Eric Sembrat's Test Bonanza

Cosmic rays are microscopic, charged particles that permanently bombard Earth from outer space. 100 years after their discovery their origin is still a mystery. It is also not clear how cosmic rays can obtain energies that are sometimes billion times larger than what can be produced in the most powerful particle accelerator on Earth, the LHC, where the Higgs particle was discovered last year. Possible particle accelerators that nature provides are very exotic sites in the universe like exploding stars, massive black holes, gamma-ray bursts, and pulsars. To find out more about these enigmatic particles and their origin a number of experiments on ground and space have been put into operation over the past ten years and provide us with stunning results.  I will give an introduction to cosmic rays, how we detect them, what we have learned from recent measurements about the origin of cosmic rays, and how cosmic rays are used to test the foundation of modern physics.

The tidal disruption of a star can serve as a diagnostic for the presence of a dormant black hole in a distant galaxy.  While such tidal disruption events are rare, they give rise to powerful flares of emission at and above Eddington luminosity, with spectral features and timescales that might reveal both the type of star and the mass (and perhaps spin) of the black hole.  In our study, we consider relativistic encounters between white dwarfs and massive black holes at the threshold of disruption.  We develop a numerical code whose central feature is the use of Fermi normal coordinates (FNC).  We characterize the mass loss from the star and provide a detailed view into the (hydro)dynamics of the remnant and debris.  In this talk, I will discuss these results and present a hybrid approach that relies on the FNC method in combination with fully general relativistic hydrodynamics to model the early accretion of debris onto the black hole with high accuracy. This new and timely development in tidal disruption studies is directly motivated by the anticipated abundance of data from the current and upcoming multi-wavelength surveys of the transient sky.

Circadian clocks rely on the alternation of light and dark to synchronize to the day/night cycle. However, a consequence of weather fluctuations and seasonal variations is that the driving signal received by the clock is highly variable not only from one day to the next but also throughout the year, which may compromise robust entrainment.

The microscopic green alga Ostreococcus tauri has recently emerged as a promising circadian model in the green lineage. Its clock is based on a central loop featuring orthologs of Arabidopsis TOC1 and CCA1 clock genes, yet seems to have a simpler architecture than Arabidopsis. The analysis of expression data from these two core clock genes and mathematical modeling have unveiled a simple yet effective strategy to protect the clock from fluctuations in daylight intensity, effectively decoupling the clock from the external cycle when it is on time. Being robust to these fluctuations appears to be sufficiently important that this strategy can be clearly evidenced for all photoperiods between 2 and 22 hours, despite the fact that the expression profiles significantly depend on day duration. This shows that a circadian clock can be both robust and flexible, using simple principles from nonlinear oscillator physics.

Whole-cell patch clamp electrophysiology of neurons in vivo enables the recording of electrical events in cells with great precision, and supports a wide diversity of cellular morphological and molecular analysis experiments. However, high levels of skill are required in order to perform in vivo patching, and the process is time-consuming and painstaking. An automated in vivo patching robot would not only empower a great number of neuroscientists to perform such experiments, but would also open up fundamentally new kinds of experiment enabled by the resultant high throughput. We discovered that in vivo blind whole cell patch clamp electrophysiology could be implemented as a straightforward algorithm, and developed an automated robotic system capable of performing this algorithm. We validated the performance of our robot in both the cortex and hippocampus of anesthetized and awake mice. Our robot achieves yields, cell recording qualities, and operational speeds that are comparable to, or exceed, those of experienced human investigators, and is simple and inexpensive to implement.  Recent developments include coupling "autopatching" to optogenetics, recording multiple neurons simultaneously, and patching deep structures including mouse brain stem.

Remoras (echeneid fish) reversibly attach and detach to marine hosts, almost instantaneously, to “hitchhike” and feed. The adhesion mechanisms that they use are remarkably insensitive to substrate topology and quite different from the latching and suction cup-based systems associated with other species at similar length scales.  Remora adhesion is also anisotropic; drag forces induced by the host’s swimming increase adhesive strength, while rapid detachment occurs when the remora reverses this shear load.  In this presentation, an investigation of the adhesive system’s functional morphology and tissue properties, carried out initially through dissection and x-ray microtomographic analyses, is discussed.  Resulting finite element models of these components have provided new insights into the adaptive, hierarchical nature of the mechanisms and a path toward a wide range of engineering applications.

PLEASE NOTE: This is a WEBINAR

We will describe the properties of dynamical systems that:

(1) possess symmetry

(2) exhibit chaotic behavior

In an initial study of such systems, Miranda and Stone projected the Lorenz attractor in a 2 to 1 locally diffeomorphic way to the ``proto''-Lorenz attractor. Then they ``lifted'' this attractor back up to n-fold covers in a locally diffeomorphic way using properties of the rotation group Cn and some complex analysis. We describe the interaction of symmetry groups with equivariant (symmetric) dynamical systems and show how invariant polynomials and an integrity basis are used to construct image dynamical systems.  There is an unexpected richness in ``lifting'' invariant dynamical systems up to equivariant dynamical systems, as different groups anddifferent singular sets can be used to construct locally diffeomorphic but topologically inequivalent covering dynamical systems. Different covers are labeled by distinct values of topological indices. These ideas will be illustrated with lots of pictures.

Sources of single photons (as opposed to sources which produce on average a single photon) are of great current interest for quantum information
processing. Perhaps surprisingly, it is not easy to produce a single photon efficiently and in a controlled way. Following earlier progress, recent experimental activity has resulted in the production of single photons by taking advantage of strong inter-particle interactions in cold atomic gases. I will show how the systematic use of the method of steepest descents can be used to understand the dynamics of the single photon source developed here at Georgia Tech and how this describes a kind of quantum scissors effect. In addition to the mathematical results, I will present the background quantum mechanics in a form suitable for a general audience. Joint work with Francesco Bariani and Paul Goldbart.

Abstract: Thermal conductivity is a basic and familiar property of materials that plays a pivotal role in a broad range of topics in energy science and engineering systems. In this talk I will emphasize recent examples of extreme behavior—and behavior under extreme conditions—in polymers and molecular solids. Our measurements of heat conduction in novel materials are enabled by variety of ultrafast optical pump-probe metrology tools developed over the past decade. At the low end of the thermal conductivity spectrum, fullerene derivatives display the lowest thermal conductivity ever observed in a fully dense solid, comparable to the conductivity of disordered layered WSe2 and only twice that of air. Extremes of high pressures (up to 60 GPa) allow us to continuous change the strength of molecular interactions in glassy polymers and test theoretical descriptions of the mechanisms for heat conduction. The thermal conductivity of aligned, crystalline and liquid crystalline polymer fibers can be surprisingly high, comparable to that of stainless-steel.  The dominate carriers of heat appear to be longitudinal acoustic modes with lifetimes dictated by anharmonic processes.

Biography: Prof. Cahill joined the faculty of the University of Illinois at Urbana-Champaign in 1991 after earning his Ph.D. in condensed matter physics from Cornell University in 1989, and working as a postdoctoral research associate at the IBM Watson Research Center. In 2005, he was named Willett Professor of Engineering and was appointed Head of the Department of Materials Science and Engineering in 2010. His research program focuses on developing a microscopic understanding of thermal transport at the nanoscale; the development of new methods of materials processing and analysis using ultrafast optical techniques; and advancing fundamental understanding of interfaces between materials and water. He received the Peter Mark Memorial Award from the American Vacuum Society (AVS); is a fellow of the AVS, American Physical Society (APS) and Materials Research Society (MRS); and is currently chair of the Division of Materials Physics of the APS.

The original concept of graphene electronics focused on carbon nanotube properties. Carbon nanotubes were known to be high mobility ballistic, phase coherent conductors and quantum confinement effects produced significant bandgaps. However, it turns out to be very difficult to develop nanotube electronics platform for a variety of reasons including fundamental physical constraints related to the quantum mechanical properties of the metal-to-nanotube contacts. Graphene electronics can in principle overcome the major problems because graphene structures can be patterned using conventional lithography and dissipation at contacts can be controlled. However, these developments rely on the premise that narrow, ballistic graphene ribbons can be produced. Experiments on conventionally patterned graphene structures produced from graphene that is deposited on insulating substrates have been discouraging. The graphene ribbon mobilities are so low due to edge roughness effects, to render this direction to be impracticable.  On the other hand, graphene produced on silicon carbide turns has been found to be more immune to edge scattering problems. Moreover, recent developments of template grown graphene structures on silicon carbide are promising. Very narrow ballistic graphene ribbons that demonstrate ballistic transport properties, have been produced with these methods which again brings the original concept of graphene based nanoelectronics back into play.

When symmetry in a system is broken, topological defects may form, with the possible defects determined by the nature of the broken symmetry.   Topological defects in an atomic Bose-Einstein condensate (BEC), such as vortices, therefore can serve as a laboratory for studying the physics of broken symmetry.  By engineering the system such that the broken symmetry changes over some boundary, one may also study the physics of topological interfaces.  In this talk I will discuss the energetic stability of vortices in spin-1 atomic BECs, identified by numerically minimizing the free energy functional, as well as proposed scheme accessible to current experiments which realizes a topological interface.

The spin-1 BEC exhibits two phases of the ground state manifold, polar and ferromagnetic (FM), with different broken symmetries.  I will present the core structures of the energetically stable singular vortices in both phases and discuss how these may be understood in terms of an energetic hierarchy of length scales.  I will then discuss recent results which show how the stable vortex structures change when the conservation of longitudinal magnetization is explicitly imposed, such as the stability of a nonsingular FM vortex when atomic interactions favor the polar phase.  Finally, I will discuss stable vortices which cross a boundary between polar and FM BECs, corresponding to a topological interface.