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.
Dennis Zaritsky, Department of Astronomy, University of Arizona
Faint Matter in Galaxies
The focus of dark matter studies must now be on galactic scales. First, this is the final domain where potential conflicts with the standard paradigm have been identified. Second, galaxies are the environment in which we expect to eventually have indirect or direct detection(s) of dark matter. Unfortunately, in this sense, the central regions of galaxies (including the region of the Milky Way that we inhabit) are dominated by baryons. Any conflict with the standard paradigm or dark matter detection/limit must be understood within the context of the baryonic physics and the baryonic contribution to the mass density. This requires a far greater understanding of the baryonic components than what we currently have. In this talk I will focus on two aspects of this challenge, the stellar initial mass function and the search for a complete accounting of stellar baryons in galaxies.
Host: A. Berlind
Kelly Holley-Bockelmann, Department of Physics and Astronomy, Vanderbilt University
Building the Black Hole in Our Own Backyard
Astronomers now know that supermassive black holes are a natural part of nearly every galaxy, but how these black holes form, grow, and interact within the galactic center is still a mystery. In theory, gas-rich major galaxy mergers can easily generate the central stockpile of fuel needed for a low mass central black hole 'seed' to grow quickly and efficiently into a supermassive one. Because of the clear theoretical link between gas-rich major mergers and supermassive black hole growth, this major merger paradigm has become a well-accepted way to form the billion solar mass black holes that power bright quasars in the early universe. It's much less clear, though, how well this paradigm works for growing the 'lightest' supermassive black holes; these million solar mass black holes tend to lie in galaxies like our own Milky Way, where the supermassive black hole is currently quiescent and major mergers were few and far between. This talk will touch on some current and ongoing work on refining our theories of black hole growth for this lightest supermassive class, and will feature the work of Fisk and Vanderbilt graduate students and postdocs.
Host: R. Scherrer
Joel Kastner, Rochester Institute of Technology
Observing Star and Planet Formation at Close Range
The past decade has seen an explosion in our knowledge of young stars near Earth. This development is generating rapid progress in the study of star and planet formation and, in particular, is driving recent and forthcoming direct imaging searches for young, warm exoplanets and proto-exoplanets. We are exploiting the recent availability of all-sky ultraviolet and infrared photometric data to compile a comprehensive list of candidate young age, 100 Myr, low-mass stars within 300 light years of Earth. Followup spectroscopy and space motion measurements are yielding the ages and distances of these candidates. In parallel, we are conducting comprehensive, multiwavelength (radio to X-ray) investigations of specific nearby, young, Sun-like stars that are orbited by dusty, gaseous ("protoplanetary") disks. These observations are yielding, among other things, insight into the effects of intense high-energy radiation from young, hyperactive host stars on the molecular chemistries within protoplanetary disks, as well as evidence for ongoing planet building.
Host: D. Weintraub
Andreas Berlind, Department of Physics and Astronomy, Vanderbilt University
From Dark Matter to Galaxies: Probing the Spatial Structure of the Universe and Testing our Cosmological Models
The last decade has seen an explosion of high precision measurements of the structure of the universe, courtesy of large galaxy surveys such as the Sloan Digital Sky Survey (SDSS). Galaxy clustering measurements encode information about the nature and abundance of dark matter and dark energy, as well as the complex physical process of galaxy formation. However, harnessing the full constraining power of the data is very challenging since it requires a detailed understanding of the statistical and systematic uncertainties in both data and models, which in turn demands significant computational effort. I will discuss my onging research program to analyze SDSS data and model it with the help of cosmological N-body simulations, highlighting several results from both recent and current ongoing projects.
Host: R. Scherrer
Robert Scherrer, Department of Physics and Astronomy, Vanderbilt University
Could Dark Matter be Electromagnetic?
One of the leading candidates for dark matter is a weakly-interacting massive particle (WIMP), with an annihilation cross section typical of the weak interaction. However, recent years have also seen a growth of interest in the possibility that dark matter interacts electromagnetically. While charged dark matter is largely ruled out, recent papers have explored the possibility of dark matter with a magnetic dipole moment, an electric dipole moment, or an anapole moment. I will review the calculation that determines the relic abundance of a given dark matter particle, and show why charged dark matter is ruled out, while higher order multipoles are not. I will then discuss the signatures of electromagnetic dark matter in direct detection experiments, and show why the anapole is favored over dipole dark matter for particles with masses greater than 10 GeV. Directions for future investigation will be outlined.
Host: T. Weiler
Bernardo Mendoza, Centro de Investigaciones en Optica, Leon, Mexico
Optical properties of nanostructured metamaterials
We present a very efficient recursive method to calculate the effective optical response of nanostructured metamaterials made up of particles with arbitrarily shaped cross sections arranged in periodic two-dimensional arrays. We consider dielectric particles embedded in a metal matrix with a lattice constant much smaller than the wavelength. Neglecting retardation our formalism allows factoring the geometrical properties from the properties of the materials. If the conducting phase is continuous the low frequency behavior is metallic. If the conducting paths are nearly bloqued by the dielectric particles, the high frequency behavior is dielectric. Thus, extraordinary-reflectance bands may develop at intermediate frequencies, where the macroscopic response matches vacuum. The optical properties of these systems may be tuned by adjusting the geometry.
Host: N. Tolk
Vaughan Jones, Department of Mathematics, Vanderbilt University
Do all subfactors come from physics?
A subfactor is a mathematical object connected to analysis, algebra, topology, geometry, combinatorics. There is a lot of circumstantial evidence that subfactors are deeply connected with physics, especially in one and two dimensions. For instance the transfer matrices of 2-D statistical mechanical methods define subfactors of some interest, the braiding of n-point functions in conformal field theory produces subfactors in a similar way and algebras of local observables in superselection sectors "are" subfactors. Many new and apparently "exotic" subfactors have been discovered recently and understood using planar algebra methods. It is tempting to speculate that a subfactor itself may produce a lattice model or conformal field theory. I will try to explain these objects, which generalize the notion of group, in an accessible way and give a crazy idea, Halloween appropriate, of taking a continuum limit what mathematicians call the "Thompson group" are the local scale transformations on a 1-dimensional lattice with sites at every dyadic rational.
Host: R. Scherrer
Vernita Gordon, University of Texas, Austin
Spatial structure in multicellular bacterial systems: how it develops, and why it matters
Microbial biofilms are communities of interacting single-celled organisms that are bound to a surface and to each other by extracellular polymers. (1) These extracellular polymers give the biofilm its structural and mechanical properties. We have recently found that extracellular polymers also influence the behavior of single cells attached to surfaces, before a biofilm has formed. This suggests that single-celled measurables could serve as a predictive readout for biofilm structure and mechanics. This would be convenient, because biofilm mechanics is important for biofilm removal yet tricky to measure. (2) Extracellular polymers also control intracellular associations, which is important for consortial interactions. We study one such interaction that depends on the population density and spatial arrangement of bacteria: in the presence of a front-line antibiotic, bacterial cells can protect each other at short range and inhibit each other at long range. This suggests ways in which co-culture with unicellular eukaryotes could benefit bacteria, and also presents a model system with which to study spatially-dependent intercellular interactions. (3) Finally, we have recently developed a method for controlling the spatial arrangement of bacteria so that the effects of spatial structure can be studied. We arrange bacteria on surfaces with single-cell resolution using laser trapping, so that native motility and the signalling effects of surface attachment are preserved unhindered. We demonstrate that this approach can be used to study spatially-dependent group behaviors of bacteria.
Host: E. Rericha
Kang Wang, Department of Electrical Engineering, UCLA
Energy Scaling of Spintronics – A new paradigm for nanoelectronics - From spin based memory to low dissipation intelligent systems
The talk will describe the physics of collective spins or nanomagnetic and the engineering of spin-orbit interaction at the interface. First, I will give a brief overview of the energy challenge of today's CMOS scaling. Then I will describe the advantages and impact of magnetic devices in terms of its low switching energy, high speed, high endurance, and scalability. The physics of spin transfer torque (STT) will be described for its application of spintronic memory. Next, we will describe in addition the physics of spin-orbit interaction and spin Hall to improve energy efficient switching via polarized spins. Only recently, it was shown to possibly use electric field to control magnetic properties of metallic ferromagnetic layers. For the latter, we will describe a couple of fundamental mechanisms of voltage control of magnetic moment and direction at the metallic surface. I will specifically describe a new concept of electric field control of metallic magnetism, that is, the use of electric field to manipulate magnetic field via engineering of the spin-orbit interactions at the metallic interface. This will lead to electric-field or voltage controlled magneto-electric (ME) memory (Me-RAM) , resulting in much reduced energy dissipation for switching. The dynamics of the switching as well as additional physical processes in improving the switching process will be outlined. The integration of such magnetic devices with CMOS will reduce the standby leakage of CMOS circuits and thus enable further scaling of CMOS with improved performance. Energy scaling will be addressed. Further advances are possible by adopting the spin wave bus concept -- the use of spin waves for logic and interconnect . With low energy, high density memory and spin wave bus, it may be possible to construct a new type of neuromorphic information processing electronics. These types of devices may be integrated directly on top of front-end processed CMOS to enable new generations of nonvolatile instant-on electronics and other systems. A potential new paradigm of intelligent nano-systems may emerge. 1. J. G. Alzate, P. Khalili Amiri, P. Upadhyaya, S.S. Cherepov, J. Zhu2, M. Lewis, J. A. Katine, J. Langer, K. Galatsis, I. Krivorotov, and K. L. Wang, "Voltage-Induced Switching of Nanoscale Magnetic Tunnel Junctions", IEDM. 2012. 2. Khitun, A., Bao, M., Lee, J.-Y., Wang, K.L., Lee, D.W., Wang, S. X. and Roshchin, I.V., "Inductively Coupled Circuits with Spin Wave Bus for Information Processing", Journal of Nanoelectronics and Optoelectronics, 3(1): 24-34. (March 2008) 3. K. L. Wang and P. Khalili Amiri, "Nonvolatile Spintronics: Perspectives on Instant-on Nonvolatile Nanoelectronics Systems", J Spin, Vol. 2, No. 2 (2012) 1250009
Host: K. Varga
Philip Kim, Department of Physics, Columbia University
Bloch, Landau, and Dirac: Hofstadter's Butterfly in Graphene
Electrons moving in a periodic electric potential form Bloch energy bands where the mass of electrons are effectively changed. In a strong magnetic field, the cyclotron orbits of free electrons are quantized and Landau levels forms with a massive degeneracy within. In 1976, Hofstadter showed that for 2-dimensional electronic system, the intriguing interplay between these two quantization effects can lead into a self-similar fractal set of energy spectrum known as "Hofstadter's Butterfly." Experimental efforts to demonstrate this fascinating electron energy spectrum have continued ever since. Recent advent of graphene, where its Bloch electrons can be described by Dirac feremions, provides a new opportunity to investigate this half century old problem experimentally. In this presentation, I will discuss the experimental realization Hofstadter's Butterfly via substrate engineered graphene under extremely high magnetic fields controlling two competing length scales governing Dirac-Bloch states and Landau orbits, respectively.
Host: K. Bolotin
James Butler, Smithsonian National Museum of Natural History
Mysteries Hidden in Diamond: Blue and Pink Diamonds
Diamond is a fascinating material to Gemologists, Geologists, and materials scientists. Now we can grow diamond of higher purity and quality than found in nature by synthetic techniques (Chemical Vapor Deposition and High Pressure High Temp. presses), there is a resurgence of interest in diamond materials for technological applications. In most cases, impurities and defects (intentional or unintentional) control the desired properties. In this talk I will explore some of these defects in natural diamonds which impact color, electrical properties, etc. examining two rare types of natural diamonds, blue and pink.
Host: N. Tolk
Mike Downer, University of Texas, Austin
Plasma-Based Particle Accelerators: There's Plenty of Room at the Bottom
Over the past few years, compact plasma-based particle accelerators have advanced sufficiently that it is no longer a pipe dream to imagine a tabletop x-ray free-electron laser in every major university in the world , or proton cancer therapy on a scale that many hospitals could afford. I will survey recent experimental highlights in the field that make these hopes more realistic than even a few years ago. These include a milestone achieved recently using the Texas Petawatt Laser: nearly mono-energetic acceleration of plasma electrons to 2 GeV with unprecedented sub-milliradian beam divergence . I will discuss near-term prospects for improving plasma-based accelerators further. Finally I will describe new holographic techniques that enable experimenters to visualize the electron density waves that lie at the heart of plasma-based accelerators [3,4]. Such 4D visualization, previously available only from intensive computer simulations, helps physicists understand how plasma-based particle accelerators work, and how to make them work better.  K. Nakajima, "Towards a table-top free electron laser," Nature Physics 4, 92 (2008).  X. Wang et al., "Quasi-monoenergetic laser-plasma acceleration of electrons to 2 GeV," Nature Communications 4, 1988 (2013).  N. H. Matlis et al., "Snapshots of laser wakefields," Nature Physics 2, 749 (2006).  Z. Li et al., "Single-shot tomographic movies of light-velocity objects," Nature Communications, in press (2014).
Host: N. Tolk
John Gore, Department of Physics and Astronomy, Vanderbilt University
Contrast mechanisms in NMR imaging
Host: E. Rericha
Yoav Kallus, Princeton Center for Theoretical Science, Princeton University
Frustrated constraint satisfaction problems
Host: R. Scherrer
Sunghwan Jung, Department of Engineering Science and Mechanics, Virginia Tech
Biofluid mechanics related to nonlinear interactions between soft bodies and their surrounding fluid
Host: E. Rericha
Raju Venugopalan, Brookhaven National Laboratory
The color glass condensate and the structure of visible matter in the Universe.
Host: J. Velkovska
Nadya Mason, Department of Physics, University of Illinois at Urbana-Champaign
Small Dots Make it Big: Studying Collective Phenomena in Arrays of Superconducting Islands
One of the most fundamental questions in physics is how the macroscopic properties of matter emerge from microscopic constituents. Often, complex electronic correlations at the micro-scale act to create remarkable macroscopic materials properties, such as superconductivity and ferromagnetism. But the parameters relevant to understanding and manipulating these correlations are difficult to access. In this talk, I will discuss a "bottom-up" approach to studying collective effects in matter via nanostructured arrays of superconducting islands. We fabricate large arrays of superconducting islands patterned on normal metal films; by changing the size and configuration of the islands, we can controllably change the superconducting correlations and thus the properties of the system. I will discuss electrical transport measurements of these systems, including characterization of the superconducting transitions, vortex dynamics in a finite magnetic-field, and evidence that the system approaches unusual metallic ground states as the island spacing is increased. This project directly explores the effects of microscopic parameters—such as correlation lengths, island size, and disorder—on macroscopic phenomena in a correlated system, and is relevant to understanding the exotic phases (e.g., the "pseudogap" in high-temperature superconductors) observed in bulk correlated materials.
Host: K. Holley-Bockelman
Junichiro Kono, Department of Electrical and Computer Engineering and Department of Physics and Astronomy, Rice University
Superfluorescence from a Quantum-Degenerate Electron-Hole Gas
Quantum particles sometimes cooperate to develop an ordered state, where macroscopic coherence appears spontaneously. Here, we demonstrate that such spontaneous appearance of coherence occurs in an optically excited semiconductor quantum well in a high magnetic field [1-3]. When we create a dense electron-hole (e-h) plasma with an intense laser pulse, after a certain delay, an ultrashort burst of coherent radiation emerges. We interpret this striking phenomenon as a manifestation of superfluorescence (SF), in which a macroscopic polarization spontaneously builds up from an initially incoherent ensemble of excited quantum oscillators and then decays abruptly, producing giant pulses of coherent radiation. SF has been observed in atomic gases, but the present work represents the first observation of SF in a semiconductor, where not only real-photon exchange but also virtual-photon exchange (Coulomb interactions) is responsible for the formation of macroscopic coherence. We found that Coulomb interactions dramatically enhance and modify the collective superradiant decay of the e-h plasma. Unlike typical spontaneous emission from semiconductors, which occurs at the band edge, the observed SF occurs at the quasi-Fermi energy of the highly degenerate carrier distribution, up to 150 meV above the band edge. As the carriers are consumed by ultrafast radiative recombination, the quasi-Fermi energy goes down, and we observe a continuously red-shifting streak of SF at zero magnetic field and a series of sequential SF bursts from higher to lower Landau levels in a magnetic field. This Coulomb enhancement allows the magnitude of the giant dipole to exceed even the maximum possible value for ordinary SF (i.e., the total sum of in-phase oscillations of individual dipoles), making e-h SF even more "super" than atomic SF. 1. G. T. Noe et al., Nature Physics 8, 219 (2012). 2. G. T. Noe et al., Fortschritte der Physik 61, 393 (2013). 3. J.-H. Kim et al., Physical Review B 87, 045304 (2013).
Host: N. Tolk
Freeman Dyson, Institute for Advanced Studies, Princeton, New Jersey
Host: R. Scherrer
John C. Angus, Department of Chemical Engineering, Case Western Reserve University
Diamond Synthesis: Then and Now
Diamond synthesis by chemical vapor deposition has been a major advance in materials science. In this talk the birth pains, current status, and unanswered questions surrounding this technology are discussed. Following World War II there was a great surge of interest in high-pressure diamond synthesis in the United States, Sweden, and the former Soviet Union, which culminated with the announcement of success by General Electric in 1955. During this time period major efforts were also made in low-pressure, metastable synthesis. These efforts were characterized by great secrecy and a considerable lack of transparency. The first public reports of diamond syntheses at low pressure appeared in the 1960's. In addition to great scepticism about the veracity of these claims, a common view was that, even if true, growth rates were far too slow to be of interest. Also, many claimed that the process violated fundamental thermodynamic laws. These attitudes changed dramatically in 1984 when a Japanese group announced growth rates in the micron per hour range. Diamond grown by chemical vapor deposition is now a well-established technology with many diverse applications. Despite this major progress, significant unresolved issues and opportunities remain.
Host: N. Tolk
Maura McLaughlin, Department of Physics and Astronomy, West Virginia University
Building a Galactic Scale Gravitational Wave Observatory
Host: K. Holley-bockelmann
Peter F. Michelson
Overview of Fermi and its window on the highly energetic universe
Host: K. Holley-bockelmann
Copyright 2010, Vanderbilt University