Events at Physics |
Events on Thursday, November 5th, 2009
- R. G. Herb Condensed Matter Seminar
- Invariant form of spin-transfer switching condition
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Yaroslaw Bazaliy, University of South Carolina
- Abstract: Spin-transfer torque is a result of a non-equilibrium magnetic interaction induced by a flow of electric current. In nano-size devices spin transfer is a mechanism for current-induced magnetic switching, an effect with many applications in computer memory and logic. Calculations of critical switching currents were performed analytically and numerically for many systems. However, a clear physical interpretation of the results was available only in special cases. We firstly derive an invariant form of the current-induced switching condition and show that there exists a figure of merit for the current ability to destabilize a minimum of magnetic energy and induce switching. The resulting approach based on the "switching ability" of the current provides an intuitive physical picture of the switching behavior in devices of any geometry. We secondly consider the problem of current-induced stabilization of the saddle points and show that it happens via a different mechanism due to the topological restrictions. Saddle points are normally stabilized by a current-induced merger with other equilibria. Finally, we present and emerging full picture of possible switching scenarios in spin torque devices.
- Host: Natalia Perkins
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- The Baryon Resonance Spectrum and the 1/Nc Expansion
- Time: 4:00 pm
- Place: 5310 Chamberlin
- Speaker: Richard Lebed, Arizona State University
- Abstract: Why do baryon resonance multiplets exist, and what controls their formation and decays? It is natural to consider them as merely excited states of some three-quark or meson-nucleon potential. But these are just simplistic quantum-mechanical pictures that recognize neither the full field-theoretical complexities of QCD nor the extremely brief lifetimes of resonances due to quark pair production. Both of these issues are addressed by the 1/Nc expansion of QCD, where Nc is the number of color charges. Constraints arising at large Nc on meson-baryon scattering amplitudes not only create linear relationships between them, thus linking distinct partial waves and their embedded resonances, but also restrict the possible resonant decay channels. I present strong experimental evidence in favor of this approach, describe the multiplet structure that it predicts, and show how to perform the analysis beyond the strict large Nc limit by incorporating 1/Nc-suppressed effects. This program has been developed to the point that now one possesses a full effective field theory formalism for physics in the baryon resonance region.
- Host: Michael Ramsey-Musolf
- Introductory Graduate Seminar
- Astrophysics II: Astroparticle Physics and Neutrinos
- Time: 5:30 pm
- Place: 2223 Chamberlin Hall
- Speaker: Faculty, University of Wisconsin Department of Physics
- Abstract: Balantekin, Barger, Chung, Halzen, Heeger, Karle, McCammon, Montaruli, Ogelman, Ramsey-Musolf, Timbie, Westerhoff, Zweibel