Events at Physics |
Events on Thursday, March 26th, 2009
- R. G. Herb Condensed Matter Seminar
- What can we learn from spin-dependent shot noise in semiconductor and graphene nanostructures?
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Branislav Nikolic, University of Delaware
- Abstract: I review recent studies of the shot noise of spin-polarized charge currents and pure spin currents in multiterminal semiconductor nanostructures with spin-orbit couplings or zigzag graphene nanoribbons attached to metallic electrodes. Akin to early utilization of noise in mesoscopic devices, where random time-dependent current fluctuations contain much more information about the transport than revealed through average current and conductance, spin-dependent shot noise emerges as a rich electrical tool to probe spin as magnetic degree of freedom. The Fano factor (noise-to-current ratio) of such noise can quantify decoherence of transported spins, or reveal the origin of the spin Hall effect (SHE) in simply-connected devices and probe interference effects in SHE induction in multiply-connected Aharonov-Casher rings. Finally, while the conventional spin-degenerate shot noise has very recently become important experimental tool to study ballistic transport through evanescent modes in graphene samples attached to metallic electrodes, in ribbons with zigzag edges which favor edge magnetic ordering when Coulomb interaction is "turned on", modification of the shot noise can be exploited to detect this unusual form of low-dimensional carbon-based magnetism.
- Host: Irena Knezevic, Elect. & Comp. Engineering
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- M-flation
- Time: 4:00 pm
- Place: 4274 Chamberlin
- Speaker: Amjad Ashoorioon, University of Michigan
- Abstract: We propose an inflationary scenario, M-flation, in which inflation is driven by three NXN Hermitian matrices Phi_i, i=1,2,3. The inflationary potential of our model in the most general form involves a commutator squared term, a cubic term involving product of a commutator with the field and a quadratic ``mass'' term. This class of potentials are strongly motivated by string theory. We show that one can consistently restrict the classical dynamics to a sector in which the Phi_i are proportional to the NXN irreducible representation of SU(2). In this sector our model effectively behaves as an N-flation with order N^2 number of fields, and therefore remove the fine-tunings associated with super-Planckian field and/or unnaturally small couplings in the chaotic type inflationry scenarios. Due to the matrix nature of the scalars 3N^2-1$ other scalar fields also affect the inflationary dynamics. These have the observational effects such as production of entropy (non-adiabatic) perturbations on cosmic microwave background. Moreover, the existence of these other scalars provides us with a natural preheating model for our setup.<br>
- Host: Dan Chung