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- Bergkvist Sylvan, Sara, 1977-
(författare)
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Numerical studies of spin chains and cold atoms in optical lattices
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- An important, but also difficult, research field in condensed matter physics is that of strongly correlated systems. This thesis considers two topics in this field. The first topic is disorder and frustration in spin models. The introduction of disorder into quantum spin chains creates a complex problem. The ground state of the random-bond spin-1 Heisenberg chain is studied by means of stochastic series expansion quantum Monte Carlo simulation, applying the concept of directed loops. It is found that this system undergoes a phase transition to the random-singlet phase if the bond disorder is strong enough. Further a frustrated spin system is investigated. The frustration is introduced by having spins positioned on a triangular lattice. Performing a quantum Monte Carlo simulation for such a frustrated lattice leads to the occurrence of the infamous sign problem. This problem is investigated and it is shown that it is possible to use a meron cluster approach to reduce its effect for some specific models. The second topic concerns atomic condensates in optical lattices. A system of trapped bosonic atoms in such a lattice is described by a Bose-Hubbard model with an external confining potential. Using quantum Monte Carlo simulations it is demonstrated that the local density approximation that relates the observables of the unconfined and the confined models yields quantitatively correct results in most of the interesting parameter range of the model. Further, the same model with the addition that the atoms carry spin-1 is analyzed using density matrix renormalization group calculations. The anticipated phase diagram, with Mott insulating regions of dimerized spin-1 chains for odd particle density, and on-site singlets for even density is confirmed. Also an ultracold gas of bosonic atoms in an anisotropic two dimensional optical lattice is studied. It is found that if the system is finite in one direction it exhibits a quantum phase transition. The Monte Carlo simulations performed show that the transition is of Kosterlitz-Thouless type.
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| 3. |
- Rosengren, Anders
(författare)
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Pathophysiology and treatment of defective insulin secretion in diabetes
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Defective insulin secretion is a central feature in diabetes mellitus and results from reduced pancreatic beta-cell mass as well as aberrant beta-cell function. The pathophysiology of diabetes is incompletely known, but a strong hereditary component is suggested. This thesis investigates the genetic and cellular basis for impaired insulin release by employing a number of cell-physiological and biochemical techniques. In addition, different potential avenues to therapeutically improve beta-cell mass and function are explored. In a specifically designed congenic animal model of defective insulin secretion, we were able to identify two genetic loci on rat chromosome 1 that confer impaired beta-cell glucose metabolism and perturbed exocytosis of insulin-containing granules, respectively. The exocytotic defect was scrutinized in further detail and demonstrated to result from genetically encoded overexpression of the alpha2a-adrenoreceptor (adra2a). Adra2a mediates adrenergic inhibition of insulin secretion, and both pharmacological receptor antagonism and adra2a gene silencing accomplished full normalization of insulin release. This illuminates a novel disease mechanism for beta-cell dysfunction and introduces adra2a as a candidate gene for defective insulin secretion. Impaired insulin release can be enhanced by the insulinotropic hormones PACAP and GLP-1. Both peptides were demonstrated to enhance insulin release by almost 200 %. This was attributed to a 50 % increase in depolarization-evoked Ca influx as well as a direct potentiation of exocytosis. The hormones acted mainly by activation of protein kinase A, which is suggested to increase the recruitment of insulin granules for release. High hopes are attached to the use of stem cell therapy to correct decreased beta-cell mass in diabetes. In our study, transplanted adult bone marrow stem cells readily engrafted the pancreas. A reduction in blood glucose levels was observed, and this was paralleled by a two-fold increase in beta-cell replication. As there was no evidence for transdifferentiation of bone marrow cells into beta-cells, we suggest that the engrafted cells have an adjuvant function in the regulation of beta-cell mass. In conclusion, defective insulin secretion in diabetes involves distinct functional defects in beta-cell exocytosis and glucose metabolism, conferred by two different genetic subloci. The exocytotic defect results from genetically encoded overexpression of the alpha2a-adrenoreceptor. Insulin exocytosis can be potentiated by the peptide hormones PACAP and GLP-1, through activation of protein kinase A. Moreover, transplanted bone marrow cells stimulate replication of endogenous beta-cells and have beneficial effects on blood glucose.
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| 4. |
- Stagnell, Alexander, 1987-
(författare)
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The Ambassador's Letter : On the Less Than Nothing of Diplomacy
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The principal aim of this dissertation is to answer the age-old question What is diplomacy? But this study approaches the question in what might, on first look, appear oblique. By employing Slavoj Žižek’s reworked notion of Ideologiekritik with respect to the history, science, and artistic explorations of diplomacy, this work begins by extracting three of its essential problems: the name, death, and representation. A presentation of the central concepts and theoretical perspectives at play in Žižek’s work is elaborated upon, focusing on his understanding of politics, ideology, and the core of the conceptual apparatus of Lacanian psychoanalysis.The overarching argument of the thesis is that diplomacy constitutes an Ideological State Apparatus, i.e. that it offers a symbolic link destined to, through ideology, repress the fundamental inconsistencies of the modern nation state, in order to secure its continuous functioning. Diplomacy is shown to constitute the name that points to the impossibility of the state to become One with itself, and the thesis aims to capture how this impossibility, and that which must be excluded from the state to keep the fantasy of fulness alive, always returns to haunt it. Empirically, this is explored by way of re-reading the history of the word diplomacy, whose modern birth coincided with the events of the French Revolution. The study seeks to retrace diplomacy through three distinct historical formations, which here are referred to as the archi-political, ultra-political, and post-political. Each of theses formations functions as a covering over of the fundamental impossibility of the state. Readings of Immanual Kant, Henry James, André Brink, Mads Brügger, and 20th century IR-theory and Diplomacy Studies are presented in order to elaborate the way in which these formations are constituted as ideological fantasies protecting the state and, for that matter, diplomacy from their abyssal ground.
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