Aspekte starker Korrelationen in niedrigen Dimensionen

The challenging intricacies of strongly correlated electronic systems necessitate the use of a variety of complementary theoretical approaches. In this thesis, we analyze two distinct aspects of strong correlations and develop further or adapt suitable techniques. First, we discuss magnetization transport in insulating one-dimensional spin rings described by a Heisenberg model in an inhomogeneous magnetic field. Due to quantum mechanical interference of magnon wave functions, persistent magnetization currents are shown to exist in such a geometry in analogy to persistent charge currents in mesoscopic normal metal rings. The second, longer part is dedicated to a new aspect of the functional renormalization group technique for fermions. By decoupling the interaction via a Hubbard-Stratonovich transformation, we introduce collective bosonic variables from the beginning and analyze the hierarchy of flow equations for the coupled field theory. The possibility of a cutoff in the momentum transfer of the interaction leads to a new flow scheme, which we will refer to as the interaction cutoff scheme. Within this approach, Ward identities for forward scattering problems are conserved at every instant of the flow leading to an exact solution of a whole hierarchy of flow equations. This way the known exact result for the single-particle Green's function of the Tomonaga-Luttinger model is recovered.Diese Dissertation diskutiert in zwei Teilen unterschiedliche Aspekte stark korrelierter Elektronensysteme. Neben der Untersuchung physikalischer Fragestellungen werden dabei Vielteilchenmethoden neu entwickelt und angepasst. Im ersten Teil behandeln wir Magnetisierungstransport in eindimensionalen Spinringen, die durch ein Heisenbergmodell in einem inhomogenen Magnetfeld beschrieben werden. In Analogie zu Ladungsdauerströmen in mesoskopischen normalleitenden Metallringen können in einer solchen Geometrie aufgrund quantenmechanischer Interferenz der Magnon-Wellenfunktionen Spindauerströme auftreten. Der zweite Teil der Arbeit beschäftigt sich mit neuen Aspekten der funktionalen Renormierungsgruppe für Fermionen. Durch eine Entkopplung der Wechselwirkung mittels einer geeigneten Hubbard-Stratonovich-Transformation führen wir kollektive bosonische Felder ein und analysieren die Hierarchie von Flussgleichungen für die gekoppelte Feldtheorie. Die Möglichkeit eines Cutoffs im Impulsübertrag der Wechselwirkung führt zu einer neuen Technik, die wir als "Wechselwirkungs-Fluss" bezeichnen. In diesem Zugang sind Ward-Identitäten für Vorwärtsstreuung zu jedem Zeitpunkt des Renormierungsgruppenflusses gültig, und liefern eine exakte Lösung für eine komplette Hierarchie von Flussgleichungen. Auf diese Weise erhalten wir das bekannte exakte Ergebnis für die Einteilchen Greensche Funktion des Tomonaga-Luttinger Modells

Das Konkurrenzverhältnis der Unionsbürgerschaft zu den wirtschaftlichen Grundfreiheiten in der Rechtsprechung des EuGH

Die Rechtsprechung des Europäischen Gerichtshofs hat sich seit der Einführung der Unionsbürgerschaft durch den Vertrag von Maastricht wesentlich geändert. Ein vereintes Europa beruhte bis dahin auf dem Gedanken, basierend auf den vier Grundfreiheiten, eine ökonomische Zweckunion zu bilden. Der wirtschaftliche Konnex war ebenso unentbehrlich, wie auch hemmend, um ein vereintes Europa der Bürger zu schaffen. Der Status der Unionsbürgerschaft hat dies entscheidend geändert. In dieser Arbeit soll das Verhältnis der speziellen wirtschaftlichen Freizügigkeitsrechte, zum allgemeinen Recht nach Art 18 EGV, sich in der Union frei zu bewegen und aufzuhalten, untersucht werden. Diese rechtswissenschaftliche Untersuchung soll im ersten Teil der Arbeit durch eine Analyse der Unionsbürgerschaft, des Aufenthaltsrechts nach Art 18 EGV und des allgemeinen Diskriminierungsverbotes vorbereitet werden. In einer begrifflichen Analyse stellt sich heraus, dass der EuGH selbst Art 18 EGV wiederholt als Grundfreiheit bezeichnet. Weiters gelten für die traditionellen vier Grundfreiheiten einerseits und dem allgemeinen Freizügigkeitsrecht nach Art 18 EGV andererseits, ähnliche primär- und sekundärrechtliche Schranken. So scheinen auch die ordre- public- Vorbehalte in verwandter Weise zur Anwendung zu kommen. Der Grundsatz der Verhältnismäßigkeit spielt dabei jeweils eine bedeutende Rolle. Unterschiede zwischen den Freizügigkeitsrechten sind unter anderem im Stellenwert des grenzüberschreitenden Sachverhalts zu erkennen. Während im Bereich der Grundfreiheiten der übernationale Tatbestand präjudiziellen Charakter hat, trifft dies beim allgemeinen Freizügigkeitsrecht nur bedingt zu. Im Verlauf meiner Arbeit hat sich gezeigt, dass die speziellen Freizügigkeitsrechte derzeit vom EuGH grundsätzlich vorrangig behandelt werden. Gleichzeitig erstreckt sich der Anwendungsbereich des Art 18 EGV jedoch mittlerweile auf Bereiche, deren Regelung, bis zur Einführung der Unionsbürgerschaft, den Nationalstaaten vorbehalten war. Dies wird vor allem in sozioökonomischen Angelegenheiten offensichtlich und zeigt, welch politischen Sprengstoff diese Regelung mit sich bringt. Von Auffälligkeitswert ist auch, dass der EuGH in Fällen, in denen er sich auf die speziellen Freizügigkeitrechte ebenso wie auf Art 18 EGV berufen könnte, zunehmend auf das allgemeine Freizügigkeitsrecht zurückgreift. Deshalb komme ich zum Schluss, dass die vier traditionellen grundfreiheitlichen Regelungen nur mehr sekundär, und zwar dann zur Anwendung kommen, wenn der allgemeine Freizügigkeitsschutz nach Art 18 EGV (noch) nicht ausreicht. Wegen der zunehmend extensiven Interpretation des unionsbürgerschaftlichen Aufenthaltsrechts, wird der allgemeine Freizügigkeitsschutz in der Rechtsprechung des EuGH künftig wohl an Bedeutung verlieren

ARE YOU AWARE OF YOUR COMPETENCIES? – THE POTENTIALS OF COMPETENCE RESEARCH TO DESIGN EFFECTIVE SETA PROGRAMS

Since the late 1990s, security education training and awareness (SETA) programs have become commonplace. Despite extensive research into the effective design of such programs and factors influencing compliance behavior, SETA programs tend not to be as effective as they should be. In order to tailor learning content as closely as possible to individual needs, vocational education relies on the modeling and measurement of competencies. We argue that this existing knowledge can be transferred to the information security domain. Therefore, we introduce a competence model from vocational education and consider it in the context of the information security domain. Subsequently, we conduct a structured literature review on conceptualization and effective SETA design and investigate to what extent the competence dimensions from vocational education are already considered in the SETA literature. Our results indicate that competence research can make an important contribution to adapting SETA programs to individual situational actions

Chasing Cyber Security Unicorns: A Taxonomy-based Analysis of Cyber Security Start-ups’ Business Models

As the number of security incidents increases, a market is emerging for established and new providers of security measures. However, we lack an idea of the business models of cyber security start-ups, which are seen as innovation and security drivers, to protect the economy from existence-threatening incidents. Due to the intangible nature of the cyber threats that security solutions aim to address, previous research on business models cannot be fully transferred. We address this research gap by developing a taxonomy following Nickerson et al. (2013) based on 90 cyber security start-ups and performing a cluster analysis to understand the business activities of cyber security start-ups concerning the protection of critical infrastructures. Our taxonomy will benefit interested decision-makers such as CISOs who want to identify custom-fit cyber security solutions for their organizations. Furthermore, investors and cyber security providers understand the market holistically and can identify innovative product approaches to adopt themselves

Effective spin-wave action for ordered Heisenberg antiferromagnets in a magnetic field

We derive the effective long-wavelength Euclidean action for the antiferromagnetic spin-waves of ordered quantum antiferromagnets subject to a uniform magnetic field. We point out that the magnetic field dependence of the spin-wave dispersion predicted by the usual O(3)-quantum nonlinear sigma model disagrees with spin-wave theory. We argue that the nonlinear sigma model does not take into account all relevant spin-wave interactions and derive a modified effective action for the long-wavelength spin-waves which contains an additional quartic interaction. At zero temperature the corresponding vertex is relevant in the renormalization group sense below three dimensions.Comment: 4 pages, 1 eps figure; proceedings of the symposium on "Spin- and charge-correlations in molecule-based materials", October 2005, Koenigstein (Taunus), German

Believing and Perceiving: Authorship Belief Modulates Sensory Attenuation

Sensory attenuation refers to the observation that self-generated stimuli are attenuated, both in terms of their phenomenology and their cortical response compared to the same stimuli when generated externally. Accordingly, it has been assumed that sensory attenuation might help individuals to determine whether a sensory event was caused by themselves or not. In the present study, we investigated whether this dependency is reciprocal, namely whether sensory attenuation is modulated by prior beliefs of authorship. Participants had to judge the loudness of auditory effects that they believed were either self-generated or triggered by another person. However, in reality, the sounds were always triggered by the participants' actions. Participants perceived the tones' loudness attenuated when they believed that the sounds were self-generated compared to when they believed that they were generated by another person. Sensory attenuation is considered to contribute to the emergence of people's belief of authorship. Our results suggest that sensory attenuation is also a consequence of prior belief about the causal link between an action and a sensory change in the environment

Bridging the Gap between Security Competencies and Security Threats: Toward a Cyber Security Domain Model

Security incidents are increasing in a wide range of organizational types and sizes worldwide. Although various threat models already exist to classify security threats, they seem to take insufficient account of which organizational assets the threat events are targeting. Therefore, we argue that conducting more job-specific IT security training is necessary to ensure organizational IT security. This requires considering which assets employees use in their daily work and for which threat events employees need to build up IT security competencies. Subsequently, we build a framework-based Cyber Security Domain Model (CSDM) for IT-secure behavior. We follow the Evidence Centered Assessment Design (ECD) to provide a deep- dive analysis of the domain for IT-secure behavior. As the leading result relevant for research and practice, we present our CSDM consisting of 1,087 cyber threat vectors and apply it to five job specifications

Landau functions for non-interacting bosons

We discuss the statistics of Bose-Einstein condensation (BEC) in a canonical ensemble of N non-interacting bosons in terms of a Landau function L_N^{BEC} (q) defined by the logarithm of the probability distribution of the order parameter q for BEC. We also discuss the corresponding Landau function for spontaneous symmetry breaking (SSB), which for finite N should be distinguished from L_N^{BEC}. Only for intinite N BEC and SSB can be described by the same Landau function which depends on the dimensionality and on the form of the external potential in a surprisingly complex manner. For bosons confined by a three-dimensional harmonic trap the Landau function exhibits the usual behavior expected for continuous phase transitions.Comment: 4 pages, 4 figures; final version to appear as a rapid communication in Physical Review A. Abstract modified and typos correcte

Correcting for the missing rich: An application to Wealth Survey Data

It is a well-known criticism that if the distribution of wealth is highly concentrated, survey data are hardly reliable when it comes to analyzing the richest parts of society. This paper addresses this criticism by providing a general rationale of the underlying methodological problem as well as by proposing a specific methodological approach tailored to correcting the arising bias. We illustrate the latter approach by using Austrian data from the Household Finance and Consumption Survey. Specifically, we identify suitable parameter combinations by using a series of maximum-likelihood estimates and appropriate goodness-of-fit tests to avoid arbitrariness with respect to the fitting of the Pareto distribution. Our results suggest that the alleged non-observation bias is considerable, accounting for about one quarter of total net wealth in the case of Austria. The method developed in this paper can easily be applied to other countries where survey data on wealth are available