Events at Physics |
Events on Thursday, March 11th, 2010
- R. G. Herb Condensed Matter Seminar
- Theory of nonequilibrium magnetooscillations in 2D electron transport in high Landau levels
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Ivan Dmitriev, Institute for Theoretical Physics, Karlsruhe University, Germany
- Abstract: We propose a unified quantum kinetic description of a family of nonequilibrium magnetooscillations recently discovered in high-mobility quantum Hall structures, including the microwave induced resistance oscillations (MIRO), Hall-field induced RO (HIRO), and phonon induced RO (PIRO). The magnetoresistivity is due to quantum oscillations in the density of states in high Landau levels and is governed by spectral and spatial resonances set by the cyclotron frequency and the cyclotron diameter in various combinations with the parameters of the external fields and phonons. External ac or dc driving leads to local population inversion in the energy distribution of electrons and modifies the scattering off impurities and phonons. In particular, PIRO are shown to be very sensitive to the applied direct current. In the supersonic regime, where the Hall velocity exceeds the sound velocity, the phonon-assisted conductivity remains finite at zero temperature, while below the supersonic transition the oscillations get exponentially suppressed at low temperature. Quite interestingly, in the microwave-illuminated inhomogeneous 2D electron gas, the photocurrent and photovoltage oscillations arise in the absence of the external dc driving, due to nonequilibrium violation of the Einstein relation between electrical current and diffusion.
[ I.A. Dmitriev et al., PRL 91, 226802 (2003); 99, 206805 (2007); JETP Lett. 85, 86 (2007); PRB 70, 165305 (2004); 71, 115316 (2005); 75, 245320 (2007); 80, 125418 (2009); 80, 165327 (2009).] - Host: Maxim Vavilov
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- Physics with ultra-cold and very cold neutrons at the Institut Laue-Langevin in Grenoble, France
- Time: 4:00 pm
- Place: 4274 Chamberlin
- Speaker: Peter Geltenbort, Institute Laue-Langevin, Grenoble
- Abstract: Due to their outstanding property to be storable and hence observable for long periods of time (several hundreds of seconds) in suitable material or magnetic traps, ultra-cold neutrons (UCN)with energies around 100 neV are an unique tool to study fundamental properties of the free neutron. Their properties and production are described in detail. Selected experiments with ultra-cold neutrons (UCN) - the measurement of its lifetime and the search for an electric dipole moment - performed at the instrument "Physique Fondamentale 2 (PF2)" of the Institut Laue-Langevinn (ILL) are highlighted. For particles to have electric dipole moments, the forces concerned in their structure must be asymmetric with regard to space-parity (P) and time reversal (T). P violation is a well-known intrinsic feature of the weak interaction which is responsible for the beta-decay of the free neutron. T violation turns out to be necessary to explain the survival of matter at the expense of antimatter after the Big Bang. By searching for an EDM of the free neutron hypothetical new channels of T-violation can be investigated. The experiments at the ILL will be compared to competing EDM projects worldwide. The measurement of the lifetime of the free neutron together with the determination of one of the correlation parameters characterizing neutron decay allows tests of the Standard Model. Furthermore, the neutron lifetime plays an important role in Big-Bang Nucleosynthesis cosmology. Up to 180 s after the big-bang nuclei with more than one nucleon are unstable. The neutron lifetime determines how many neutrons have decayed up to this moment and hence the relative helium abundance in the universe. The different methods to measure the lifetime of the free neutron are reviewed and the latest experiments using storage of UCN at the ILL are described in detail. A brief outlook on future projects worldwide will be given.
- Host: Michael Ramsey-Musolf