Spring 2014 Seminar Series

Organizer: Dave Soper

Seminars are at 4pm, Tuesdays, in 472 Willamette Hall and are preceded by cookies at 3:45.

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1-Apr-14 Vortices and A Depinning Transition In Magnetic Superconductors: The example of ErNi B C
Lev Boulaevski
Los Alamos National Laboratory

In borocarbide, ErNi2B2C, the phase transition to the commensurate spin-density wave at 2.3 K leaves 1/20 part of Ising-like Er spins practically free. Vortices polarize these spins nonuniformly and repolarize them when moving. At a low spin relaxation rate, and at low bias currents, vortices carrying magnetic polarization clouds become polaron-like and their velocities are determined by the effective drag coefficient which is significantly bigger than the Bardeen-Stephen (BS) one. As the current increases, at a critical current Jc the vortices release polarization clouds and the velocity as well as the voltage in the I-V characteristics jump to values corresponding to the BS drag coefficient. The nonuniform components of the magnetic field and the magnetization drop as the velocity increases, resulting in weaker polarization and a discontinuous dynamic dissociation depinning transition. As the current decreases, on the way back, vortices are retrapped by polarization clouds at a current Jr < Jc.
Dietrich Belitz
8-Apr-14 Supernovae New and Old: A Primer and Discussion of a Recent Event in M82
Scott Fisher
University of Oregon, Physics

In mid-January 2014 a supernova in the nearby galaxy M82 was discovered by a team of undergraduate students observing with a small telescope in central London. Initial analyses showed that the so named SN2014J is the closest Type-1a supernova in the last 42 years. Given its ‘nearby’ distance of only 15 million light-years, SN2014J has been a boon for both professional and amateur astronomers around the globe. The professional community is studying the event in unprecedented detail with large 8-10 meter telescopes, while amateurs are observing the object with small telescopes and even binoculars as its brightness peaked at a magnitude of ~11. In this informal talk I will give an overview of supernovae, discuss their importance in modern astrophysics, and talk about SN2014J, including the imaging of it that was completed at our own Pine Mountain Observatory.
15-Apr-14 Renormalization of Jamming Transition In Frictionless Spheres
Eric Corwin
University of Oregon, Physics

Packing problems are ubiquitous, ranging from oil extraction through porous rocks to grain storage in silos and the compaction of pharmaceutical powders into tablets. At the "jamming" density, particulate systems pack into a mechanically stable and amorphous jammed state. Because of this onset of mechanical stability jamming has traditionally been studied as a mechanical phenomenon and characterized with mechanical order parameters. However, the mechanical vacuum of systems below jamming is surprisingly rich in structure. Using geometric quantities derived from the Voronoi tessellation we report on the discovery of a new phase transition preceding the mechanical jamming transition. This phase transition corresponds to the appearance of a new kind of symmetry hidden in the shape of the Voronoi cells. We characterize this symmetry by looking at properties related to the maximum inscribed sphere in each cell, moments of the volume distribution of cells, and the aspect ratios of cells. Each contains a very different signature of the jamming transition with various scaling laws. We offer several possible routes towards renormalization of this system and discuss whether a field theory could be made to explain the various phases.
None This Week

6-May-14 An Example of the Application of Theory to Astronomy: Star Formation History of Galaxies
Jim Schombert
University of Oregon, Physics

The study of the star formation history in galaxies is closely linked to one of the more successful theoretical accomplishments in astronomy, stellar evolution. I report on a constructive interface between theory and observation to unravel the history of star formation in galaxies, and how these techniques lead to a new form of discovery science through the use of digital sky surveys.
13-May-14 On Planet Formation
Jim Imamura
University of Oregon, Physics

To Be Announced
20-May-14 On the Nature of Singularities in Cosmological Solutions of Einstein's Equations
Jim Isenberg
University of Oregon, Mathematics

The Hawking-Penrose theorems tell us that cosmological solutions of Einstein's equations are generally singular, in the sense of the incompleteness of causal geodesics (the paths of physical observers). These singularities might be marked by the blowup of curvature and therefore crushing tidal forces, or by the breakdown of physical determinism. Penrose has conjectured (in his "Strong Cosmic Censorship Conjecture") that it is generically unbounded curvature that causes singularities, rather than causal breakdown. The verification that BKL behavior (marked by the domination of time derivatives over space derivatives) is generically present in a family of solutions has proven to be a useful tool for studying Strong Cosmic Censorship in that family. We discuss what is known about BKL behavior and Strong Cosmic Censorship in families of solutions defined by varying degrees of isometry, and discuss new results which we believe will extend this knowledge and provide new support for Strong Cosmic Censorship.
27-May-14 The interplay between condensation and chemistry in the
formation of solid particles in the ejecta of supernova explosions
Davide Lazzati

Dust is a minor component of the interstellar medium but plays a major role in the evolution of the universe and on our capability to observe it. Despite recent advances, some fundamental aspects of the physics of formation of cosmic dust are still poorly understood. This leads, for example, to theoretical predictions of dust yields from supernova explosions that are at least two orders of magnitude larger than observed. In this talk I will use carbonaceous dust formation in core-collapse supernovae as a case study to discuss the current status and recent advances on our understanding of the physical and chemical mechanisms at the base of the gas-solid phase transition in the astrophysical environment. I will discuss different approaches to the problem and compare their results to available observations. I will conclude by introducing a new framework for studying the outcomes of collisions between dust agglomerates to study their compactification, growth, and fragmentation.
Dave Soper
3-Jun-14 Split SUSY Radiates Flavor
Matthew Baumgart
Carnegie Mellon University

Radiative flavor models where the hierarchies of Standard Model (SM) fermion masses and mixings are explained via loop corrections are elegant ways to solve the SM flavor puzzle. Here we build such a model in the context of Mini-Split Supersymmetry (SUSY) where both flavor and SUSY breaking occur at a scale of 1000 TeV. This model is consistent with the observed Higgs mass, unification, and WIMP dark matter. The high scale allows large flavor mixing among the sfermions, which provides part of the mechanism for radiative flavor generation. In the deep UV, all flavors are treated democratically, but at the SUSY breaking scale, the third, second, and first generation Yukawa couplings are generated at tree level, one loop, and two loops, respectively. Save for one, all the dimensionless parameters in the theory are O(1), with the exception being a modest and technically natural tuning that explains both the smallness of the bottom Yukawa coupling and the largeness of the Cabibbo angle.
Spencer Chang
Last modified: Fr Aug 27 17:54:31 PDT 2010
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