Events Archive

Apr
30
2013
Soft Matter Physics and Bacteria
One of the long term challenges in human health and disease is the control of pathogens, such as antibiotic-resistant forms of bacteria. In this talk, we will briefly describe two directions where soft condensed matter physics based approaches have been useful.
Apr
29
2013
Anomalous Electron Spin Decoherence in an Optically Pumped Quantum Dot
Recently, there is revived interest in non-equilibrium dynamics of the nuclear spins partially due to the decoherence issue of the electron spin qubit in semiconductor quantum dots for quantum computation. In this talk, I will first introduce a microscopic theory for the non-equilibrium nuclear spin dynamics controlled by a closed feedback loop mediated by the electron and/or the hole under continuous wave pumping in a quantum dot [W. Yang and L. J. Sham, Phy. Rev. B 85, 235319(2012)], and then present a study on the nuclear-spin-fluctuation induced spin decoherence of an electron (SDE) in an optically pumped quantum dot....
Apr
25
2013
Astrophysics and particle physics with cosmic-ray electrons and positrons
Cosmic rays are predominantly nuclei, in particular protons.  However, the less abundant cosmic-ray electrons and positrons are also important probes of open questions in astrophysics and particle physics.  The Fermi Gamma Ray Space Telescope, designed to study the high-energy universe with gamma rays, is also an excellent electron and positron detector.  Ground-based imaging atmospheric Cherenkov telescopes have also measured cosmic-ray electrons and positrons up to several TeV.  PAMELA, Fermi, and AMS have discovered a surprising excess of positrons between 10 GeV and 350 GeV.  I will describe these measurements and their...
Apr
25
2013
Experiments with Atoms in Optical Lattices
Atoms in optical lattices are very versatile experimental systems. They can be used to study many-body quantum physics, to aid precision measurements and tests of fundamental symmetries, and for quantum computation. I will describe ongoing efforts along these lines in my research group.
Apr
24
2013
Optimizing Undulatory Swimming
Most organisms live in aqueous environments and propel themselves by swimming. A large subclass is micro-organisms that have slender rod-like shapes, e.g. sperm. These organisms propel themselves using undulations that follow certain waveforms depending on the type of desired...
Apr
23
2013
Membranes, Interfaces, and the Interplay Between Curvature and Tension
Nature and technology abound with fluid interfaces such as the surfaces of oil droplets in water or the membrane surfaces of living cells.  These interfaces are typically crowded with adsorbed particles, proteins or other large molecules, which are effectively confined to a two-dimensional fluid.  This two-dimensional system, though, has a twist: it can spontaneously change its curvature and thereby substantially alter the interactions among the bound particles or proteins. ...
Apr
22
2013
The Astrophysics of Black Hole Spin
In addition to providing vital clues as to the formation and evolution of black holes, the spin of black holes may be an important energy source in the Universe.  Over the past couple of years, tremendous progress has been made in the realm of observational measurements of spin.   I will describe these efforts with particular focus on the use of X-ray spectroscopy to probe the spin of supermassive black holes in active galactic nuclei (AGN).   I shall describe results from the Suzaku AGNSpin Survey, a Suzaku Key Project that targets five bright and well-known AGN...
Apr
18
2013
Non-equilibrium Spin Domains in Quenched Sodium Spinor Bose-Einstein Condensates
Understanding dynamics of pattern formation, following a symmetry breaking quantum phase transition is an area of active interest. Spontaneous spin domain is formed in sodium Bose-Einstein condensates that are quenched, i.e. rapidly tuned, through a quantum phase transition from polar to antiferromagnetic phases. A microwave ``dressing'' field globally shifts the energy of the mF= 0 level below the average of the mF= ±1 energy levels, inducing a dynamical instability . We use local spin measurements to quantify the spatial ordering kinetics in the vicinity...
Apr
15
2013
Was Mars Ever Habitable?
NASA has followed the water on Mars, and has now progressed further to asking: is or was this environment ever actually habitable by life as we know it?  All such life has some common physical and chemical requirements, which we can look for via orbital remote sensing and missions to the Martian surface.  Recent results from the Curiosity rover are providing a delightfully nuanced answer to this question.  The talk will discuss how we arrived at Gale crater (versus the many other interesting places on Mars), what we have been doing there, and what...
Apr
09
2013
Anticipating sudden transitions in biological populations: Cooperation, cheating, and collapse
Natural populations can suffer catastrophic collapse in response to small changes in environmental conditions, and recovery after such a collapse can be exceedingly difficult. We have used laboratory microbial ecosystems to study early warning signals of impending population collapse. Yeast cooperatively breakdown the sugar sucrose, meaning that below a critical size the population cannot sustain itself. We have demonstrated experimentally that changes in the fluctuations of the population size can serve as an early warning signal that the population is...

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