Events Archive

Mar
29
2011
"Laser-Cooled Primary Frequency Standards at NIST" by Dr. Steven Jefferts
Mar
17
2011
"Supermassive Black Holes: Rulers of the Universe?" by Gordon Richards
nce the realm of philosophers, black holes have now been shown to exist. Indeed, black holes as massive as 1 million to 1 billion Suns populate the cores of essentially all massive galaxies. Contrary to popular thought, these super-massive black holes are messy eaters, spewing out nearly as much (in the form of mass and energy) as they consume. This "feedback" process has been postulated to be the valve that controls the growth and evolution of their host galaxies, shaping the very evolution of our Universe. I will discuss how statistical analyses of active galactic nuclei and quasars from the Sloan Digital Sky Survey (SDSS) can be used to test the hypothesis that...
Mar
16
2011
"Why Meissner was wrong (or a classical explanation of flux expulsion from superconductors)" by Hanno Essen
The discovery by Meissner and Ochsenfeld in 1933  that the magnetic field inside a conductor is expelled when it is cooled down to become  superconducting was considered very surprising at the time. Meissner wrote that this has no classical explanation. Since then virtually every textbook stresses that this means that a superconductor is in fact not just a zero resistivity perfect conductor but in addition has the mysterious property that it can expel internal magnetic flux. In this colloquium I will present evidence that this is all a misunderstanding based on insufficient knowledge of what classical physics in fact predicts about the...
Mar
15
2011
"Normal metal contacts and small-scale graphene two-terminal junctions"
Graphene has been proposed as a viable replacement for silicon on a number of electronic applications due to its high mobility. At Georgia Tech we have been studying charge transport through graphene junctions contacted to two normal metal contacts, aiming for a quantitatively accurate description. In this talk I will present an overview of the problem, making an emphasis on crucial differences between theory and experiment, that so far preclude quantitative modeling. Then I will present our results, including some very recent developments concerning graphene junctions attached to titanium contacts (titanium has been the metal of choice for contacting graphene in experiments)....
Mar
14
2011
"Random walks, DNA elasticity, and the motion of proteins" by Meredith Betterton
A particle undergoing a random walk is a classic physics problem that underlies our understanding of diffusion, the molecular nature of matter, polymer conformations, and the fluctuation-dissipation theorem. In addition to its conceptual importance in physics, a random walk is a surpisingly good model for some biophysical problems. This talk will present several examples, including DNA conformation and protein motion along a biopolymer, that are well described by a biased or unbiased random walk. I will discuss the physical theory and biophysical applications of several problems from our recent research: finite-length effects in DNA elasticity, the coupling...
Mar
07
2011
"Relating single motor function to ensemble transport in cells" by Jing Xu
Cells are highly ordered and organized. Much of the cell’s order relies on the active transport of material by molecular motors. Disruption in intracellular transport can be detrimental to cells, and is a common early theme in neurodegeneration. While molecular motors have been studied in isolated, cell-free system, how they act in groups in cells, and how their group functions are regulated or disrupted, are not yet understood. To address these questions in a concrete, experimentally tractable system, we studied the effects of a neurodegenerative mutation (“Legs at Odd Angles”, or Loa) on the major molecular motor, dynein. Combining single...
Mar
02
2011
"Microbial Interaction Networks in Soil and in Silico" by Kalin Vetsigian
Soil harbors a huge number of microbial species interacting through secretion of antibiotics and other chemicals. What patterns of species interactions allow for this astonishing biodiversity to be sustained, and how do these interactions evolve? I used a combined experimental-theoretical approach to tackle these questions. Focusing on bacteria from the genus Steptomyces, known for their diverse secondary metabolism and production of antibiotics, I isolated 64 natural strains from several individual grains of soil and systematically measured all pairwise interactions among them. Quantitative measurements on such scale were never possible before. They were...
Feb
28
2011
"Dynamics of bacterial propulsion: Behavior of the flagellar rotary motor near zero load" by Junhua Yuan
Flagellated bacteria swim by rotating thin helical filaments, each driven at its base by a reversible rotary motor, powered by an ion flux. Studies of the physiology of the bacterial flagellar rotary motor have been limited to the regime of relatively high load due to technical limitations. Here, we developed a new technique that allows systematic study of the motor near zero load. Sixty-nanometer-diameter gold spheres were attached to motors lacking flagellar filaments, and a novel laser darkfield setup was used to monitor the sphere rotation. Resurrection experiments were carried out near zero load: paralyzed motors without torque...
Feb
25
2011
"Induction of Cellular Responses by Nanoscopic Environments" by Joachim P. Spatz
Our approach to engineer cellular environments is based on self-organizing spatial positioning of single signaling molecules attached to inorganic or polymeric supports, which offers the highest spatial resolution with respect to the position of single signaling molecules. This approach allows tuning cellular material with respect to its most relevant properties, i.e., viscoelasticity, peptide composition, nanotopography and spatial nanopatterning of signaling molecule. Such materials are defined as “nano-digital materials” since they enable the counting of individual signaling molecules, separated by a biologically inert background. Within these...
Feb
23
2011
"Advances in chaotic dynamics and chaos synchronization" by Hongtao Zhang
Chaotic system, characterized by sensitivity to initial conditions, handles abundant dynamics, sometimes leading to unimagined results in reality. In the past two decades, dynamical analysis and control of chaos attracted a lot of interests of scientists. Specifically, since chaos synchronization was found in 1991, generating and synchronizing chaotic systems has become a hot issue and been intensively studied. In this presentation, I would like to introduce some advances in generating new chaotic attractors and synchronizing chaos. Initially, starting from designing new chaotic systems, I demonstrate three types of systematic approaches to generate multi-scroll attractors and...

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