6301 Stevenson Center
VU Station B #351807
Nashville, TN 37235
Colloquia are held on Thursdays at 4pm 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 3:30pm in Stevenson 6333.
Fall Faculty Assembly, no colloquium
J. Davy Kirkpatrick, Caltech
From Nashville to the Solar Neighborhood: The Discovery and Characterization of Brown Dwarfs
Brown dwarfs are a type of star that have insufficient mass to sustain thermonuclear fusion in their cores and burn steadily for billions of years as true stars do. They enable the study of star formation efficiency at the lowest masses and provide ideal laboratories for exoplanet atmospheric research due to their similarity in temperature and chemical makeup to planets. Although first predicted to exist in the early 1960s, brown dwarfs eluded detection for decades because of their intrinisic faintness. Their discovery in the mid- to late-1990s led to the establishment of spectral classes L and T, the first additions to the time-honored stellar classification system of OBAFGKM spectral types. These discoveries have helped bridge the gap in temperature between the lowest mass stars (~1700K) and planets (~125K for Jupiter), but until recently no brown dwarfs cooler than ~500K had been identified. The recent launch of NASA's Wide-field Infrared Survey Explorer (WISE) has now enabled the discovery of these colder objects, which have been designated as "Y dwarfs". Early results hint at there possibly being as many brown dwarfs as true stars in the Milky Way, meaning that WISE may have already imaged a brown dwarf closer to the Sun than our current nearest neighbor, Proxima Centauri.
Host: R. Scherrer
Hod Lipson, Cornell University
Automating Science: Distilling Natural Laws from Experimental Data
Can machines discover analytical laws automatically? For centuries, scientists have attempted to identify and document analytical laws that underlie physical phenomena in nature. Despite the prevalence of computing power, the process of finding natural laws and their corresponding equations has resisted automation. A key challenge to finding analytic relations automatically is defining algorithmically what makes a correlation in observed data important and insightful. By seeking dynamical invariants and symmetries, we show how we can go from finding just predictive models to finding deeper conservation laws. We demonstrated this approach by automatically searching motion-tracking data captured from various physical systems, ranging from simple harmonic oscillators to chaotic double-pendula. Without any prior knowledge about physics, kinematics, or geometry, the algorithm discovered Hamiltonians, Lagrangians, and other laws of geometric and momentum conservation. The discovery rate accelerated as laws found for simpler systems were used to bootstrap explanations for more complex systems, gradually uncovering the “alphabet” used to describe those systems. Application to modeling physical and biological systems will be shown.
Schmidt M., Lipson H. (2009) "Distilling Free-Form Natural Laws from Experimental Data," Science, Vol. 324, no. 5923, pp. 81 - 85. Try it on your own data.
BIO: Hod Lipson is an Associate Professor of Mechanical & Aerospace Engineering and Computing & Information Science at Cornell University in Ithaca, NY. He directs the Creative Machines Lab, which focuses on novel ways for automatic design, fabrication and adaptation of virtual and physical machines. He has led work in areas such as evolutionary robotics, multi-material functional rapid prototyping, machine self-replication and programmable self-assembly. Lipson received his Ph.D. from the Technion - Israel Institute of Technology in 1998, and continued to a postdoc at Brandeis University and MIT. His research focuses primarily on biologically-inspired approaches, as they bring new ideas to engineering and new engineering insights into biology. For more information visit http://www.mae.cornell.edu/lipson.
Host: J. Wikswo
Sandra Rosenthal, Vanderbilt University
Host: S. Hutson
Alexey Petrov, Wayne State University
The physics of flavor
Abstract: We know of three generations of matter particles: there are 12 types (flavors) of quarks and leptons. Their observed properties, such as patterns of masses and mixing angles, however, remain a mystery. This mystery constitutes the "flavor problem." In this talk, I will discuss how this flavor puzzle is (not) solved in the Standard Model of particle physics. I will provide an overview of flavor physics and its implications for physics beyond the Standard Model, as well as what hints about the solution to the flavor puzzle could be given by upcoming results from experiments at the Large Hadron Collider at CERN and beyond from the point of view of a theorist.
Host: P. Sheldon
Kalman Varga, Vanderbilt University
Quantum dynamics at the nanoscale
Recent experimental advances of ultrafast laser pulses and ultrafast high-resolution imaging allow the study of nanoscale dynamics. Electrons and nuclei play an equally important role in the dynamical behavior of matter at the nanoscale. In this talk we show how time-dependent density functional theory can be used to simulate the electron and nuclear dynamics in nanostructures probed by time-dependent external fields.
Host: R. Scherrer
An-Ping Li, Oak Ridge National Laboratory
Probing Electron Transport at the Nanoscale: Application of the Four-Probe Scanning Tunneling Microscopy
Electron transport in low-dimensional materials is the key to the novel applications of nanomaterials in electronic and energy technologies. Due to the restricted dimensionality, one distinctive character of these systems is that the transport properties are critically dependent on the structural details. Therefore, an important requirement for transport research of a specific low dimensional system is to examine its structures and properties in a coherent manner. As a “nano” version of a four-probe station, ORNL Four-probe STM combines STM local imaging and spectroscopy functions with four-point contact electrical transport capability in a well-controlled sample environment to allow for simultaneous measurements of transport and local structure on the same nanomaterials . In this talk, I will give a brief overview on this unique facility, and then present a few examples to demonstrate how we are using this platform to study the electron transport properties and the structure relationships over multiple length scales, from individual atoms, molecules, to nanowires and mesoscopic systems[2-5]. My focus will be on the measurements of individual grain boundary resistance in copper interconnect nanowires  and the manipulations of electronic phases near the Mott metal-insulator transition in a ruthenate surface. The goal of this research is to establish the relationship between transport functionalities and local structural and electronic properties down to atomic scale. This research was sponsored by the Office of Basic Energy Sciences, U.S. Department of Energy. 1.Tae-Hwan Kim, Zhouhang Wang, John F. Wendelken, Hanno H. Weitering, Wenzhi Li, and An-Ping Li, Rev. Sci. Instrum. 78, 123701 (2007). 2.C. Zeng, P.R.C. Kent, Tae-Hwan Kim, An-Ping Li, Hanno H. Weitering, Nature Mater., 7, 539 (2008). 3.Tae-Hwan Kim, M. Angst, B. Hu, R. Jin, X. G. Zhang, J. F. Wendelken, E. Ward Plummer, and An-Ping Li, Proc. Nat. Acad. Sci., 101, 5272 (2010). 4.Tae-Hwan Kim, X. G. Zhang, Don M. Nicholson, Boyd M. Evans, Nagraj S. Kulkarni, B. Radhakrishnan, Edward A. Kenik, and An-Ping Li, Nano Lett., 10, 3096 (2010). 5.Wangyang Fu, Shengyong Qin, Lei Liu, Tae-Hwan Kim, Sondra Hellstrom, Wenlong Wang, Wenjie Liang, Xuedong Bai, An-Ping Li, and Enge Wang, Nano Lett., 11, 1913 (2011).
Host: Y. Xu
Michael Lubell, CUNY and APS Director of Public Affairs
Science: What the Public is Thinking, What Congress is Doing, How You Can Contribute
Recent polling of public attitudes toward science contains very mixed results. By a margin of 93 to 7 the public believes the U.S. should be a global leader in science, and 2 out of 3 people approve using taxpayer funds to support research. But 46 percent of the respondents in the sample of 1200 give the federal government a grade of C, D, or F for its efforts to promote technological innovation, and 50 percent believe the federal science budget is too large, with a significant fraction of the respondents naming science as their number one target for cutting. In their actions and public statements Washington policymakers also reflect mixed attitudes. The president has been a champion of science, as have many lawmakers. But with Congress and the White House having struck a deal on deficit reduction, science funding could fall victim to major cuts in discretionary spending scheduled to begin in FY 2013. Whether lawmakers elect to make science funding an exception to fiscal austerity measures will depend in part on public attitudes. The polling data suggest a rocky road ahead and point to the need for scientists to engage with the public in far more effective ways than the have until now.
Host: R. Haglund
Thomas J. Weiler, Vanderbilt University
Phantoms at the OPERA -- Faster-than-Light Neutrinos (!?) and the Negation of Cause and Effect"
The original purpose of the OPERA neutrino-detection experiment at Gran Sasso National Laboratory in Italy was to observe oscillations of muon neutrinos into tau neutrinos over the 732 km baseline between Gran Sasso and the neutrino source at CERN, in Geneva, Switzerland. Remarkably, OPERA has recently released a paper (arXiv:1109.4897) which claims a space-like separation between neutrino production at CERN and the "subsequent" detection at Gran Sasso. Their interpretation of this result is that the neutrinos travel superluminally, i.e. faster than light. If this experimental result is reproduced by other experiments, then profound alterations to our understanding of cause and effect result. After a brief discussion of the OPERA paper, I will discuss the meaning of their result in the context of relativity, and then discuss the myriad of models proposed to accommodate the result. Constraints on model-building will also be discussed. My own conclusion is that most probably the data is wrong, or else we are forced to consider baroque models such as sterile neutrinos propagating in extra dimensions.
Zhenyu Zhang, Oak Ridge National Laboratory and U. Tennessee
“Quantum Energy” refers to the emerging discipline that utilizes quantum mechanics as the primary working principle in the precise design and control of the microscopic quantum states of novel functional materials, so as to maximize the macroscopic energy outputs to meet the energy needs of the society for sustainable development. The potential thriving success of “Quantum Energy” demands revolutionary conceptual breakthroughs in the respective fields. In this talk, we will first give a brief overview of the advances and challenges in the utilization of solar energy as a clean and sustainable energy source. Next we focus on some recent examples of quantum design of advanced materials for enhanced solar energy conversion, including intermediate-band solar cell materials, photolysis for hydrogen generation, and plasmonic solar cell materials. Those progresses hopefully will serve as steppingstones on our marching towards the ultimate goal of solving the energy crisis.
Host: S. Pantelides
Michael Fuhrer, U. Maryland
Graphene: Scratching the Surface
Graphene is of interest for its unique electronic structure: electrons in graphene obey the Dirac equation for massless particles, complete with a two-component spinor degree of freedom that mimics the spin of a relativistic particle. But graphene is also composed entirely of surface atoms, making the techniques of surface science useful in studying its properties. I will discuss experiments which combine ultra-high vacuum (UHV) surface science with electronic transport measurements to understand graphene and the adsorbed species on its surface. Surface science techniques can be used to controllably modify graphene's properties: potassium atoms can be deposited to form charged impurity scatterers; ice can be deposited to modify the dielectric environment of graphene and tune the electron-electron interaction strength; and ion irradiation can be used to create atomic vacancies which act as Kondo impurities. Graphene's transport properties are extraordinarily sensitive to surface adsorbates, and can be used to detect e.g. correlations in the positions of potassium atoms at concentrations below 1/1000th of a monolayer, and phase transitions in few-monolayer water.
Host: K. Bolotin
Abbas Ourmazd, U. Wisconsin
Structure and Dynamics from Random Snapshots
Structure often determines function, and there is increasing evidence that structure is neither immutable, nor static. The study of structural variability and dynamics represents a crucial but difficult frontier in biology, soft condensed matter science, and atomic, molecular, and optical physics. I describe how advanced graph-theoretic techniques, augmented with concepts from Riemannian geometry and general relativity, can be used to determine the structure and dynamics of evolving systems from a random collection of ultra-low-signal snapshots emanating from unknown orientations and conformations. * In collaboration with D. Giannakis, G.N. Phillips, Jr., P. Schwander, and C.H. Yoon
Host: S. Pantelides
Rocky Kolb, U. Chicago
The Decade of the WIMP
Most of the mass of the universe is dark. The best explanation at present is the dark matter is a new species of elementary particle. The hypothesis that the dark matter species is a massive relic particle should be confirmed or refuted in the next decade by a combination of experiments and observations that attempt to detect directly the relic WIMP, to detect the relic WIMP through its present-day annihilation products, and produce and detect it at accelerators.
Host: R. Scherrer
Thanksgiving Holidays, no colloquium
Jan Tobochnik, Kalamazoo College
Physical Insight From Computational Algorithms
After a brief discussion of the importance of introducing computational physics into the undergraduate curriculum, I will provide a number of examples of how computational algorithms can aid in providing physical insight, particularly about abstract concepts that students frequently have difficulty appreciating. ADDITIONAL TALK: The ``Scandal" of Quantum Mechanics - Wednesday November 30. After providing a very simple explanation of Bell's Theorem, I will provide a variety of points of view concerning quantum interpretation drawn from a series of Letters to the Editor of the American Journal of Physics and a Reference Frame essay by David Mermin in Physics Today. I will end my presentation with some analogies to other controversial topics in science.
Host: S. Hutson
Bharat Ratra, Kansas State University
Dark Energy: constant or time variable? (... and other open questions)
Experiments and observations over the last decade have persuaded cosmologists that (as yet undetected) dark energy is by far the main component of the energy budget of the universe. I review a few simple dark energy models and compare their predictions to observational data, to derive dark energy model-parameter constraints and to test consistency of different data sets. I conclude with a list of open cosmological questions.
Host: R. Scherrer
Copyright 2010, Vanderbilt University