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ItemNon-equilibrium current fluctuations in graphene(Georgia Institute of Technology, 2012-12-20) Wiener, Alexander DavidWe analyze experimental evidence of transport through evanescent waves in graphene, reconciling existing experimental data with theory. We propose novel experimental geometries that provide even more compelling evidence of evanescent waves. We investigate the shot noise generated by evanescent modes in graphene for several experimental setups. For two impurity-free graphene strips kept at the Dirac point by gate potentials, separated by a long highly doped region, we find that the Fano factor takes the universal value F=1/4. For a large superlattice consisting of many strips gated to the Dirac point, interspersed among doped regions, we find F=1/(8ln2). These results differ from the value F=1/3 predicted for a disordered metal, providing an unambiguous experimental signature of evanescent mode transport in graphene. For a graphene nano-ribbon transistor geometry, we explain that the experimentally observed anomalous voltage scale of the shot noise can arise from doping by the contacts to the electrical circuit. These observations provide strong evidence of evanescent mode transport in graphene.
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ItemAn FPGA-based microarchitecture for the implementation of quantum gates with trapped ions(Georgia Institute of Technology, 2012-12-18) Nichols, Charles SpencerQuantum computing promises to revolutionize computing by providing exponential speed improvements to classically difficult problems. Over the past 30 years, experimental research has progressed from manipulating quantum systems to creating elementary gates in many quantum mechanical systems. One of the most successful media for implementing quantum gates is trapped ions. Current trapped-ion quantum computing architectures have very high gate fidelities and long coherence times, but creating quantum gates with low error rates with trapped ions is challenging since it requires precise trap and laser control. In order to implement quantum gates with trapped ions, I have created a field-programmable-gate-array- (FPGA) based microarchitecture for constructing laser-pulse sequences and controlling ancillary equipment. The microarchitecture is centralized to minimize experimental timing errors and is programmable to provide the generality necessary for implementing a vast range of experiments.
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ItemUltra-high Energy Cosmic Rays from Blazars(Georgia Institute of Technology, 2012-12-10) Dermer, CharlesBlazar astronomy is rapidly progressing thanks in large part to the successes of the Fermi Gamma-ray Space Telescope and the ground-based gamma-ray telescopes. More than 1000 active galaxies have been detected at GeV energies, and nearly 50 at Very-High Energies (VHE, > 100 GeV). We can now explore multiwavelength and multi-messenger connections in unprecedented detail, and derive the astroparticle implications of those results. In this presentation, leptonic and hadronic spectral modeling of blazars is reviewed with the intent of identifying ultra-high energy cosmic rays (UHECRs) in the spectral energy distributions of these objects. We consider a number of unusual results that could be explained by UHECRs in blazars: (1) distinct spectral components revealed by deabsorption of blazar VHE spectra; (2) flattening at moderate redshift in the Stecker-Scully relation showing the GeV - TeV spectral index difference versus redshift; (3) conflicting results for the location of the gamma-ray emission site in blazars; (4) the unusually short variability times of luminous blazars. The arguments for and against radio galaxies and blazars being the sources of the UHECRs are reviewed, and predictions for UHECR composition is made if BL Lac objects accelerate most of the UHECRs. Unusual effects of UHECR acceleration in blazars is illustrated by the strange case of 4C +21.35. We also discuss effects of hypothetical axions, a dark matter candidate, in the interpretation of unusual blazar behavior, and a recent Fermi-LAT search for axions in occultations of bright AGNs by the Sun.
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ItemMechanics of undulatory swimming in a frictional fluid(Georgia Institute of Technology, 2012-12) Ding, Yang ; Sharpe, Sarah S. ; Masse, Andrew ; Goldman, Daniel I.The sandfish lizard (Scincus scincus) swims within granular media (sand) using axial body undulations to propel itself without the use of limbs. In previous work we predicted average swimming speed by developing a numerical simulation that incorporated experimentally measured biological kinematics into a multibody sandfish model. The model was coupled to an experimentally validated soft sphere discrete element method simulation of the granular medium. In this paper, we use the simulation to study the detailed mechanics of undulatory swimming in a ‘‘granular frictional fluid’’ and compare the predictions to our previously developed resistive force theory (RFT) which models sand-swimming using empirically determined granular drag laws. The simulation reveals that the forward speed of the center of mass (CoM) oscillates about its average speed in antiphase with head drag. The coupling between overall body motion and body deformation results in a non-trivial pattern in the magnitude of lateral displacement of the segments along the body. The actuator torque and segment power are maximal near the center of the body and decrease to zero toward the head and the tail. Approximately 30% of the net swimming power is dissipated in head drag. The power consumption is proportional to the frequency in the biologically relevant range, which confirms that frictional forces dominate during sand-swimming by the sandfish. Comparison of the segmental forces measured in simulation with the force on a laterally oscillating rod reveals that a granular hysteresis effect causes the overestimation of the body thrust forces in the RFT. Our models provide detailed testable predictions for biological locomotion in a granular environment.
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ItemFrom Social Intelligence of Bacteria to Cyber-war on Cancer(Georgia Institute of Technology, 2012-11-26) Ben-Jacob, EshelCancer continues to elude us. Metastasis, relapse and drug resistance are all still poorly understood and clinically insuperable. Evidently, the prevailing paradigms need to be re-examined and out-of-the-box ideas ought to be explored. Recently, has become acknowledged that transformative convergence of physical sciences with life sciences can bring forth new perspectives for addressing major questions and challenges relating to cancer. Drawing upon recent discoveries demonstrating the parallels between collective behaviors of bacteria and cancer, I will present a new picture of cancer as a society of smart communicating cells motivated by the realization of bacterial social intelligence. There is growing evidence that cancer cells, much like bacteria do, rely on advanced communication, social networking and cooperation to grow, spread within the body, colonize new organs, relapse and develop drug resistance. I will address the role of communication, cooperation and decision-making in bacterial collective navigation, swarming logistics and colony development. This will lead to a new picture of cancer cell migration, metastasis colonization and cell fate determination. I will reason that the new understanding calls for “cyber war” on cancer – the developments of drugs to target cancer communication and control.
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ItemHow Worms Wiggle(Georgia Institute of Technology, 2012-11-19) Samuel, AraviDirected locomotion requires coordinated motor activity throughout an animal’s body. The nematode C. elegans, with only 302 neurons, offers an opportunity to understand how locomotion is organized by an entire motor system. We discovered that the mechanism that organizes undulatory locomotion in C. elegans is a novel form of sensory feedback within the motor circuit. Stretch-sensory feedback simply compels each body segment to bend in the same direction and shortly after the bending of the adjacent anterior segment. Remarkably, the entire sensorimotor loop operates is contained within a single (particularly sophisticated) type of neuron. We used microfluidics, optogenetics, calcium imaging, and modeling to show how stretch sensory feedback is integrated into the motor circuit and how it explains the propagation of undulatory waves from head to tail. Our results point to a new framework for the organization of swimming and crawling gaits in worm undulatory locomotion.
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ItemUltracold sodium and rubidium mixtures: collisions, interactions and heteronuclear molecule formation(Georgia Institute of Technology, 2012-10-31) Raman, Chandra
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ItemLift-off dynamics in a simple jumping robot(Georgia Institute of Technology, 2012-10-26) Aguilar, Jeffrey ; Lesov, Alex ; Wiesenfeld, Kurt ; Goldman, Daniel I.We study vertical jumping in a simple robot comprising an actuated mass-spring arrangement. The actuator frequency and phase are systematically varied to find optimal performance. Optimal jumps occur above and below (but not at) the robot’s resonant frequency f0. Two distinct jumping modes emerge: a simple jump, which is optimal above f0, is achievable with a squat maneuver, and a peculiar stutter jump, which is optimal below f0, is generated with a countermovement. A simple dynamical model reveals how optimal lift-off results from nonresonant transient dynamics.
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ItemTopological Insulator Heterostructures: Searching for Exotic Particles on a Bench Top(Georgia Institute of Technology, 2012-10-08) Samarth, NitinA triumph of contemporary physics is the highly successful description of the most fundamental constituents of Nature and their excitations. Recent theories of “topological insulators” [1,2] have shown that in the complex and emergent world of condensed matter physics, one can engineer the interplay between fundamental symmetries, band structure and spin-orbit coupling to create novel energy-spin-momentum relationships for band electrons and to yield effective realizations of exotic particles predicted but yet unobserved in Nature. This Colloquium will describe the experimental routes we are pursuing in this context to build "detectors" for such particles, by coupling the surface states of a topological insulator with the gauge symmetry breaking effects of superconductivity [3] and the time-reversal symmetry breaking effect of magnetism [4.5]. 1. M. Z. Hasan and C. L. Kane, Rev. Mod. Phys. 82, 3045 (2010). 2. Xiao -Liang Qi and Shou-Cheng. Zhang, Rev. Mod. Phys. 83, 1057 (2011). 3. Duming Zhang et al., Phys. Rev. B 84, 165120 (2011). 4. Su-Yang Xu et al., Nature Physics 8, 616 (2012). 5. Duming Zhang et al., arxiv: 1206.2908.
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ItemPhysics of the Piano(Georgia Institute of Technology, 2012-10-01) Giordano, NickWhy does a piano sound like a piano? A similar question can be asked of virtually all musical instruments. A particular note, such as middle C, can be produced by a piano, a violin, and a clarinet. Yet, it is easy for even a musically untrained listener to distinguish between these instruments. One would like to understand why the sound of the “same” note depends greatly on the instrument. In particular, we would like to understand what aspects of the piano are most critical in producing its musical tones. The questions we will address in the talk include: Who invented the piano and why? Why does the piano have 88 keys and not more or fewer? How and why is the tone color of a loud note different from that of a soft note, and why is this important? Why are the bass strings on a piano made by wrapping a coil of wire around a central wire core? A piano tone is the sum of components that can be described by sine waves. The frequencies of these sine waves deviate a small amount from a simple harmonic series. What is the source of these deviations and why are they important? After we have addressed all of these questions, we’ll be able to understand why a piano sounds like a piano.