Events at Physics |
Events on Thursday, May 22nd, 2008
- R. G. Herb Condensed Matter Seminar
- Order by distortion and chiral magnetism in frustrated pyrochlore magnet
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Gia-Wei Chern, Johns Hopkins University
- Abstract: Flexible pyrochlore antiferromagnet is known to be unstable
against lattice distortion which relieves the magnetic frustration
and makes long-range magnetic order possible. In many cases
the distortion breaks the inversion symmetry, thus making the
crystal structure chiral. The handedness is transferred to the
magnetic order by the relativistic spin-orbit coupling, which is manifested as the Dzyaloshinskii-Moriya (DM) interaction between nearest-neighbor spins. In this talk, I will discuss the basic mechanism of degeneracy-breaking through spin-lattice coupling in pyrochlore antiferromagnet. The chiral nature of the DM interaction is important to the determination of the resulting ground state magnetic order. Our theoretical model successfully describes the ground state of spinel CdCr2O4. I will also give other more complicated examples and our speculations on the magnetic order of ZnCr2O4. - Host: Natalia Perkins
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Measurement of muon neutrino charged current quasielastic (CCQE) scattering on carbon in MiniBooNE
- Time: 4:00 pm
- Place: 4274 Chamberlin Hall
- Speaker: Teppei Katori, Indiana University
- Abstract: The observation of neutrino oscillations is clear evidence for physics beyond the standard model. To make precise measurements of this phenomenon, neutrino oscillation experiments, including MiniBooNE, require an accurate description of neutrino charged current quasielastic (CCQE) cross sections to predict signal samples. Using a high-statistics sample of muon neutrino CCQE events, MiniBooNE finds that a simple Fermi gas model, with appropriate adjustments, accurately characterizes the CCQE events observed in a carbon-based detector. The extracted parameters include an effective axial mass, MA=1.23 +- 0.20GeV, that describes the four-momentum dependence of the axial-vector form factor of the nucleon, and a Pauli-suppression parameter, kappa=1.019 +- 0.011. Such a modified Fermi gas model may also be used by future accelerator-based experiments measuring neutrino oscillations on nuclear target.
- Host: Teresa Montaruli