Department Colloquium

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Physics & Astronomy Department
Vanderbilt University
PMB 401807
2401 Vanderbilt Place
Nashville, TN 37240-1807

(615) 322-2828
(615) 343-7263

Physics and Astronomy Colloquium, 2014-2015

Colloquia are held on Thursdays at 3PM in room 4327 (building 4) of the Stevenson Science Center unless otherwise noted. Click here for directions, or phone the department. A reception with the speaker is held at 2:30pm in Stevenson 6333.

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Fall 2014

Thursday, September 11---FORMAN LECTURE

Rhett Allain, Department of Chemistry & Physics, Southeastern Louisiana University

Real vs. Fake Videos: The Physics of Video Analysis   (show abstract)

We have all seen videos on television or online, and wonder "Is that real?" Most fake videos have either unrealistic physics or show an event with a very small probability of success. In this talk, I will share some of my favorite online videos along with the tools and physics used to analyze them.

Host: R. Scherrer

Thursday, September 18

Kate Scholberg, Department of Physics, Duke University

Neutrinos from the Sky and Through the Earth   (show abstract)

The progress in neutrino physics over the past fifteen years has been tremendous: we have learned that neutrinos have mass and change flavor. I will pick out one of the threads of the story-- the measurement of flavor oscillation in neutrinos produced by cosmic ray showers in the atmosphere, and further measurements by long-baseline beam experiments. In this talk, I will present the latest results from the Super-Kamiokande and T2K (Tokai to Kamioka) long baseline experiments, and will discuss how the next generation of high-intensity beam experiments will address some of the remaining puzzles.

Host: David Ernst

Thursday, September 25

Marcelo Gleiser, Department of Physics & Astronomy, Dartmouth College

Emergent Complexity in the Universe: An Information-Entropic Approach   (show abstract)

From atoms to stars, physically-bound systems result from the interplay between attractive and repulsive interactions. In this lecture, I will present a new measure of complexity called "Configurational Entropy". Inspired by Shannon's information entropy, I will show how the configurational entropy encodes information about the shape and the stability of various physical objects, and how it can be used as an efficient measure of emerging complexity during nonequilibrium phenomena. Applications will include solitons, compact astrophysical objects, spontaneous symmetry breaking, and inflationary cosmology.

Host: R. Scherrer

Thursday, October 2

David Snoke, Department of Physics & Astronomy, University of Pittsburgh

Superfluid photons: Bose-Einstein condensation of polaritons in microcavities    (show abstract)

In specially designed solid microcavities, the photon properties can be altered to have effective mass and repulsive interactions; these new states are called "polaritons". The polaritons act like atoms, and because they are bosons, they can undergo Bose-Einstein condensation. The experiments on polariton condensation have shown truly remarkable progress in recent years, with new results showing superfluidity and quantized vorticity in a ring geometry. I will review the state of the art in the field, including results from our lab in Pittsburgh which show quantized vorticity, and measurements of the phase diagram for the polariton condensation.

Host: R. Haglund

Thursday, October 9

Kirill Bolotin, Department of Physics & Astronomy, Vanderbilt University

Weirdness in two dimensions    (show abstract)

The discovery of two-dimensional atomic crystals (2DACs)-- materials only a few atoms thick -- sparked an ongoing revolution in condensed matter physics. We can now isolate, cut, move, and stack single atomic sheets for dozens of different materials. The diverse family of 2DACs includes graphene, an allotrope of carbon with massless relativistic charge carriers, monolayer molybdenum disulfide (MoS2), a semiconductor with very strong interactions between electrons, and boron nitride, a near-perfect insulator. In this talk, I will explain how reduced dimensionality of 2DACs dramatically changes properties of these materials. First, we will discuss the effects due to unique out-of-plane "lexural" 2D phonons in graphene. We will see how graphene gets softer, more thermally conductive, and less "bendy" due to these phonons. Second, we see how reduced screening in two dimensions leads to formation of extremely tightly bound hydrogenic electron-hole pairs, or excitons, in monolayer MoS2. We will discuss the approach for precise measurements of the binding energy of these excitons.

Host: R. Scherrer

Thursday, October 16---Fall Break

Thursday, October 23

Ingmar Riedel-Kruse, Department of Bioengineering, Stanford University

Playful interactions and biophysics of multi-cell patterns    (show abstract)

Dynamic multi-cell patterns such as generated by micro-swimmers or during development are fascinating to watch as well as challenging to understand. In my talk I will describe how we can design and engineer platforms that enable the open-ended interaction with and exploration of such micro-biological systems, for example via biotic video games or online experiments. These new interactive media hold significant promise to facilitate education, research, and other applications with the vision to provide a first hand experience (and understanding) of modern life sciences to everyone.

Host: S. Hutson

Thursday, October 30

William C. Keel, Department of Physics & Astronomy, University of Alabama, Tuscaloosa

Citizen Science, Giant Ionized Clouds, and the History of Galactic Nuclei   (show abstract)

The signature discovery of the Galaxy Zoo citizen-science project has been Hanny's Voorwerp, a galaxy-sized gas cloud ionized by a quasar which has faded so rapidly that we no longer see it when observing the galaxy nucleus directly. Project participants have helped find a sample of 20 similar objects, giving our first look at the history of active galactic nuclei on timescales from 30,000-120,000 years. About 40% of these clouds require much more energy input thanthe nuclear source can provide, indicating that dramatic variability of active nuclei is common on these timescales. This is faster than simple models indicate for accretion disk changes; signs of gaseous outflow and triggered star formation may mean that the rate of accretion itself is changing less strongly than its byproducts, switching to kinetic rather than radiative-energy dominance. Current surveys show similar cases in both high- and low-power regimes; our snapshot of the population of accreting supermassive black holes will be incomplete without including these faded objects.

Host: K. Holley-Bockelman

Thursday, November 6---HOLLADAY LECTURE

Rene Lopez, Department of Physics & Astronomy, University of North Carolina, Chapel Hill

Bio-inspired electro-photonic structures for alternative solar cells   (show abstract)

A major challenge in solar cell technology is simultaneously achieving an efficient absorption of photons and effective carrier extraction. In all cases, light absorption considerations call for thicker modules while carrier transport would benefit from thinner ones - a fundamental problem limiting the efficiencies of most photovoltaics. One way to overcome this problem is to decouple light absorption from carrier collection. We present solutions to this problem applying bio-inspired nanostructures to three different types of systems: organic photovoltaic (OPV) and dye sensitized (DSSC) and quantum dot (QD) solar cells. For OPV devices, we describe a 2-D photonic crystal geometry that enhances the absorption of polymer-fullerene photonic cells ~ 20% relative to conventional planar cells. In DSSCs we introduce a new structural motif for the photoanode in which the traditional random nanoparticle oxide network is replaced by vertically aligned bundles of TiO2 nanocrystals; the direct pathways provided by the vertical structures provide for enhanced collection efficiency for carriers generated throughout the device. The most striking potential enhancement is found in PbS QDs solar cells where simple photonic structures could double their performance from current heterojunction cells, and even surpass the 20 % limit for a viable disruptive entry in the photovoltaic commercial energy landscape.

Host: R. Haglund

Thursday, November 13

Hong-Jun Gao, Institute of Physics, Chinese Academy of Sciences

Construction of 2D Atomic Crystals on Transition Metal Surfaces: Graphene, Silicene, and Hafnene    (show abstract)

The novel properties of graphene-like honeycomb structure have spurred tremendous interest in investigating other two-dimensional (2D) layered structures beyond graphene. In this talk, I will present the construction of graphene, silicene, and hafnene honeycomb lattices on transition metal surfaces (TMS) (for example, Ru(0001), Pt(111), and Ir(111)). Molecular beam epitaxial growth technique is used to form the large scale 2D atomic crystals on TMS. Low electron energy diffraction (LEED) and scanning tunneling microscopy/spectroscopy (STM/S) together with density functional theory (DFT) calculations are employed to confirm the formed structure on the TMS. We expect that on the TMS more new 2D crystals could be found and these materials will show very interesting physical property and its promising potential applications in nanoscale devices

Host: S. Pantelides

Thursday, November 20

David Weintraub, Department of Physics & Astronomy, Vanderbilt University

Exoplanets, Extraterrestrial Life, and Religion    (show abstract)

Astronomers have now discovered thousands of planets in orbit around other stars. I will briefly describe those discoveries and predict the progress astronomers are likely to make in their studies of these planets over the next fifty years, as we begin to study these planets in detail, looking for evidence for the presence or absence of life. Then we will consider some of the consequences of those potential discoveries. Specifically, if astronomers develop convincing evidence that life exists beyond the Earth, how will that discovery impact terrestrial religions and our understanding of our place in the universe? Are any of humanities’ religions universal, or does a particular religion only make sense for earthlings?? Would Roman Catholicism or Judaism or Islam or Mormonism or Buddhism work or make sense on another planet? Could a Klingon be a Southern Baptist?

Host: R. Scherrer

Thursday, Novmember 27---Thanksgiving

Thursday, December 4

Larry Taber, Department of Biomedial Engineering, Washington University

How the Embryo Uses Mechanics to Construct Organs   (show abstract)

Although the molecular and genetic aspects of embryonic development are becoming clear, the physical mechanisms that create tissues and organs remain poorly understood. Using a combination of theoretical modeling and laboratory experiments, we study the mechanics of heart and brain morphogenesis in the early embryo. This talk focuses on two problems: (1) cardiac looping, which transforms the initially straight heart tube into a curved tube to lay out the basic plan of the mature heart; and (2) the response of the early heart and brain to changes in mechanical loads. Our general approach is to conduct experiments that perturb development in chick embryos. Morphogenesis is altered mechanically using microdissection as well as chemically using drugs that alter cytoskeletal activities. For each condition, morphogenetic strains, tissue stresses, and mechanical properties are measured. Next, finite element models are used to determine the specific combination of processes (e.g., differential growth and cytoskeletal contraction) that can create the observed shape changes in normal embryos. Finally, the proposed mechanism is tested by simulating our perturbation experiments and comparing numerical predictions of morphogenesis with experimental results. This approach has led to new insights into the forces that drive heart and brain development. In addition, our results suggest that widely disparate tissues in the embryo respond similarly to perturbations in mechanical loads. Understanding the mechanics of development could one day lead to new strategies for tissue engineering, tissue regeneration, and the prevention and treatment of congenital malformations.

Host: S. Hutson

Spring 2015

Thursday, January 15

Joshua Lui, MIT

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Host: R. Scherrer

Thursday, January 22

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Thursday, January 29

Ilija Zeljkovic, Boston College

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Host: R. Scherrer

Thursday, February 5

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Thursday, February 12

Alfred Leitenstorfer, Department of Physics, Universitaet Konstanz

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Host: R. Haglund

Thursday, February 19---SLACK LECTURE

David Awschalom, Institute for Molecular Engineering, University of Chicago

Beyond electronics: abandoning perfection for quantum technologies    (show abstract)

Our technological preference for perfection can only lead us so far: as traditional transistor-based electronics rapidly approach the atomic scale, small amounts of disorder begin to have outsized negative effects. Surprisingly, one of the most promising pathways out of this conundrum may emerge from recent efforts to embrace defects and construct 'quantum machines’ to enable new information technologies based on the quantum nature of the electron. Recently, individual defects in diamond and other materials have attracted interest as they possess an electronic spin state that can be employed as a solid state quantum bit at and above room temperature. Research at the frontiers of this field includes creating and manipulating these unusual states in a new generation of nanometer-scale structures. These developments have launched technological efforts aimed at developing applications ranging from secure data encryption to radical improvements in computation speed and complexity. We will describe recent advances towards these goals, including the surprising ability to control atomic-scale spins for communication and computation within materials surrounding us for generations.

Host: R. Scherrer

Thursday, February 26

Farhan Rana, School of Electrical and Computer Engineering, Cornell University

Graphene Plasmonics: From Physics to Devices    (show abstract)

Host:N. Tolk

Thursday, March 5---Spring Break

Thursday, March 12

Kartik Sheth, NRAO/NAASC

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Thursday, March 19

Anirudha V. Sumant, Center for Nanoscale Materials, Argonne National Laboratory

Graphene Triboloby    (show abstract)

Host:N. Tolk

Thursday, March 26---FORMAN LECTURE

Chad Orzel, Department of Physics & Astronomy, Union College

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Thursday, April 2

Stefano Profumo, Department of Physics, University of California, Santa Cruz

New Physics and Astrophysical Searches for Dark Matter    (show abstract)

Can we learn about New Physics with astronomical and astro-particle data? Understanding how this is possible is key to unraveling one of the most pressing mysteries at the interface of cosmology and particle physics: the fundamental, particle-physics nature of dark matter. I will discuss some of the recent puzzling findings in astro-particle and astronomical observations that might be related to signals from dark matter. I will first review the status of explanations to the cosmic-ray positron excess, emphasizing how we might be able to discriminate between astrophysical sources and dark matter. I will then discuss the evidence for an X-ray line at 3.5 keV, and present new results on systematic effects and on the role of previously underestimated astrophysical lines. Finally, I will briefly discuss a reported excess of gamma rays from the central regions of the Galaxy. I will address the question of whether we are possibly observing a signal from dark matter annihilation, how to test this hypothesis, and which astrophysical mechanisms constitute the relevant background.

Host: R. Scherrer

Thursday, April 9

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Thursday, April 16

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Thursday, April 23

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