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 3:40pm in Stevenson 6333.
Thursday, September 1
David Kipping, Department of Astronomy, Columbia University
Hunting for Exomoons and Other Cool Worlds
Despite thousands of exoplanets now known, the detection of moons around these objects has proven elusive. Exomoons may be habitable worlds in their own right and affect the habitability and history of the planets they orbit. Whilst their detection requires pushing modern instruments to their limits, these objects hold a great potential to revolutionize our understanding of other planetary systems. I will present the methods and results from the Hunt for Exomoons with Kepler (HEK) project, which remains the only systematic survey for these objects. Statistically robust upper limits, derived for each planet surveyed, are beginning to provide meaningful constraints on the exomoon population. In a new strategy I’ll briefly outline, we are greatly increasing the sample of objects surveyed from dozens to hundreds. From exomoons to other cool worlds, I will also introduce my new group at Columbia, highlighting some of the exciting research from my graduate students, spanning Bayesian inference to SETI. Finally, I will add a brief update on our space-based photometry with MOST of Proxima Centauri.
Host: K. Stassun
Thursday, September 8
Joshua Shaevitz, Department of Physics, Princeton University
Self-driven phase transitions in living matter
The soil dwelling bacterium Myxococcus xanthus is an amazing organism that uses collective motility to hunt in giant packs when near prey and to form beautiful and protective macroscopic structures comprising millions of cells when food is scarce. I will present an overview of how these cells move and how they regulate that motion to produce different phases of collective behavior. Inspired by recent work on the thermodynamics of active matter, I will discuss experiments that reveal how these cells generate nematic order and how they actively tune the Péclet number of the population, a quantity that plays the role of an active temperature in generating fluctuations, to drive a phase transition from a gas-like flocking state to an aggregated liquid state during starvation.
Host: S. Hutson
Thursday, September 22
Jens Meiler, Department of Chemistry, Vanderbilt University
Innovative Computational Methods for Challenging Biological Problems
I will cover new computational algorithms for protein structure prediction, drug discovery, and the design of therapeutic antibodies.
Host: S. Hutson
Thursday, September 29
Nia Imara, Harvard-Smithsonian Center for Astrophysics
A Story of Stellar Nurseries
All stars are observed to form in molecular clouds (MCs). Since these “stellar nurseries” set the stage for star formation in the Milky Way and other galaxies, astronomers would like to understand how they form and evolve. In this presentation, I will discuss different ways of investigating the environments and evolution of MCs. Millimeter observations, for example, show that local and extragalactic MCs have systematic velocity gradients, possibly indicating the large scale of rotation of clouds. In my study of MCs in the Milky Way and M33, I demonstrate that any viable theory of cloud formation must agree with a number of interesting trends pertaining to cloud kinematics. For instance, I find that many clouds may be counter-rotating with respect to overall galactic rotation, which has important consequences for theory and galactic simulations. I will also discuss how investigating both the dense gas and diffuse gas associated with MCs —- via far-infrared and 21-cm observations —- provides unique insight into how these environments evolve and eventually form stars.
Host: K. Stassun
Thursday, October 6
Brian Fields, Department of Astronomy, U. Illinois
When Stars Attack! Near-Earth Supernova Explosion Threat and Evidence
The most massive stars are the celebrities of the cosmos: they are rare, but live extravagantly and die in spectacular and violent supernova explosions. These awesome events take a sinister shade when they occur close to home, because an explosion very nearby would pose a grave threat to Earthlings. We will discuss these cosmic insults to life, and ways to determine whether a supernova occurred nearby over the course of the Earth's existence. We will present evidence that one or more stars exploded near the Earth about 3 million years ago. Radioactive iron atoms have been found globally in deep-ocean material, and very recently reported in lunar samples as well. These are supernova debris, transported to the Earth as a "radioactive rain." With these data, for the first time we can use sea sediments and lunar cores as telescopes, probing the nuclear fires that power exploding stars and possibly even indicating the direction towards the event(s). Furthermore, an explosion so close was probably a "near-miss" that exposed the biosphere to intense and possibly harmful ionizing radiation.
Host: R. Scherrer
Thursday, October 13---Fall Break
Thursday, October 20
Jacob Khurgin, Department of Electrical and Computer Engineering, The Johns Hopkins University
Using Plasmonics to Enhance Optical Nonlinearity: What Works and What Does Not
Metallic plasmonic structures allow one to confine optical fields into sub-wavelength volumes, achieving high spatial concentrations of field strength. In theory, a high degree of field enhancement should lead to the enhancement of many nonlinear optical effects, such as intensity-dependent absorption and refraction. In practice, however, these enhancements are often rendered ineffective due to large losses in the metals. Nevertheless, we can identify a few promising niches - such as thermal nonlinearities - in which plasmonics can indeed lead to fast all-optical switching and signal modulation.
Host: R. Haglund
Thursday, October 27
Fernando Reboredo, Materials Sciences and Techology Division, Oak Ridge National Laboratory
The physics world is changing. I feel it in the water… I smell it in the supercomputers.
Perhaps since the discovery of Neptune using mathematics, the ultimate success of a theory is often recognized when it goes beyond what is known from observation to predict what has not been ever measured. However, not every calculation can be carried out due to limited computational resources. Since grams of any solid have 10^23 nuclei plus their electrons, no material can be calculated with Quantum Mechanics without approximations. Yet, the horizon of what is possible to include in the calculation has been expanding dramatically during the last decades, and it is expected to continue growing at least for the next one. Not every stablished method of calculation, however, can take advantage of current computer architectures. Old methods that have been impractical until recently, may become practical in the near future. Perhaps today’s students may find in the near future alternative avenues to solutions to unsolved problems with new computers. Therefore, we should be aware of the changes in computing power to adapt or to choose a new research area or professional path. - In this talk, we will discuss some of the progress made in the area of computational prediction of materials. During the last quarter of a century, the prediction and modeling of materials has been largely carried out in the framework of density functional theory (DFT) and its approximations. The popularity of this approach is due to its ability to reduce an extremely complex many-body problem to a simpler single particle one using a bold approximation that saves a considerable amount of computation. This approach has been successful enough for a broad class of materials (e.g. metals and simple semiconductors) but continues to present outstanding problems in others. The success in the area of metals, however, has motivated initiatives like the material-genome where the discovery of new materials is largely carried out using computations. - The oxides made by the same metals, however, are much more difficult to predict and understand within the single-particle DFT framework. The complex family of transition metal oxides shows experimentally a remarkable set features that range from superconductivity and magneto resistance to multi-ferroic behavior. Their potential applications include energy transport, generation, and storage as well as information storage and processing. Their properties are significantly altered by subtle changes in their composition and by defects. It is believed that many-body methods are required not only to predict new oxides but also to understand and describe the ones that we already know. - In our team we have focused on this class of highly correlated oxides. But we avoid DFT approximations solving the many-body problem in real solids. We have been using a statistical many-body method denoted as Diffusion Monte Carlo. The research of our team in the area started from simple binary oxides, going next to ternary oxides and finally we focus on super-lattices of different oxides and their defects. This is a proof of principle that showcases what can be achieved with current supercomputers and provides a hint to the near future.
Host: S. Pantelides
Thursday, November 3
Dmitry Kharzeev, Department of Physics, Stony Brook University and Brookhaven National Laboratory
Chirality: from particles and nuclei to quantum materials
Chirality is an ubiquitous concept in modern science, from particle physics to biology. In quantum physics, chirality is intimately linked to the topology of gauge fields due to the quantum chiral anomaly. While the quantum anomaly is usually associated with the short-distance behavior, recently it has been realized that it affects also the macroscopic behavior of fluids with chiral fermions. In particular, the local imbalance between left- and right-handed fermions in the presence of magnetic field induces the non-dissipative transport of electric charge ("the Chiral Magnetic Effect"). In heavy ion collisions, there is an evidence for the effect from the experiments at the Relativistic Heavy Ion Collider and the Large Hadron Collider. Very recently, the Chiral Magnetic Effect has been discovered in ZrTe5, a material possessing chiral quasi-particles. The manifestations of chirality in condensed matter systems open a path towards new applications.
Host: J. Velkovska
Thursday, November 10
Jerry Bernholc, Department of Physics, North Carolina State University and Oak Ridge National Laboratory
Computational materials science: the era of applied quantum mechanics
It is already possible to predict the properties of new and artificially structured materials entirely by computations, using atomic numbers as the only input. The rapid progress in computational materials science is expected to continue, which should eventually enable the “design” of materials with tailor-made properties largely on a computer, with only relatively few final candidates being evaluated experimentally. Although this goal is still some time in the future, several strategies have emerged, including creating massive “materials genome” databases of pre-computed properties of various classes of materials, more limited explorations focused on “already hot” materials, as well as efforts to combine computed and measured data to pre-“validate” predictions for some types of materials. I will discuss some recent examples and the generally bright outlook for this field, accelerated by the rapid growth in computer power, the emergence of petaflops and beyond computation, and the growing importance of “big data” methods.
Host: S. Pantelides
Thursday, November 17
Knicole Colón, NASA Ames Research Center
Investigating the Structure and Climate of Scorching Hot Exoplanet Atmospheres
Exoplanet surveys have revealed that short-period gas giant planets, also known as hot Jupiters, are intrinsically rare in the Galaxy. These extreme examples of extrasolar planets have been the subject of many studies to date, but their formation and evolution are still shrouded in mystery. Studying the atmospheres of these planets can provide some clues to their formation environments. It is possible to study the atmospheres via either transit observations, which probe the limb of the planet, or eclipse observations, which measure the thermal emission directly from a planet’s day-side. In this talk, I will present results from a large ground-based survey to study the atmospheres of hot Jupiters via their eclipses in the near-infrared. Such observations allow us to probe the connection between the atmospheric structure and climate deep in their atmospheres, as well as the irradiation from their host star. The sample of hot Jupiters observed to date in our survey spans a range of planetary parameters (e.g. temperatures and densities) and also includes several new exotic discoveries from the KELT transit survey. These observations will ultimately allow a comprehensive statistical analysis of the diversity of hot Jupiter atmospheres via their near-infrared eclipses. In addition, I will discuss opportunities to measure the metallicity of exoplanet atmospheres via transit observations. When combined with eclipse observations, we can obtain a more complete view of exoplanet atmospheres. All of this sets the stage for future observations to be made with facilities like the upcoming James Webb Space Telescope (JWST).
Host: K. Stassun
Thursday, November 24---Thanksgiving
Thursday, December 1
Chunlei Guo, Institute of Optics, University of Rochester
Material functionalization with femtosecond lasers
Femtosecond laser materials processing is a rapidly advancing field due to its high precision and versatility. In this talk, I will discuss a number of techniques developed in my lab that allow us to functionalize material surfaces through femtosecond laser processing. The techniques led to the creation of the so-called black and colored metals, brighter incandescent light sources, superhydrophillic and superhydrophobic surfaces. Possible applications of the functionalized materials will also be discussed.
Host: R. Haglund
Thursday, December 8
Dave Spiegel, Data Scientist, StitchFix.com
From Exo-Worlds to Health Monitoring to Clothing Personalization
A (first) career studying exoplanets turns out to be great preparation for a (second) career in data science. I'll describe how I went from an astrophysicist studying planets around other stars to working at a startup where we were building a health and fitness tracking device, to now working at Stitch Fix, a personalization clothing company that's rethinking how retail can and should work. Along the way, I'll describe some of the machine learning and data science challenges that I've worked on at these various stops. In particular: how can we use data to learn what exoplanet atmospheres are made of? How can we infer what people are doing and how their bodies are behaving from noninvasive monitoring of physiological data? And how can we manage inventory in order to delight our clients through personalized clothing selections? I'll highlight how experience as an astrophysicist has enabled me to contribute meaningfully in various business contexts.
Host: K. Holley-Bockelmann
Thursday, January 19
Xiaoqin "Elaine" Li, Department of Physics, University of Texas - Austin
Fundamental Excitations in Solids
Fundamental excitations (e.g. plasmons, excitons, phonons and magnons) determine both the equilibrium and non-equilibrium properties of solids such as metals, semiconductors, and magnetic materials. In this talk, I will give a few examples of our recent work on investigating how such fundamental excitations and particularly coupling between different types of excitations can be probed and controlled. In one example, we develop a temperature sensing method capable of characterizing phonons and magnons temperatures separately. Using such a temperature sensor, we demonstrate that magnons and phonons in a magnetic insulator can be driven out of local equilibrium. This experiment provides quantitative information on phonon-magnon coupling strength, which is essential to a host of newly discovered phenomena in the emerging field of spin caloritronics. In another example, we observe that the scattering spectrum of a plasmonic nanoparticle is modified by a single semiconductor quantum dot even though their scattering cross sections differ by four orders of magnitude. The coupling between the classical plamonic resonance and the exciton resonance (a true quantum two-level system) is manifested as a Fano resonance mediated by single photon absorption and scattering.
Host: S. Rosenthal
Thursday, January 26
Natalia Litchinitser, University at Buffalo, The State University of New York
Nonlinear and Singular Optics in Photonic Meta-Structures
The emergence of synthetic photonic media, or metamaterials, has revealed many unique electromagnetic phenomena, including negative index of refraction, which has never been found in nature, magnetism at optical frequencies, backward propagating waves with antiparallel phase and energy velocities and extreme field enhancement effects giving rise to entirely new regimes of nonlinear optical interactions. In a context of nonlinear optics, the emergence of these novel photonic media necessitates a reconsideration of many, if not all, fundamental processes, including second harmonic generation, soliton propagation, four-wave mixing, modulation instability, and optical bistability to name a few. These recent developments in the field of nanostructured optical materials enable unprecedented control over light propagation and a possibility of "tailoring" the space for light propagation. In particular, the emergence of novel optical materials opens a new paradigm in studies of singular optics (or structured light), which is a fascinating emerging area of modern optics that considers spin and orbital angular momentum (OAM) properties of light and brings a new dimension to optical physics. We will discuss fundamental optical phenomena at the interface of singular and nonlinear optics in novel optical media and show that the unique optical properties of optical nanostructures open unlimited prospects to “tailor” light itself.
Host: S. Rosenthal
Thursday, February 2
Marc Bockrath, Department of Physics & Astronomy, University of California - Riverside
Nanoscale Electronics and Mechanics in Low-Dimensional Material Systems
We will discuss a number of our ongoing research projects aimed at understanding the properties of low-dimensional systems such as graphene and two-dimensional material heterostructures. We first measure the quality factor Q of electrically-driven few-layer graphene drumhead resonators, providing an experimental demonstration that Q ~ 1/T, where T is the temperature. Because the resonators are atomically thin, out-of-plane fluctuations are large. As a result, we find that Q is mainly determined by stochastic frequency broadening rather than frictional damping, in analogy to nuclear magnetic resonance. In addition, recently several research groups have demonstrated placing graphene on hexagonal BN (hBN) with crystallographic alignment. This not only creates a protected environment yielding high-mobility devices, but also due to the resulting superlattice formed in these heterostructures, an energy gap, secondary Dirac Points, and Hofstadter quantization in a magnetic field have been observed. In these systems, we observe a Berry’s phase shift in the magneto-oscillations when tuning the Fermi level past the secondary Dirac points, originating from a change in topological pseudospin winding number from odd to even when the Fermi-surface electron orbit begins to enclose the secondary Dirac points. We also observe a distinct hexagonal pattern in the longitudinal resistivity versus magnetic field and charge density, resulting from a systematic pattern of replica Dirac points and gaps, reflecting the fractal spectrum of the Hofstadter butterfly. Finally, we study the properties of additional graphene/hBN layer electrostatically gated structures such as twisted trilayers that are comprised of AB-stacked bilayer graphene contacting a graphene monolayer through a twist angle, and hBN-encapsulated graphene bilayers with large applied perpendicular electric field. In the twisted trilayers, which couple the massive bilayer spectrum to that of the massless monolayer spectrum, the interlayer interactions and screening produce a nonlinear monolayer graphene gate capacitance and renormalize the bilayer band structure. In the encapsulated bilayers, we perform Landau level spectroscopy, measure the layer polarizability of the electrons, and observe easy-axis quantum Hall ferromagnetism. Our latest results will be discussed.
Host: S. Rosenthal
Thursday, February 9
Jeanie Lau, Department of Physics & Astronomy, University of California - Riverside
Quantum Transport and Electron Interactions in Few-Layer Atomic Membranes
Two dimensional materials constitute an exciting and unusually tunable platform for investigation of both fundamental phenomena and electronic applications. Here I will present our results on transport measurements on high mobility few-layer graphene and phosphorene devices. In bilayer and trilayer graphene devices with mobility as high as 400,000 cm2/V, we observe intrinsic gapped states at the charge neutrality point, arising from electronic interactions. This state is identified to be a layer antiferromagnetic state with broken time reversal symmetry. In another few-layer graphene system, ABA-stacked trilayer graphene consists of multiple Dirac bands, where crystal symmetry protects the spin degenerate counter-propagating edge modes resulting in σxx = 4e2/h. At even higher magnetic fields, the crystal symmetry is broken in by electron-electron interactions and the n=0 quantum Hall state develops an insulating phase. Our findings indicate the role of crystal and spin symmetry in generation of topological phases in multiple Dirac bands. At finite doping, we explore the tunable integer and fractional quantum Hall states and Landau level crossings in these few-layer systems. Finally, I will present our recent results on weak localization and quantum Hall effect in air-stable, few-layer phosphorene devices. Our results underscore the fascinating many-body physics in these 2D membranes.
Host: S. Rosenthal
Thursday, February 16
Marta Verweij, CERN
Jetting through a hot QCD medium
In collisions between relativistically accelerated lead ions a dense medium is formed: the Quark Gluon Plasma (QGP). These type of collisions are used to study matter in phases where quarks and gluons are no more confined into hadrons and where chiral symmetry is restored. Hard scattered partons are used to map out the properties of the QGP. As a parton passes through the QCD medium, induced energy loss from elastic and radiative interactions leads to a modification of the parton shower; this modification is used to deduce medium properties. In this colloquium, recent jet measurements in heavy-ion collisions at the LHC are discussed. I will also discuss how these measurements improve our understanding of the properties of hot QCD matter and give an outlook of ideas for the future.
Host: J. Velkovska
Thursday, February 23---FRANCIS G. SLACK LECTURE
Paul McEuen, John A. Newman Professor of Physical Science at Cornell University and Director of the Kavli Institute at Cornell for Nanoscale Science
The Future of Small
Over fifty years ago, the physicist Richard Feynman gave a remarkably prescient talk about the coming revolution in miniaturization. For the next half a century, the ever-shrinking integrated circuit brought his dreams to fruition in the realms of data and computing. But the third leg of Feynman’s dream, the miniaturization of machines, is only just getting underway. The next 50 years promise even bigger changes if we can create functional devices at the smallest scales, mimicking the complexity and functionality of life. In this talk, we will discuss both the past and the future of miniaturization, with a particular emphasis on strategies to construct machines at the micro and nano scale. It is a problem that touches on everything from the mathematics of origami to the origin of life, and solving it is one of the great challenges of the twenty-first century.
Host: S. Rosenthal
Thursday, March 2
Sarah Campbell, Department of Physics, Columbia University
Using direct photons to shed light on the quark gluon plasma
The quark gluon plasma, a plasma of strongly-correlated quarks and gluons, is created in the laboratory by colliding heavy ions at relativistic speeds. These collisions generate the high energy densities and temperatures needed to free the quarks and gluons bound inside nuclei. The resulting QGP is both a medium exhibiting emergent phenomena and a high energy density environment in which quantum chromodynamics, the fundamental theory describing quark and gluon interactions, is investigated. Direct photons are clean probes of nuclear collision systems because they are produced throughout the collision and do not re-interact with the system after emission. In this talk, I will present current and future direct photon measurements at RHIC and the LHC, highlighting how they are used to understand key properties of the QGP such as energy loss.
Host: J. Velkovska
Thursday, March 9---Spring Break
Tuesday, March 14 @ 4:00pm---Note Special Day/Time
Ross King, School of Computer Science, University of Manchester
The Automation of Science
Ross D. King is Professor of Machine Intelligence at the University of Manchester, UK. His main research interests are in the interface between computer science and biology/chemistry. The research achievement he is most proud of is originating the idea of a “Robot Scientist”: using laboratory robotics to physically implement a closed-loop scientific discovery system. His Robot Scientist “Adam” was the first machine to hypothesize and experimentally confirm scientific knowledge. His new robot “Eve” is searching for drugs against neglected tropical diseases. His work on this subject has been published in the top scientific journals, Science and Nature, and has received wide publicity. He is also very interested in NP problems, computational economics, and computational aesthetics.
Host: J. Wikswo
Thursday, March 23---WENDELL HOLLADAY LECTURE
Mickel Holcomb, Department of Physics, West Virginia University
Investigating Complex Systems via Element Specific X-ray Absorption
While my group also grows high quality complex oxides and performs ultrafast optical measurements, one of our core strengths is the utilization of synchrotron radiation techniques to provide element specific information, such as atomic valence, magnetization, uncompensated spin, bond lengths and neighbor elements. I will highlight some of these efforts in order to illustrate our capabilities. While the majority of our work has been in perovskites (ABO3), my group at WVU has also invested a wide range of other systems, including a variety of delafossites (ABO2), doped and undoped topological insulators, ferroelectrics, multiferroics and magnetic systems with more than one magnetic element. For example, we have used these techniques to measure depth-dependent atomic valence and image and quantify for the first time uncompensated spin at magnetic/ferroelectric interfaces. Both of these examples are critical for understanding how to manipulate magnetic, multiferroic and likely many other surfaces and interfaces. As I have done for several other researchers, I am always happy to discuss your specific research goals and how synchrotron (or other) measurements might be able to help.
Host: R. Scherrer
Thursday, March 30
Robert Grober, Renaissance Technologies
< Physics | Finance >
This talk will attempt to make clear why physicists, mathematicians and computer scientists have had profound impact on the field of statistical arbitrage by providing simple examples which emphasize the aspects of their academic disciplines that are particularly relevant to the field of quantitative finance.
Host: R. Scherrer
Thursday, April 6
Ed Daw, Department of Physics and Astronomy, University of Sheffield
Gravitational Waves and Axions - Aspects of the Dark Universe
The quest for direct detection of gravitational waves, and the hunt for dark matter have been amongst the most pressing problems in science. I will describe the LIGO gravitational wave detectors, and the search for the axion, a dark matter candidate of increasing interest as the space for new electroweak physics is squeezed by ever-more-stringent null results. In my own work on LIGO and on ADMX, I have found that there are surprisingly large and beneficial areas of overlap between these seemingly disparate fields. In theoretical terms, both fields are probing aspects of the poorly understood dark Universe; in experimental terms, both experiments exploit high quality classical resonators. I will describe my research work in connection with these experiments, especially where the same techniques have found applications in both fields.
Host: R. Scherrer
Thursday, April 13
Jonathan Feng, Department of Physics & Astronomy, University of California - Irvine
Dark Matter and the Search for a Fifth Force
The possibility of a fifth fundamental force, induced by a new force-carrier particle, has been the topic of intense interest at various times in the past several decades. Recently the search for a fifth force has found new motivations from the need to explain dark matter. I will discuss the colorful history of fifth force searches, the connection between new forces and dark matter, the worldwide program now searching for such new forces, and the preliminary, but striking, evidence that a fifth force may already have been seen in an experiment at the boundary of nuclear and particle physics.
Host: R. Scherrer
Monday, April 17---FRANCIS G. SLACK LECTURE---NOTE SPECIAL DAY & TIME
David J. Wineland, National Institute of Standards and Technology (NIST), Boulder, CO
Superposition, Entanglement, and Raising Schrödinger’s Cat
Research on precise control of quantum systems occurs in many laboratories throughout the world, for fundamental research, new measurement techniques, and more recently for the development of quantum computers. I will briefly describe experiments on quantum state manipulation of atomic ions at the National Institute of Standards and Technology (NIST), which serve as examples of similar work being performed with many other atomic, molecular, optical (AMO) and condensed matter systems around the world. This talk is, in part, the “story” of my involvement in these subjects that I presented at the 2012 Nobel Prize ceremonies.
Host: R. Scherrer
Thursday, April 20
Dean Lee, Department of Physics, North Carolina State University
Nuclear structure from lattice simulations
Recent ab initio lattice simulations using effective field theory find evidence that the nuclear many-body system is close to a quantum phase transition. The transition is between a nuclear liquid and a Bose-Einstein condensate of alpha particles. I discuss this result and its significance for ab initio nuclear structure theory. I then discuss new theoretical and computational progress in studies of neutron-rich nuclei and nuclear clustering.
Host: S. Umar