1. Separability of the Dirac equation in the STU black hole spacetime : Pairwise-equal charge case studyMirjam Cvetič, M. M. Stetsko, 2023, izvirni znanstveni članek Opis: We study the separability of the Dirac equation in the background of four dimensional charged rotating asymptotically flat black hole solution of supergravity and the low energy string theory known as STU black hole. In particular, we analyze in detail the separability conditions in the pairwise equal charge STU black hole space-time [M. Cvetič and D. Youm, Phys. Rev. D 54, 2612 (1996).]. While in the latter case the minimally coupled Dirac equation is not separable, the introduction of a specific torsion term ensures the separability. The source of the torsion is the Kalb-Ramond field, which is an integral part of string theory, but further aspects of its properties and coupling to fermionic fields remain to be studied. To derive the torsion, two different approaches are used in conformally related frames, showing that the torsion is not unique. The correspondingly modified Dirac equations in the Einstein and string frames are shown to be separable. Furthermore, the massless radial and angular wave equations are examined; they show close similarity with corresponding equations for the standard Kerr background. A generalization of the Teukolsky equation for the pairwise equal case is conjectured. We also briefly analyze a technically sophisticated radial equation in the massive case. Ključne besede: quantum fields of spacetime, supergravity, curved spacetime, Dirac equation Objavljeno v DKUM: 19.07.2024; Ogledov: 160; Prenosov: 4 Celotno besedilo (373,36 KB) Gradivo ima več datotek! Več... |
2. Statistics of limit root bundles relevant for exact matter spectra of F-theory MSSMsMartin Bies, Mirjam Cvetič, Mingqiang Liu, 2021, izvirni znanstveni članek Opis: In the largest, currently known, class of one quadrillion globally consistent F-theory Standard Models with gauge coupling unification and no chiral exotics, the vectorlike spectra are counted by cohomologies of root bundles. In this work, we apply a previously proposed method to identify toric base threefolds, which are promising to establish F-theory Standard Models with exactly three quark doublets and no vectorlike exotics in this representation. The base spaces in question are obtained from triangulations of 708 polytopes. By studying root bundles on the quark-doublet curve Cð3;2Þ1=6 and employing well-known results about desingularizations of toric K3 surfaces, we derive a triangulation independent lower bound Nˇ ð3Þ P for the number Nð3Þ P of root bundles on Cð3;2Þ1=6 with exactly three sections. The ratio Nˇ ð3Þ P =NP, where NP is the total number of roots on Cð3;2Þ1=6 , is largest for base spaces associated with triangulations of the eighth three-dimensional polytope Δ∘ 8 in the Kreuzer-Skarke list. For each of these Oð1015Þ threefolds, we expect that many root bundles on Cð3;2Þ1=6 are induced from F-theory gauge potentials and that at least every 3000th root on Cð3;2Þ1=6 has exactly three global sections and thus no exotic vectorlike quark-doublet modes. Ključne besede: astrophysics, compactification, string theory models, geometry, higher-dimensional field theories, mathematical physics, quantum fields in curved spacetime, string phenomenology, supersymmetric models, topology Objavljeno v DKUM: 16.10.2023; Ogledov: 310; Prenosov: 25 Celotno besedilo (444,13 KB) Gradivo ima več datotek! Več... |
3. Impact of curvature on nematic topological defectsLuka Mesarec, 2018, doktorska disertacija Opis: Topological defects (TDs) appear almost unavoidably in continuous symmetry breaking phase transitions. The topological origin makes their key features independent of systems' microscopic details; therefore TDs display many universalities. Because of their strong impact on numerous material properties and their significant role in several technological applications it is of strong interest to find simple and robust mechanisms controlling the positioning and local number of TDs. There are strong evidences that in physics the fields are fundamental entities of nature and not particles. If this is the case then topological defects (TDs) might play the role of fundamental particles. An adequate testing ground to study and gain fundamental understanding of TDs are nematic liquid crystals. We present a numerical study of TDs within effectively two dimensional closed soft films exhibiting in-plane orientational ordering. Popular examples of such class of systems are liquid crystalline shells and various biological membranes.
We analyze the impact of extrinsic and intrinsic curvature on positions of topological defects (TDs) in two-dimensional (2D) nematic films. We demonstrate that both these curvature contributions are commonly present and are expected to be weighted by comparable elastic constants. A simple Landau-de Gennes approach in terms of tensor nematic order parameter is used to numerically demonstrate impact of the curvatures on position of TDs on 2D ellipsoidal nematic shells. We introduce the Effective Topological Charge Cancellation mechanism controlling localised positional assembling tendency of TDs and the formation of pairs (defect,antidefect) on curved surfaces. Furthermore, we estimate a critical depinning threshold to form pairs (defect,antidefect) using the electrostatic analogy. Finally, we show how one could efficiently switch among qualitatively different structures by using a relative volume of ordered shells, which represents a relatively simple naturally accessible control parameter.
In doctoral thesis, we developed theoretical model of erythrocyte membrane by using a hybrid Helfrich-Landau type mesoscopic approach, taking into account in-plane membrane ordering. We demonstrate that the derived extrinsic membrane energy term, which strongly depends on the local orientations of the molecules, is essential for the predicted broadening of the range of the relative volumes corresponding to the stable discocyte shapes, which is otherwise very narrow if only intrinsic curvature energy term dominates. Ključne besede: Topological defects, Continuum fields, Nematic liquid crystals, Biological membranes, Nematic shells, Landau-de Gennes formalism, Topological charge, Nanoparticles, Gaussian curvature, Electrostatic analogy, Intrinsic curvature, Extrinsic curvature, Crystal growth nucleation, Relative
volume Objavljeno v DKUM: 09.03.2018; Ogledov: 2379; Prenosov: 240 Celotno besedilo (23,66 MB) |
4. The heterogeneous accentuation of economic content in vocational education : a starting point for occupation-specific human resource developmentChristoph Maus, 2015, izvirni znanstveni članek Opis: Background and Purpose: Since the successive introduction of “learning field“ oriented curricula in Germany, teaching at vocational schools as part of the dual system has been based on concrete actions. The underlying curricula describe business-oriented learning fields. The translating of these learning fields into pedagogical learning situations is the responsibility of the vocational curriculum conferences at the individual vocational school level, whereby con-siderable leeway is afforded them in these activities. This means that, despite there being an identical curricular basis, it is possible for individual schools to apply heterogeneous content emphases within the education programmes for commercial vocations. This study examines the correlation between such heterogeneous content with regard to the subject of economics and the person characteristics of the relevant teachers.
Design/Methodology/Approach: In order to determine the individual significance of eco-nomics curricula as part of the education programme for commercial occupations on a learning field basis, an online survey was conducted among commerce teachers. Descriptive statements about the significance of topics of content are generated with the help of the relevance ratings provided by the respondent teachers.
Results: The findings for the correlations indicate a relationship between the relevance of economics content and individual experience of teaching staff with regard to that particular academic discipline, because a high relevance rating correlates with individual tangencies within the teacher's educational biography
Conclusion: The shifting of lesson content design onto the individual vocational schools has resulted in human resource determinant factors coming into force. Apart from a (fully desira-ble) heterogeneity of content, these factors have also led to a decrease of economics content. Ključne besede: education, economic education, curriculum, learning fields, institutionalized human resource development Objavljeno v DKUM: 22.01.2018; Ogledov: 1033; Prenosov: 142 Celotno besedilo (721,66 KB) Gradivo ima več datotek! Več... |
5. Rate-dependent extensions of the parametric magneto-dynamic model with magnetic hysteresisSimon Steentjes, Martin Petrun, G. Glehn, Drago Dolinar, Kay Hameyer, 2017, izvirni znanstveni članek Opis: This paper extends the parametric magneto-dynamic model of soft magnetic steel sheets to account for the phase shift between local magnetic flux density and magnetic field strength. This phase shift originates from the damped motion of domain walls and is strongly dependent on the microstructure of the material. In this regard, two different approaches to include the rate-dependent effects are investigated: a purely phenomenological, mathematical approach and a physical-based one. Ključne besede: magnetic hysteresis, eddies, magnetic flux, viscosity, magnetic fields, magneto-dynamic models, parametric Objavljeno v DKUM: 03.08.2017; Ogledov: 1260; Prenosov: 430 Celotno besedilo (567,72 KB) Gradivo ima več datotek! Več... |
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8. Influence of a random field on particle fractionation and solidification in liquid-crystal colloid mixturesVlad Popa-Nita, Paul van der Schoot, Samo Kralj, 2006, izvirni znanstveni članek Opis: The influence of a random-anisotropy (RA) type disorder on the phase separation of nematogen-colloid mixtures is studied theoretically by combiningthe phenomenological Landau-de Gennes, Carnahan-Starling, and hard-sphere crystal theories. We assume that the colloids enforce the RA disorder on the surrounding thermotropic liquid-crystal (LC) molecules. We adopt the Imry-Ma argument according to which the lower-temperature phase exhibits a domain-type pattern. The colloids impose a finite degree of orientational ordering even in the isotropic (paranematic) phase. In the ordered phase they give rise to a domain-type structure, resulting in the distorted nematic (speronematic) phase. The RA field opposes the phase separation tendency. With increasing disorder the difference between the paranematic and speronematic ordering decreases. Consequently there is a critical disorder, above which both phases become identical from the orientation point of view, but have different concentrations of colloids. We have also estimated another characteristic value of disorder above which the isotropic phase can exist only in a liquid state, the crystal phase being suppressed completely. Ključne besede: liquid crystals, transitions, segregation, mixing, random fields, solidification, complex fluids Objavljeno v DKUM: 07.06.2012; Ogledov: 1519; Prenosov: 94 Povezava na celotno besedilo |
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