Physics & Astronomy Department
2401 Vanderbilt Place
Nashville, TN 37240-1807
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.
Rhett Allain, Department of Chemistry & Physics, Southeastern Louisiana University
Real vs. Fake Videos: The Physics of Video Analysis
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
Kate Scholberg, Department of Physics, Duke University
Neutrinos from the Sky and Through the Earth
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
Marcelo Gleiser, Department of Physics & Astronomy, Dartmouth College
Emergent Complexity in the Universe: An Information-Entropic Approach
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
David Snoke, Department of Physics & Astronomy, University of Pittsburgh
Superfluid photons: Bose-Einstein condensation of polaritons in microcavities
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
Kirill Bolotin, Department of Physics & Astronomy, Vanderbilt University
Weirdness in two dimensions
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
Ingmar Riedel-Kruse, Department of Bioengineering, Stanford University
Playful interactions and biophysics of multi-cell patterns
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
William C. Keel, Department of Physics & Astronomy, University of Alabama, Tuscaloosa
Citizen Science, Giant Ionized Clouds, and the History of Galactic Nuclei
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
Rene Lopez, Department of Physics & Astronomy, University of North Carolina, Chapel Hill
Bio-inspired electro-photonic structures for alternative solar cells
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
Hong-Jun Gao, Institute of Physics, Chinese Academy of Sciences
Construction of 2D Atomic Crystals on Transition Metal Surfaces: Graphene, Silicene, and Hafnene
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
David Weintraub, Department of Physics & Astronomy, Vanderbilt University
Exoplanets, Extraterrestrial Life, and Religion
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
Larry Taber, Department of Biomedial Engineering, Washington University
How the Embryo Uses Mechanics to Construct Organs
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
Joshua Lui, Department of Physics, MIT
Shedding light on two-dimensional electrons in graphene and beyond
Graphene, a single layer of carbon atoms, has stimulated intense scientific interest due to its distinctive electronic and mechanical properties. Graphene also exhibits strong interactions with light over a broad spectral range. This enables us to examine its electronic and vibrational properties through optical spectroscopy. In addition to gaining understanding of the properties of single-layer graphene, we can also probe the behavior of electrons in few-layer graphene. This reveals the unique electronic and vibrational properties for graphene of each layer thickness and stacking order, as well as their distinct capability to induce an electrically tunable band gap. I will also highlight recent development of 2D materials beyond graphene.
Host: R. Scherrer
Marija Zanic, Department of Cell & Developmental Biology, Vanderbilt University
Microtubule Dynamic Instability: Understanding the Molecular Switch
Microtubules are dynamic cytoskeletal polymers essential for cell division, motility, shape and intracellular transport. Remodeling of the microtubule cytoskeleton relies on precise regulation of switching between growth and shrinkage of individual polymers, behavior known as microtubule dynamic instability. We use bottom-up in vitro reconstitution approaches with purified proteins and total-internal-reflection-fluorescence microscopy to elucidate the molecular mechanisms underlying dynamic instability and its regulation. Our research showed that microtubules age: the switch between growth and shrinkage requires multiple steps, allowing for greater control of microtubule length distributions. Additionally, we identified a minimal system needed for reconstitution of physiological microtubule growth rates, never previously obtained using purified components. The acceleration of growth is achieved through collective effects of two microtubule-regulating proteins, leading to fast and dynamic microtubule behavior typically observed in cells.
Host: S. Hutson
Ilija Zeljkovic, Department of Physics, Boston College
Symmetry protected Dirac electrons in topological crystalline insulators
In Dirac materials such as graphene and topological insulators, electrons behave like relativistic particles with zero mass, which is a direct consequence of the form of the low energy effective Hamiltonian describing these electrons. Topological crystalline insulators are a recently discovered class of topological materials, in which topology and crystal symmetries intertwine to create relativistic massless electrons. One of the unique characteristics of these systems is that crystalline symmetry breaking is theoretically predicted to impart mass to otherwise massless surface state (SS) Dirac electrons. In this talk, I will discuss our recent experimental investigations of a topological crystalline insulator Pb1-xSnxSe. We acquire and analyze two types of scanning tunneling microscopy (STM) data: Fourier transforms of interference patterns of SS electrons and Landau level spectroscopy. Our experiments reveal the coexistence of zero mass Dirac electrons protected by crystal symmetry and massive Dirac electrons arising from crystal symmetry breaking. Additionally, we discover that the measured electron mass scales with alloying composition, which we attribute to one of the fundamental properties of any topological SS – SS penetration depth.
Host: R. Scherrer
Darius Torchinsky, Department of Physics, California Institute of Technology
Revealing hidden symmetry breaking in strongly correlated matter
Essential to a microscopic understanding of strongly correlated materials is a clear picture of the relationship between their myriad quantum ground states. However, in phenomena ranging from unconventional magnetism to high temperature superconductivity, this picture is often obscured by the presence of broken symmetries hidden from view of existing experimental techniques. This may include hidden structural symmetries or tensor order parameters representing complex spatial arrangements of multipolar electric and magnetic moments. It may even include electronic forms of order which come in and out of existence on ultrashort timescales, invisible to static probes. I will demonstrate how ultrafast time resolved and nonlinear optical methods can reveal hidden symmetry breaking in some of the most intensely researched strongly correlated materials of the past decade, including high-temperature superconductors, spin-orbit coupled transition metal oxides and heavy fermion materials, and I will discuss how the newly uncovered symmetries play a fundamental role in their physics.
Host: R. Scherrer
Farhan Rana, School of Electrical and Computer Engineering, Cornell University
Graphene Plasmonics: From Physics to Devices
Plasmons in graphene are collective charge density oscillations of electrons. Although most materials with free electrons have plasmons, the unique properties of graphene make graphene plasmons not just tremendously interesting from a science perspective but also very suitable for different applications. Plasmons in graphene have frequencies in the 1-100 terahertz range and wavelengths in the 10-1000 nanometer range enabling extreme confinement of the electromagnetic energy. Tailored modes in patterned graphene micro- and nano-scale plasmon resonators can be studied with terahertz and optical probes. Plasmon frequencies are tunable over almost 3 octaves. Plasmons in graphene are strongly coupled to interband electronic transitions and plasmon emission dominates electron-hole recombination resulting in sub-picosecond recombination times. Plasmon oscillations generate mechanical forces between adjacent resonators that can approach values as large as 1 micro-Newton/micron allowing for the possibility of manipulating atomic membranes with plasmonic forces. Plasmons in graphene interact with the optical phonons resulting in hybrid collective modes that exhibit EIT-like features in radiation absorption. This talk will present our results in this area and discuss applications of graphene plamsons in different devices, such as hyperspectral detectors, terahertz sources, and sensors.
Host: N. Tolk
Victor Brar, California Institute of Technology
Surface Plasmons and Impurity States in Graphene
Graphene, an atomically thin sheet of carbon atoms, provides a unique platform to study quasiparticle interactions and to create optical metamaterials with novel properties. The 2D nature and linear band structure of graphene give rise to a semi-metal behavior, with tunable dielectric properties and a high electron mobility. In the DC regime, these properties allow graphene to display novel screening behavior in the presence of charged impurities, which I will show can be probed directly using scanning tunneling spectroscopy (STM). At optical (mid-IR) frequencies, the graphene dielectric function is strongly perturbed by the presence of collective excitations known as surface plasmon polaritons (SPPs), which are optical modes - supported by free carriers - that are bound to the graphene sheet. These modes show a number of remarkable properties, including a carrier density dependent dispersion relation, and wavelengths that are more than 100 times shorter than freespace. I will show how the surface plasmons in graphene can be harnessed to create infrared metasurfaces that have tunable optical properties. These materials will be shown to exhibit extreme light-matter interactions that can control the macroscopic absorption and emission properties of nanostructured dielectric stacks.
Host: R. Scherrer
Kartik Sheth, NRAO/NAASC
Constraining the Assembly of Galaxy Disks over the last 12 Billion Years with ALMA, HST and Spitzer
Bars are a key signpost in the evolutionary history of a disk galaxy. When a disk is sufficiently massive, dynamically cold and rotationally supported, and sufficient time has elapsed for the baryonic matter to exchange energy and angular momentum with the dark matter halo or the outer disk, the formation of a bar is inevitable. Therefore understanding the evolution of the bar fraction as a function of the host galaxy properties and as a function of redshift provides important clues to the evolutionary history of galaxies. I will present the latest results on local bars from the Spitzer Survey of Stellar Structure in Galaxies (S4G) and discuss the observations for the declining bar fraction with redshift from the COSMOS survey. A plausible reason for the decline in the bar fraction may be that galaxy disks were too dynamically hot to host bars at higher redshift which we have investigated using the DEEP2 / AEGIS data. Together these data are beginning to provide a coherent and consistent picture for the assembly history of disks on the Hubble sequence. The star formation in these disks is also now being understood with the latest results from ALMA. I will show the latest results on the cosmological evolution of the molecular gas content in a mass-selected sample of galaxies at three epochs, z=2.2, z=1, and z=0.3 and discuss planned Cycle 1 observations of the molecular gas environment in the prototypical barred spiral NGC 1097.
Host: R. Scherrer
Anirudha V. Sumant, Center for Nanoscale Materials, Argonne National Laboratory
Exploring the flatland of 2D materials for tribological manipulation
Macroscopic friction and wear remain the primary modes of mechanical energy dissipation in moving mechanical assemblies. With recent advances in the development of 2D materials and understanding of their mechanical and tribological properties, we have now began to unfold a completely new perspective on how they behave as a lubricant as compared to traditional thin film or bulk solid lubricants. In my talk, I’ll mostly talk about exceptional wear/friction properties of few layer and single atom thick graphene from nano to macroscale and will discuss their frictional response in different environmental conditions. With the help from reactive molecular dynamic simulations, we elucidate the role of different molecular species and their interaction with graphene at the tribological interface. Our studies demonstrate that tuning the atomistic scale chemical interactions holds the promise of realizing extraordinary tribological properties of monolayer graphene coatings. These findings may offer a direct pathway for designing smart frictionless tribological systems for practical applications of industrial interest.
Host: N. Tolk
Chad Orzel, Department of Physics & Astronomy, Union College
Talking Dogs and Galileian Blogs: Social Media for Communicating Science
Modern social media technologies provide an unprecedented opportunity to engage and inform a broad audience about the practice and products of science. Such outreach efforts are critically important in an era of funding cuts and global crises that demand scientific solutions. In this talk I'll offer examples and advice on the use of social media for science communication, drawn from more than a dozen years of communicating science online.
Host: R. Scherrer
Stefano Profumo, Department of Physics, University of California, Santa Cruz
New Physics and Astrophysical Searches for Dark Matter
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
What is Relativity? An Intuitive Introduction to Einstein’s Ideas and Why They Matter
This year (2015) marks the 100th anniversary of Einstein's completion of his General Theory of Relativity, yet the vast majority of students, teachers, and the public know very little about the critical importance of either the special or general theories to modern understanding of the universe. In this presentation, based on his book What is Relativity? from Columbia University Press, Dr. Bennett will introduce you to the basic ideas of Einstein's theories and discuss why they are important to everyone. (He’ll also help you understand the movie Interstellar.) No prior knowledge of physics or relativity will be assumed. Note: This presentation is part of Dr. Bennett’s “national relativity tour” in honor of a century of general relativity.
Bio: Astrophysicist and educator Jeffrey Bennett’s extensive experience includes teaching at every level from preschool through graduate school, proposing and helping to develop the Voyage Scale Model Solar System on the National Mall in Washington, DC, and serving two years as a Visiting Senior Scientist at NASA Headquarters, where he helped create numerous programs designed to build stronger links between the research and education communities. He is the lead author of bestselling college textbooks in astronomy, astrobiology, mathematics, and statistics; of critically acclaimed titles for the general public including Beyond UFOs (Princeton University Press, 2008/2011), Math for Life (Big Kid Science 2014), What is Relativity? (Columbia University Press, 2014), and On Teaching Science (Big Kid Science, 2014). His five books for children are currently orbiting Earth and being read by astronauts aboard the International Space Station for the new “Story Time From Space” program (storytimefromspace.com). Dr. Bennett was recently honored with the American Institute of Physics Science Communication Award. His personal web site is www.jeffreybennett.com.
Host: D. Weintraub
Orlando Auciello, Department of Materials Science & Engineering, Department of Bioengineering, University of Texas-Dallas
Host: Y. Xu
David Awschalom, Institute for Molecular Engineering, University of Chicago
Beyond electronics: abandoning perfection for quantum technologies
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
Angela Speck, Department of Physics & Astronomy, University of Missouri
Host: D. Weintraub
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