Theory Seminar

Våren 2010

  • 9.6. Jens Andersen:
    A flavor of two-color QCD: What QCD-like theories can tell us about the world

    Lattice calculations in QCD cannot be performed at finite baryon chemical potential. One therefore studies QCD-like theories such as two-color QCD where one can. This opens up the possibility of a direct test of model calculations in hot and dense matter. In this talk I will discuss the phase diagram of two-color QCD as a function of baryon and isospin chemical potential using effective models. Differences between real (three-color) QCD and QCD-like theories will be highlightened.

  • 31.5. Arturo Amador:
    Geometrical methods in the modeling of physical and biological systems

    The relativistic particle remains to be one of the most interesting dynamical systems to investigate if one wishes to understand some aspects of physics at a fundamental level. One of the many reasons for this, is that the theory of the relativistic particle possesses many characteristics present among objects of higher dimension in the ambit of relativistic extended objects and even in general relativity. Concerning biological systems, a fascinating and still unsolved topic challenging physics, biochemistry, and even geometry is the presence of highly regular motifs such as the alpha helixes and beta sheets in the folded states of biopolymers and proteins. Differential geometry provides us with a direct relationship between physics and mathematics and therefore it is possible to apply a mathematical analysis to this problem. In this talk we review some of the geometrical methods used in the modeling of the physical and biological systems mentioned above, ranging from relativistic particles to relativistic extended objects in the field of theoretical physics, and from polymer chains to the Golgi complex on the field of biology.

  • 10.5. Gaetano Annunziata:
    Charge and spin transport through spin bandwidth asymmetry ferromagnet/insulator/singlet superconductor junctions

    Charge and spin transport through ballistic ferromagnet/insulator/singlet superconductor junctions are analyzed by means of the Bogoliubov-de Gennes equations. The possibility for ferromagnetism in the first electrode to be driven by a mass renormalization of oppositely polarized carriers, i.e. a spin bandwidth asymmetry, rather than by a rigid splitting of up- and down-spin electron bands as in a standard Stoner ferromagnet, is taken into account. Differences in transport features for the two kind of magnetic eletrodes are analyzed for conventional (s-wave) and unconventional (pure d-wave, time reversal breaking d-wave) order parameter symmetries in the superconducting one. It is shown how these differences can be useful to get information about the mechanism of ferromagnetism in the first electrode and order parameter symmetry in the second one. Circumstances under which a spin bandwidth asymmetry ferromagnet can support a larger spin current compared to a standard Stoner ferromagnet are highlighed.

  • 3.5. Lars Kyllingstad:
    Probing the phase diagram of the universe with QCD-like models

    I will give an overview of our current understanding of the phase diagram of the strong interaction, and outline some of the difficulties one encounters when trying to investigate it by direct use of QCD. Then I will describe a selection of simpler field theories that can be used to model various aspects of QCD, and describe how and why they can provide a better understanding of the phase diagram.

  • 19.4 Marius Solberg:
    On the role of electroweak bremsstrahlung for indirect dark matter signatures

    For indirect dark matter (DM) searches, knowing the standard model final state products of their annihilation or decay is important to optimize the detection strategy. Usually, only states accessible at tree level are considered in applications, with the few exceptions of the loop-suppressed gamma-ray lines or the electromagnetic bremsstrahlung process (especially important when two-body channels are suppressed). For TeV mass scale candidates engineered to produce only leptons in final states, we show here that this restriction may lead to severely biased results due to the neglected role of the (unavoidable) $Z$ and $W$ boson radiation. In particular, we work out the consequences of including these processes for two situations: i) The case where tree level annihilation final states are electrons. The latter case is typical of many recently popular models trying to fit cosmic ray e^- and e^+ data, especially the excess of e^+ in the 10-90 GeV range observed in the PAMELA experiment. ii) The idealized case where DM annihilates at tree level with 100% branching ratio into neutrinos. For a given cross section, this leads eventually to ``minimal yields'' of photons, electrons, positrons and antiprotons. Both the spectral shapes and the multi-messenger signatures can be significantly modified with respect to results presented in the literature.

  • 22.3. Kenate Nigussa:
    Electronic structure based on DFT calculation

    I will talk briefly on the theoretical background of the density functional theory field and how information is extracted from calculated results for systems of interest. Then next I will present some calculation results from the ongoing project as part of example of electronic structure in the titanium-nickelide system."

  • 11.3. Zlatko Tesanovic:
    Superconductivity at Dawn of the Iron Age

    Recent discovery of iron-based high temperature superconductors hints at a new pathway to the room temperature superconductivity. The new materials feature FeAs layers instead of the signature CuO2 planes of much studied cuprate superconductors. The antiferromagnetism also appears to be involved, although the d-electrons in FeAs seem considerably more mobile than their cuprate cousins. This high mobility, facilitated by a large overlap between atomic orbitals of Fe and As, plays a crucial role in warding off Hund's rule and the large local moment magnetism of Fe ions, the archrival of superconductivity. A pedagogical review of the current status of the field will be presented, highlighting similarities and differences between Fe-pnictides and cuprates and emphasizing the importance of the multiband nature of magnetism and superconductivity in these new materials.

  • 22.2. Tomas Brauner:
    Confinement, chiral symmetry breaking, and Bose-Einstein condensation in QCD-like theories

    I will discuss the phase diagram of QCD-like theories - two-color QCD and QCD with adjoint quarks - using the phenomenological Nambu--Jona-Lasinio model augmented with the effective field for the Polyakov loop. The topics covered include: relation of confinement and chiral symmetry breaking, critical behavior near the deconfinement phase transition, and Casimir scaling of the Polyakov loops. In the end, I will suggest an explanation of recent lattice data for the thermodynamics of dense two-color QCD.

  • 15.2. Jacob Linder:
    Hybrid structures of ferromagnets and superconductors: Spin-active interfaces and inhomogeneous magnetization textures

    An introduction to proximity-effects in hybrid structures of ferromagnets and superconductors is provided. In particular, we focus on recent developments in the field such as the role of spin-active interfaces and inhomogeneous magnetization textures and highlight a few key research results.

  • 8.2. Askhat Gazizov:
    Cosmic rays in magnetic fields - between rectilinear and diffusion regime

    The general problem of superlumial signal in solutions of parabolic partial differential equations, such as heat and diffusion equations, is demontrated for the case of ultrahigh energy cosmic rays propagating through intergalactic magnetic fields. In the lack of exact relativistic diffusion equation, a phenomenological approach based on the generalization of Juettner propagator is proposed. The propagator coincides with the rectilinear one at highes energies and for weak magnetic fields and reproduces the diffusion one at lower energies and for strong magnetic fields. The natural parameter measuring the rate of 'diffusivity' of propagation is derived. Calculated cosmic ray spectra smoothly connect earlier found solutions for both extreme cases.