Events at Physics |
Events on Thursday, April 17th, 2008
- R. G. Herb Condensed Matter Seminar
- Dynamic Nuclear Polarization in Double Quantum Dots
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Jason Petta, Princeton University
- Abstract: Trapped electrons in GaAs quantum dots are coupled to approximately one million lattice nuclei through the contact hyperfine interaction. Random hyperfine fields drive rotations from the singlet state (S) to the triplet state (T_O ), leading to an ensemble averaged spin dephasing time T_2 * of 10 ns [1]. In this talk, I will describe how fast electrical control of two-electron spin states near the anti-crossing of the singlet (S) and triplet (T_+ ) can be used to polarize the nuclear spin bath. An initialized S state is cyclically brought into resonance with the T_+ state, where hyperfine fields drive rapid rotations between S and T_+ , `flipping' an electron spin and `flopping' a nuclear spin [2]. The resulting Overhauser field approaches 80 mT, in agreement with a simple rate-equation model. A self-limiting pulse sequence is developed that allows the steady-state nuclear polarization to be set using a gate voltage. Lastly, I will describe efforts at Princeton to fabricate semiconductor nanowire double quantum dots. These devices will limit nuclear spin diffusion, resulting in increased nuclear polarization.
[1] J. R. Petta /et al./, Science *309*, 2180 (2005).
[2] J. R. Petta, J. M. Taylor /et al./, Phys. Rev. Lett. *100*, 067601 (2008). - Host: Vavilov
- Special Joint Nuclear Physics and High Energy Physics Seminar
- Polarized Antiprotons
- Time: 2:45 pm - 3:45 pm
- Place: 5310 Chamberlin
- Speaker: Hans-Otto Meyer, Indiana University
- Abstract: Particle physics has made remarkable advances when stored, cooled beams of antiprotons became available. Soon after, scientists called for polarized antiprotons in order to explore the spin degrees of freedom in antiproton-induced reactions. In the past 20 years, many clever ideas on how to satisfy this request have been produced, however, only recently has there been experimental activity in this field. I will review some of the history, what is going on at present, and where this might possibly lead.
- Host: Haeberli
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Colliders to the Cosmos: Dark Matter and Electroweak Phase Transition
- Time: 4:00 pm - 5:00 pm
- Place: 4274 Chamberlin
- Speaker: Maxim Perelstein, Cornell
- Abstract: Experiments at the Large Hadron Collider (LHC) will begin in the second half of this year. This machine, along with the proposed International Linear Collider (ILC), will directly probe physics at the TeV energy scale. The discoveries and measurements made by these experiments can have interesting implications for cosmology. I will discuss two examples of this connection. First, if dark matter is composed of weakly interacting massive particles, these particles may be directly produced and studied at the LHC and the ILC. Second, precise measurements of the Higgs boson properties may allow us to understand the dynamics of the electroweak phase transition in the early universe, with important implications for baryogenesis. In both cases, I will stress the aspects of the connection that do not depend on the (presently unknown) details of the fundamental theory at the TeV scale.