Physics Seminar Series
(Refreshments at 3:15 p.m. in 245 Fronczak)
Contact: Prof. Andrea Markelz, (716) 645-2017 x124;
Prof. Avto Kharchilava, (716) 645-2017 x245
September Go to Aug. '09 Oct. '09 Nov. '09 Dec. '09
09/15/2009, Tuesday, 3:30 p.m., 201 NSC
Quantum phase transitions and stochastic switching in superconducting nanowires
Dr. Nayana Shah
Department of Physics, University of CincinnatiBy using carbon nanotubes or DNA molecules as templates to fabricate nanowires, it is now becoming possible not only to pose, but also to experimentally answer questions about the very essence of superconductivity in reduced dimensions, and to explore the potential of superconducting nano-circuitry. By sketching the recently developed theoretical picture of a certain class of quantum phase transitions and stochastic switching in superconducting nanowires and its experimental counterpart, I will address such questions and report our findings about the behavior of superconducting fluctuations in the normal state as well as phase-slip fluctuations in the superconducting state. By the end of my talk, I would also hope to have convinced the audience that nanowires provide an ideal setting for systematically studying quantum phase transitions and quantum fluctuations at the nano-scale.
09/29/2009, Tuesday, 3:30 p.m., 201 NSC
Coherent control of intense terahertz radiation
Dr. Kiyong Kim
Institute for Research in Electronics and Applied Physics, University of MarylandSandwiched between the traditional optical and microwave regimes, far infrared or terahertz (THz) frequency has recently drawn special attention due to its ubiquitous nature, as well as its potential for molecular sensing, biomedical imaging and spectroscopy, security scanners, and plasma diagnostics. For these applications, there is a present and growing need for high-energy, compact THz sources at a tabletop-scale. In this effort, I will present our recent demonstration of high-energy (>5 microjoule), super-broadband (>75 THz) THz radiation generation using a tabletop femtosecond laser [1]. In this scheme, an ultrafast pulsed laser's fundamental and second harmonic fields are mixed in a gas of atoms or molecules, causing them to ionize. The resulting plasma can generate a directional electron current and simultaneous far-field THz radiation, all coherently controlled by the laser field amplitudes and relative phase. By controlling the relative phase, we can also switch the output energy between THz and harmonics [1].
[1] K. Y. Kim et al., Nature Photonics 2, 605 (2008)
October Go to Aug. '09 Sept. '09 Nov. '09 Dec. '09
10/06/2009, Tuesday, 3:30 p.m., 201 NSC
Microscopic Route to Nematicity in Sr3Ru2O7 and Avoided Quantum Criticality
Dr. Hae-Young Kee
Department of Physics, University of TorontoIn correlated electron systems, electrons can organize themselves to form states analogous to classical liquid crystal phases. The search for such phases in solid state systems, in particular for the quantum version of an anisotropic liquid crystal state, dubbed electronic nematic phase, has been of great interest. Anisotropic metal bounded by two consecutive meta-magnetic transitions was reported in bilayer Ruthenates, Sr3Ru2O7. It was proposed that the nematic phase -- anisotropic metal -- accompanying the meta-magnetic transitions is driven by an effective momentum-dependent quadrupole-type interaction. In this talk, I will first review recent experimental and theoretical findings, and then present a microscopic origin of such an effective interaction. Experimental consequences, other competing orders, and avoided quantum criticality will be also discussed.
10/13/2009, Tuesday, 3:30 p.m., 201 NSC
Wild Universe!
Black Holes and Gamma-Ray Bursts, Quasars and Neutron Stars
Dr. Jason Kendall
NASA/JPL Solar System AmbassadorOur night sky looks peaceful, placid and wondrous, but scientists and astronomers have recently discovered jut how amazingly violent the Universe can be. There are cataclysmic explosions and extreme environments beyond our imaginations. The Chandra X-Ray Telescope, The Hubble Space Telescope, and the Fermi Gamma Ray Telescope all give us pictures of the most violent places in the Cosmos. Come see what happens to stuff near a Black Hole. Learn how supernova explosions create super-dense stars. And see beacons of light so bright that they can be seen literally across the entire Universe. Come join us for a safe view of these wild corners of the sky.
Jason Kendall holds a Master of Science in Astronomy from New Mexico State University. He has taught Astronomy at the high school and college level. Currently, he is also part an Earth and Space Explainer at the American Museum of Natural History. He has led numerous "starwatching parties" at New Mexico State University, Minnesota State University in Mankato, and The University of Texas at Austin. He also put together the historic Inwood Star Fest during the 100 Hours of Astronomy IYA Cornestone Event. During the StarFest, New York City Parks turned off the lights for a major Manhattan park. By day, Jason works on Wall Street with Cantor-Fitzgerald, and by night Jason shares his enthusiasm about the wonders of the night sky to his neighborhood in northern Manhattan and the Bronx as part of a public outreach project of the International Year of Astronomy.
Jason is a volunteer with the NASA/JPL Solar System Ambassadors Program. This program has a cadre of enthusiastic and knowledgeable volunteers who are skilled in educating the public about the exciting and adventurous work being done by our nation's most talented scientists and engineers. These volunteers communicate the excitement of JPL's space exploration missions and information about recent discoveries to people in their local communities. The Solar System Ambassadors Program is sponsored by the JET PROPULSION LABORATORY in Pasadena, CA, an operating division of the California Institute of Technology (Caltech) and a lead research and development center for the National Aeronautics and Space Administration (NASA).
10/27/2009, Tuesday, 3:30 p.m., 201 NSC
Lipid rafts reach a critical point
Dr. Sarah Veatch
Department of Physics, Cornell UniversityMulticomponent lipid bilayer membranes can contain two coexisting liquid phases, named liquid-ordered and liquid-disordered. Recently, we demonstrated that large (micron-scale) and dynamic critical fluctuations are found in "simple" ternary bilayer membranes prepared with critical compositions. Remarkably, robust critical behavior is also found in compositionally complex vesicles isolated directly from living cell plasma membranes. This finding strongly suggests that cells tightly regulate plasma membrane protein and lipid content to reside near a critical point and that critical fluctuations provide a physical basis of functional membrane heterogeneity in living cells at physiological temperatures. We are currently probing for critical fluctuations in intact cells using high resolution imaging techniques (scanning electron microscopy and super-resolution fluorescence localization microscopy). In addition, we are investigating possible structural and functional consequences of plasma membrane criticality using computational approaches, and are testing these predictions experimentally using the model system of IgE mediated signaling in RBL mast cells.
November Go to Aug. '09 Sept. '09 Oct. '09 Dec. '09
11/03/2009, Tuesday, 3:30 p.m., 201 NSC
Punching Holes into Membranes: How Pro-apoptotic Proteins Cooperate to Form Aqueous Channels in Lipid Bilayers
Dr. Cécile Fradin
McMaster UniversityApoptosis is a form cell suicide, which plays an essential role in the growth and maintenance of multi-cellular organisms. Many of the known apoptotic pathways involve the permeabilization of mitochondria, which commits the cell to dying. This key step is controlled by the Bcl-2 family of proteins, and revolves around the action of pore-forming family member Bax, which inserts in the outer mitochondrial membrane in response to apoptotic stimuli, oligomerizes and form pores. Although the sequence of events leading to pore formation is now established, the exact molecular mechanisms involved remain obscure. We used an in vitro system consisting of small liposomes prepared with a lipid composition mimicking the mitochondrial membrane, in which purified Bax was inserted via activation with purified tBid, another Bcl-2 family protein. We looked at the activation and pore-formation mechanisms both with the help of fluorescence methods (to resolve protein-protein and protein-lipid interactions) and small-angle neutron scattering (to resolve protein membrane conformations). Our study sheds light on the multiple conformations adopted by the different proteins involved at the membrane, and on the cooperativity of the pore formation. One of our most intriguing finding is that pore-formation is accompanied by a thinning of the lipid bilayer, an effect reminiscent of the action of antimicrobial peptides on membranes.
11/10/2009, Tuesday, 3:30 p.m., 201 NSC
Using colloidal suspensions to investigate statistical mechanics phenomena
Dr. Itai Cohen
Cornell UniversityColloidal suspensions consist of micron sized solid particles suspended in a solvent. The particles are Brownian so that the suspension as a whole behaves as a thermal system governed by the laws of statistical mechanics. The thermodynamic nature of these systems has allows scientists to use colloidal suspensions as models for investigating numerous processes that typically take place on the atomic and nano scale but are often very difficult to investigate. In this talk I will describe how we use various experimental techniques to investigate the structure and dynamics of these systems and gain an understanding of epitaxial growth, defect nucleation, and defect translation in colloidal crystals.
11/17/2009, Tuesday, 3:30 p.m., 201 NSC
Multi-scale models of biomolecular processes: developments and applications
Dr. Qiang Cui
Department of Chemistry and Theoretical Institute of Chemistry, University of Wisconsin-MadisonIn this presentation, I'll discuss two areas in our research that represent two distinct types of biological problems that we have been working toward to understand. The first area concerns proton pumping in biomolecules such as cytochrome c oxidase. The discussion will focus on, however, microscopic pKa calculations using brute-force QM/MM simulations. In particular, I'll explain the relevance of such calculations to the investigation of proton pumping, and some of the technical challenges that we face. A simple enhanced sampling method is used to improve the convergence of such calculations and will be illustrated with a few examples. The second topic concerns the mechanism of mechanosensation, where the mechanical perturbation can be introduced either in the form of membrane tension or addition of small molecules into the membrane. To shed light into specific questions of interest, coarse-grained models at both particle and continuum levels have been found valuable.
December Go to Aug. '09 Sept. '09 Oct. '09 Nov. '09
12/08/2009, Tuesday, 3:30 p.m., 201 NSC
TBA
Dr. Elisabeth Gwinn
Department of Physics, UC Santa Barbara