, distortion, the chemical bond or the relevance of electron correlation. In the last part of this work, the importance of corrections that go beyond the EFG are discussed. Such corrections arise for any multipole order of the hyperfine interactions, and are due to electron penetration into the nucleus. A correction similar to the isomer shift, coined here the &amp;quot;quadrupole shift&amp;quot; is examined in detail.