Eric Sembrat's Test Bonanza

The unification of the four fundamental forces remains one of the most important issues in theoretical particle physics. In this talk, I will first give a short introduction to Non-Commutative Spectral Geometry, a bottom-up approach that unifies the (successful) Standard Model of high energy physics with Einstein's General theory of Relativity. The model is built upon almost-commutative spaces and I will discuss the physical implications of the choice of such manifolds. I will show that even though the unification has been obtained only at the classical level, the doubling of the algebra may incorporate the seeds of quantization. I will then briefly review the particle physics phenomenology and highlight open issues and current proposals. In the last part of my talk, I will explore consequences of the Gravitational-Higgs part of the spectral action formulated within such almost-commutative manifolds. In particular, I will study modifications of the Friedmann equation, propagation of gravitational waves and the onset of inflation. I will show how current measurements (Gravity Probe, pulsars, and torsion balance) can constrain free parameters of the model. I will conclude with a short discussion on open questions.

Computational modeling of eukaryotic cells moving on substrates is an extraordinarily complex task: many physical processes, such as actin polymerization, action of motors, formation of adhesive contacts concomitant with both substrate deformation and recruitment of actin etc., as well as regulatory pathways are intertwined. Moreover, highly nontrivial cell responses emerge when the substrate becomes deformable and/or heterogeneous. Here we extended a computational model for motile cell fragments, based on an earlier developed phase field approach, to account for explicit dynamics of adhesion site formation, as well as for substrate compliance via an effective elastic spring. Our model displays steady motion vs. stick-slip transitions with concomitant shape oscillations as a function of the actin protrusion rate, the substrate stiffness, and the rates of adhesion. Implementing a step in the substrate’s elastic modulus, as well as periodic patterned surfaces exemplified by alternating stripes of high and low adhesiveness, we were able to reproduce the correct motility modes and shape phenomenology found experimentally. We also predict the following nontrivial behavior: the direction of motion of cells can switch from parallel to perpendicular to the stripes as a function of both the adhesion strength and the width ratio of adhesive to non-adhesive stripes.

The effects of vibrations on fluids are important in a wide range of scientific and engineering applications such as liquid storage, mixing, convection, pattern formation, and the study of basic fluid instabilities.

Vertical vibrations are the most studied case because the basic (unexcited) state is quiescent in a co-moving reference frame. Horizontally or obliquely vibrated systems, although more resistant to theoretical analysis, may be more relevant to the question of general fluid behavior than the more popular vertically forced Faraday system.

We present the new results on interface instability between miscible liquids when vibrations act either parallel to the interface or under 5-7° angle. The interface is represented as a transitional layer of small but nonzero thickness.  The considered mixtures represent the wide class of fluids: water-alcohol. We demonstrate both experimentally and theoretically not only that interface instability exists in miscible liquids but also strongly affect by the gravity.  The dependence of pattern formation and mixing on vibration forcing is discussed.

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.

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.

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.

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.

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.

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.

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.