Physics and Astronomy Colloquium Series
Colloquia are presented on Thursdays at 3:45 p.m. in the Physics Research Building, room 245 (unless otherwise noted). Refreshments are served at 3:30 p.m.
Past and present colloquia
April 8, 2021
Prof. Eric Weeks, Department of Physics, Emory University
Flowing and clogging of soft particles and droplets
We study the flow and clogging of soft particles: micron-sized oil droplets, centimeter-sized hydrogel particles, and simulated soft particles. We find that softness is a key factor controlling clogging: with stiffer particles or a weaker driving force, clogging is easier. Softer particles form less stable arches and thus reduce the probability of clogging. Our results suggest that prior studies using hard particles were in a limit where the role of softness is negligible, which causes clogging to occur with significantly larger openings. In addition to softness, we also have some understanding of the roles of friction, vibrations, hydrostatic pressure, viscosity, and geometry… there’s a lot of physics behind clogging!
Eric Weeks earned his undergraduate degree in engineering physics at the University of Illinois at Urbana-Champaign. In 1997 he graduated with a Ph.D. in physics from the University of Texas at Austin, working in the Center for Nonlinear Dynamics with Prof. Harry Swinney. His dissertation was on experiments studying anomalous diffusion and atmospheric phenomena. He started a postdoctoral fellowship at the University of Pennsylvania with Prof. David Weitz and Prof. Arjun Yodh, and finished his postdoctoral work at Harvard University when the Weitz lab moved there. In January 2001 he joined the faculty of Emory University, where he is currently a Dobbs Professor of Physics. Since July 2018 he has also been the Director of Emory's Center for Faculty Development and Excellence.
March 25, 2021
Prof. Mary B. James, Dean for Institutional Diversity & A.A. Knowlton Professor of Physics, Reed College
TEAM_UP Report: The Time is Now Charting a Course to 2030
The AIP National Task Force to Elevate African American Representation in Undergraduate Physics & Astronomy (TEAM-UP) spent two years investigating the reasons for the persistent underrepresentation of African Americans in physics and astronomy and produced a report with its findings: “The Time Is Now: Systemic Changes to Increase African Americans with Bachelor’s Degrees in Physics and Astronomy.” In this groundbreaking report, TEAM-UP uncovers long-term systemic issues within the physics and astronomy communities that contribute to the underrepresentation of African Americans in these fields and makes important, actionable recommendations for community wide efforts to reverse this trend.
March 11, 2021
Dr. Saptarshi Das, Department of Engineering Science and Mechanics, Materials Science and Engineering, Materials Research Institute, Pennsylvania State University
Smart Sensors and Computing Devices for Hardware Artificial Intelligence
Many animals outsmart humans in sensory skills. In fact, animals can do much more than just see, smell, touch, taste, and hear. For example, octopuses possess polarized vision, bats use ultrasound to echolocate, hyper touch sensitive spiders can trace the origin of micro-vibrations and sharks can detect electric fields as weak as nanovolts per centimeter. The extraordinary sensing ability of these animals are mostly attributed to the evolutionary success of their respective and specialized sensory organs. However, less emphasis is laid on the connectivity, association, and organization of neurons inside the brainstem of these animals. What is even more humbling is the fact that the tiny brains of these animals allocate very limited neural resources in terms of area and energy for executing these high-level computations.
Drawing inspiration from natural intelligent sensor design, we have developed a number of solid-state biomimetic devices that provide unprecedented energy and area benefits for sensory computations. In particular, we have mimicked auditory information processing in barn owl (Nature Communications, 10, 3450, 2019), collision avoidance by locust (Nature Electronics, 2020), and subthreshold signal detection by paddlefish and cricket using stochastic resonance (Nature Communications, 2020). We have also mimicked probabilistic computing in animal brains using low-power Gaussian synapses (Nature Communications, 10, 4199, 2019) and realized a biomimetic device that can emulate neurotransmitter release in chemical synapses (ACS Nano, 11, 3, 2017). We use novel two-dimensional (2D) nano materials, nano devices, and in-memory computing architectures to demonstrate this new paradigm of sensing and computing. Our goal is to deploy theses low-power and smart biomimetic devices at remote, inaccessible, and resource constrained locations.
Dr. Das was appointed as an Assistant Professor of Engineering Science and Mechanics (ESM) and member of the Materials Research Institute (MRI) and the Intercollege Graduate Degree Program in Materials Science and Engineering at Penn State University in January 2016. He received his B.Eng. degree (2007) in Electronics and Telecommunication Engineering from Jadavpur University, India, and Ph.D. degree (2013) in Electrical and Computer Engineering from Purdue University. Before joining Penn State, he was a Postdoctoral Research Scholar (2013-2015) and Assistant Research Scientist (2015-2016) at Argonne National Laboratory (ANL). Dr. Das was the recipient of Young Investigator Award from United States Air Force Office of Scientific Research in 2017 and National Science Foundation (NSF) CAREER award in 2021. Das Research Group at Penn State leads a new multidisciplinary area of science, namely biomimetic sensing and neuromorphic computing inspired by the neurobiological architecture and neural computational algorithms found inside various animal brains allowing evolutionary success of the species.
February 18, 2021
Dr. Mark Moldwin, University of Michigan
The STEM Academic System: Feedbacks Limiting Diversity and Driving Inequality
Despite deliberate and concerted efforts over the years, the science community has not made significant progress in increasing the representation of the discipline with respect to marginalized communities (or what will be called the “new majority” community in this talk). Using a systems approach, this presentation will discuss how academic culture promotes inequality and will describe some leading practices that can be implemented at the individual, academic department, and professional community level to help create a diverse and inclusive science community.
February 11, 2021
Prof. Lilia M. Woods, University of South Florida
From 2D graphene to its “cousin” 3D Weyl semimetal: Casimir effects
The Casimir force is a universal interaction originating from electromagnetic fluctuations between objects, however, its magnitude, sign, scaling laws, and other dependencies are strongly affected by the interacting systems and their boundaries. The expansion of the graphene family of materials by adding silicene, germanene, and stanene has created new opportunities for probing Dirac-like physics and nontrivial band topology. Due to the finite buckling and significant spin-orbit interaction in these newly added to the materials library systems, various quantum Hall phase states can be realized by applying external fields. These in turn cause Casimir force phase transitions with many unusual features, including repulsion and quantization. Weyl semimetals, considered to be 3D graphene analogs, display very different Casimir behavior despite the common Dirac-like energy spectra. While nontrivial topology may play a dominant role in graphene interactions, such effects are secondary in the Weyl semimetal Casimir forces. These materials present an excellent platform for light-matter interactions, and, in particular, to show that the universal Casimir force has non-universal dependence upon distance, sign, magnitude, and various fundamental constants.
Lilia M. Woods has obtained her Ph.D. in condensed matter physics from the University of Tennessee, Knoxville with Prof. Gerald D. Mahan as a Ph.D. advisor. After graduation, she was a postdoc at Oak Ridge National Lab, followed by a second postdoc at the Naval Research Lab, where she held the prestigious Director’s funded NRC Fellowship. In 2003 she became an Assistant Professor at the University of South Florida and in 2012 she was promoted to Full Professor. Lilia M. Woods has established a vigorous research program in theoretical and computational condensed matter physics, which has been continuously funded by the National Science Foundation and the Department of Energy since 2006. She has been recognized by the USF Outstanding Research Achievement Award (three times) and by the Jewell Faculty Excellence Award. She is a member of the National Academy of Inventors. She has received the International Association of Advanced Materials Medal for 2018. Lilia Woods was also elected as an APS Fellow in 2017 and an AAAS Fellow in 2019.
December 3, 2020
Dr. Laura Grego, Union of Concerned Scientists
The Growing Danger of Nuclear Weapons (and how physicists can help reduce it)
While nuclear weapons might sound like Cold War relics, in truth the immense risks they pose to all humanity are still very much with us. In fact, trends indicate the risks may be growing with the abandonment of arms control agreements and the development of new types of strategic weapons. Physicists have historically constructively engaged policymakers and their communities to help reduce nuclear dangers. This talk will explain the current nuclear crisis, provide feasible remedies, and introduce a new project sponsored by the American Physical Society to help physicists once again get involved.
Laura Grego is a senior scientist in the Global Security Program at the Union of Concerned Scientists. She focuses her analysis and advocacy on missile defense, outer space security, and nuclear weapons. She has authored or co-authored numerous papers on a range of topics, including cosmology, space security, and missile defense, and has testified before Congress and addressed the UN General Assembly and the UN Conference on Disarmament on security issues. Before joining UCS, Dr. Grego was a postdoctoral researcher at the Harvard-Smithsonian Center for Astrophysics. She earned a Ph.D. in experimental physics at the California Institute of Technology, and a BS in physics and astronomy at the University of Michigan.
November 19, 2020
Dr. Rachel Henderson, Michigan State University
Advancing Assessment in Physics Education: Bringing Psychometrics to Physics Education Research with an Eye to Equity and Inclusion
The field of Physics Education Research (PER) has made major contributions to various educational practices and materials to reform instruction in order to recruit and retain more students. However, while many research-based instructional strategies in physics have continued to advance, reform in undergraduate physics assessment has had limited space in these conversations and needs more attention. As educators, we would like to prepare our physics students for 21st-century careers. In addition to providing the robust set of skills that students are expected to have after receiving a degree in physics, the physics community seeks to increase diversity within our classrooms. Overall, to ensure all students will become successful scientists, physics departments need to be able to provide evidence to make sure that we are reaching these goals. The importance of fairness and equity in assessment for undergraduate Science, Technology, Engineering and Mathematics (STEM) education is greater than ever. In this talk, I will motivate, through my prior research experiences, the need for the next generation of physics assessments. Advancing the practices and tools of assessment in physics, so that they are valid, fair, and effective, can better provide equal educational access and better ensure success for our students.
Rachel earned her Bachelor of Science degree in physics from Slippery Rock University in Pennsylvania. She then went on to do her graduate work at West Virginia University where she completed her Master’s and Ph.D. in physics. A little over two years ago, she moved here to Michigan where she did her postdoc work in collaboration with Danny Caballero in the Physics Education Research Lab at Michigan State University. Rachel is currently the newest Assistant Professor in the Department of Physics and Astronomy and the CREATE for STEM Institute at MSU. She also serves as a member-at-large for the APS Topical Group on Data Science and the APS Topical Group on Physics Education Research. She was most recently nominated to serve on the American Association of Physics Teacher Physics Education Research Leadership and Organizing Council (PERLOC). In general, Rachel’s research focuses on developing and implementing inclusive and equitable assessment tools that can be used to improve learning for all students within the physics classroom.
November 12, 2020
Dr. Valerica Raicu, University of Wisconsin-Milwaukee
Distance- and diffusion-based methods for probing macromolecular association in living cells
Fluorescence-based methods for probing the association of proteins or other biological macromolecules within living cells fall roughly within two groups. One family of methods, based on Förster Resonance Energy Transfer (FRET), probes molecular association by measuring relative distances between molecules within a complex (or oligomer) via transfer of energy from an optically excited to un-excited fluorescent tags attached to the molecules of interest. A second class of methods, generically known as Fluorescence Fluctuation Spectroscopy (FFS), probes fluctuations in fluorescence intensities from pixel to pixel in an image (i.e., in space) or from measurement to measurement (in time series) to determine whether the molecules that produced the fluctuations diffuse around as monomers, dimers, or higher-order oligomers. In this talk, I will overview contributions of my research group to the development of such methods and their application to the study of oligomerization of membrane receptors in the presence and absence of their natural or artificial ligands. Our studies aim to provide (i) biophysicists and life scientists with tools for understanding life processes, such as cellular signaling, and (ii) pharmacologists with in-cell drug screening assays for the discovery and characterization of new drugs.
Valerică Raicu is a Professor in the Physics Department and affiliated faculty in the Biological Sciences Department at the University of Wisconsin-Milwaukee (UWM). Professor Raicu’s main research interests span the development of spectroscopic and micro-spectroscopic technology and applications to the study of protein-protein and cell-cell interactions. His focus over the past decade has been the investigation of G-Protein coupled receptor oligomerization in living cells and the biological role it may play. He has authored or co-authored numerous peer-reviewed articles, book chapters, and books on various topics in physics, biophysics, and bioengineering, as well as several patents in the area of optical micro-spectroscopy. He also currently serves as the Director of the UWM Small Businesses Collaboratory and Biophysical Spectroscopy Laboratory.
November 5, 2020
Dr. Kevin Pedro, Fermilab
Coprocessors as a service to accelerate machine learning inference for particle physics
New heterogeneous computing paradigms on dedicated hardware with increased parallelization offer exciting solutions with large potential gains. The growing applications of machine learning algorithms in particle physics for simulation, reconstruction, and analysis are naturally deployed on such platforms. The acceleration of machine learning inference as a web service represents a heterogeneous computing solution for particle physics experiments that requires minimal modification to the current computing model. Coprocessors deployed as an edge or cloud service for the particle physics computing model can have a higher duty cycle and are potentially much more cost-effective. Initial results with Microsoft Brainwave FPGAs and Nvidia GPUs show more than an order of magnitude reduction in inference latency and high throughput. The demonstrated performance is suitable to address the computing challenges faced by both energy frontier and intensity frontier experiments, including the HL-LHC detectors and DUNE.
- B.S. Physics from Rensselaer Polytechnic Institute (Daya Bay, CLIC, CMS, ATLAS)
- Ph.D. Physics from University of Maryland (CMS)
- Postdoc at Fermilab (CMS)
- (now) Associate Scientist in Scientific Computing Division at Fermilab
- Member of CMS collaboration; informally doing some computing work for DUNE
- Interests: dark matter from hidden sectors (semi-visible jets, emerging jets, soft unclustered energy patterns), software and computing, detector simulation, machine learning
November 4, 2020
Dr. Abhijit Majumdar, Wayne State University
A Framework for Precision Exploration of the Quark-Gluon Plasma
Over the last several years there has been a sea change in the study of the Quark-Gluon-Plasma (QGP) produced in high-energy heavy-ion collisions. There is now a considerable amount of high statistics data in both the high and low momentum sector. Theoretical descriptions have attained a much higher level of sophistication. This has necessitated the incorporation of Bayesian inferencing methods in the comparison between theory and data. In this presentation, I will focus on the high momentum sector and outline how the theory has evolved over the last several years, into a multi-stage, multi-faceted formalism that describes the modification of QCD jets in the QGP. Particular focus will be placed on the work of the JETSCAPE Collaboration and the need for extensive event generator frameworks. I will highlight several upcoming studies and demonstrate how these provide a much more detailed picture of the QGP.
- BSc Physics Indian Institute of Technology at Kharagpur 1995
- MSc Physics Indian Institute of Technology at Kharagpur 1997
- Ph.D. in Theoretical Nuclear Physics, McGill University, 2002
- Post doc 1: Lawrence Berkeley National Lab. 2002 - 2005
- Post doc 2: Duke University, 2005 - 2008
- Visiting assistant prof: Ohio State University 2008 - 2011
- Associate professor: Wayne State University 2011-Present, Tenured 2015
Study of the Quark-Gluon Plasma in Relativistic Heavy-Ion Collisions, with a focus on jets and jet quenching. Recent work has been more computational and interdisciplinary (with statistics and computer science as part of the JETSCAPE Collaboration). Have been involved with the design of extensive computer simulations of heavy-ion collisions. Carried out theoretical calculations of jet quenching both using analytic techniques as well as static simulations of the QGP using Lattice QCD.
October 29, 2020
Dr. Omer Blaes, University of California at Santa Barbara
Magnetohydrodynamics and Convection in Accretion Disks: From Dwarf Novae to Luminous Quasars
One of the most powerful sources of energy in the universe is the liberation of gravitational binding energy as plasma falls into a central object, such as a black hole. Such accretion flows are dynamically complex, involving significant rotational support against gravity, transport of angular momentum by magnetic turbulence, and turbulent dissipation. Unfortunately, simulations of these processes have had a checkered history of explaining observed accretion-powered sources in the universe. In this talk, I will show how an additional complicating factor, opacity-driven convection, exerts a powerful backreaction on the dynamics of these flows and might provide an explanation for a variety of observed phenomenology in luminous accretion flows around white dwarfs and supermassive black holes.
After spending his formative years growing up in the UK, Omer Blaes earned his doctorate at the International School for Advanced Studies (SISSA) in Trieste, Italy. He then did postdoctoral work at the California Institute of Technology and the Canadian Institute of Theoretical Astrophysics in Toronto. He finally joined the Physics Department faculty at the University of
California, Santa Barbara in 1993 and has been there ever since. He has worked on a variety of problems in theoretical high energy astrophysics, particularly on the theory of so-called accretion disks: rapidly rotating flows around black holes and other compact objects that liberate gravitational energy into energetic outflows and radiation that we observe in a variety of spectacular phenomena across the universe.
October 22, 2020
Dr. Annika Peter, Ohio State University
Testing the nature of dark matter with galaxies
The nature of dark matter is unknown. The leading paradigm for dark matter is that it consists of at least one species of non-relativistic ("cold"), weakly interacting particles, the cold dark matter (CDM) paradigm. One of the strongest predictions of CDM is the hierarchy of structure down to Earth-mass scales. However, individual self-bound clumps of dark matter--"halos"--are difficult to detect directly. Instead, we use galaxies, which grow at the centers of halos, as lampposts for these halos. By counting galaxies, we can measure the underlying population of dark matter halos and test the nature of dark matter. In this talk, I describe two results that seem completely at odds with each other in measuring the population of small halos. I argue that the resolution to the problem is a better mapping between galaxies and halos. I will show what my group is doing so far to address the problem, and what opportunities lie ahead in the wide-field surveys of the 2020s.
I received a Ph.D. in physics from Princeton University in 2008. I was a Moore postdoctoral fellow at Caltech 2008-2010, and a McCue postdoctoral fellow at UC Irvine 2011-2013. Since 2013, I have been a faculty member in the physics and astronomy departments at The Ohio State University, and a member of the CCAPP Science Board. I received tenure in 2019. My work straddles the line of physics and astronomy, and theory and observation: I use the tools of astronomy and astrophysics to reveal the fundamental physics of dark matter.
October 15, 2020
Dr. Zhixian Zhou, Wayne State University
Two-Dimensional Layered Semiconductors beyond Graphene: Electronics and Device Physics
The successful isolation of two-dimensional (2D) graphene has stimulated research on a broad range of other 2D materials, among which layered transition metal dichalcogenides (TMDs) have attracted particular attention. The semiconducting members of the TMD family including MoS2, MoSe2 and WSe2 have not only demonstrated many of the "graphene-like" properties desirable for electronic applications such as a relatively high mobility, mechanical flexibility, chemical and thermal stability, and the absence of dangling bonds but also have a substantial band gap (1 ~ 2 eV depending on the material and its thickness), which is absent in 2D graphene but required for mainstream logic applications. However, a major bottleneck in electronic applications of TMDs is their tendency to form a substantial Schottky barrier with most electrode metals, which severely limits their performance. In this talk, I will discuss our recent work aiming to overcome this fundamental challenge and subsequently explore the intrinsic transport properties of TMDs.1-5
1. Perera, M. M.; Lin, M.-W.; Chuang, H.-J.; Chamlagain, B. P.; Wang, C.; Tan, X.; Cheng, M. M.-C.; Tománek, D.; Zhou, Z. Improved Carrier Mobility in Few-Layer MoS2 Field-Effect Transistors with Ionic-Liquid Gating. ACS Nano 2013, 7, 4449-4458.
2. Chuang, H.-J.; Tan, X.; Ghimire, N. J.; Perera, M. M.; Chamlagain, B.; Cheng, M. M.-C.; Yan, J.; Mandrus, D.; Tománek, D.; Zhou, Z. High Mobility WSe2 p- and n-Type Field-Effect Transistors Contacted by Highly Doped Graphene for Low-Resistance Contacts. Nano Letters 2014, 14, 3594-3601.
3. Chuang, H.-J.; Chamlagain, B.; Koehler, M.; Perera, M. M.; Yan, J.; Mandrus, D.; Tománek, D.; Zhou, Z. Low-Resistance 2D/2D Ohmic Contacts: A Universal Approach to High-Performance WSe2, MoS2, and MoSe2 Transistors. Nano Letters 2016, 16, 1896-1902.
4. Guan, J.; Chuang, H.-J.; Zhou, Z.; Tománek, D. Optimizing Charge Injection across Transition Metal Dichalcogenide Heterojunctions: Theory and Experiment. ACS Nano 2017.
5. Andrews, K.; Bowman, A.; Rijal, U.; Chen, P.-Y.; Zhou, Z. Improved Contacts and Device Performance in MoS2 Transistors Using a 2D Semiconductor Interlayer. ACS Nano 2020, 14, 6232-6241.
Dr. Zhixian Zhou received his Ph.D. from Florida State University/National High Magnetic Field Laboratory in 2004. After working at the Oak Ridge National Laboratory as a postdoctoral research associate, he joined Wayne State University as an assistant professor in 2007 and was promoted to associate professor with tenure in 2013.
October 8, 2020
Dr. Xiaoming Wang, University of Michigan
Topological floppy modes in aperiodic networks and a mechanical duality theorem
Topological states of matter have been intensively studied in crystals, leading to fascinating phenomena such as scattering-free edge current in topological insulators. However, the power of topological protection goes well beyond ordered crystal lattices. In this talk, we explore how topology protects mechanical edge modes in messy, noncrystalline, systems. We will use disordered fiber networks and quasicrystals as our examples, to demonstrate how topological edge floppy modes can be induced in these structures by controlling their geometry. Fiber networks are ubiquitous in nature and especially important in bio-related materials. Establishing topological mechanics in fiber networks may shed light on understanding robust processes in mechanobiology. Quasicrystals show unusual orientational order with quasiperiodic translational order. We found that a bulk topological polarization can be defined for mechanics of quasicrystals that are unique to their non-crystallographic orientational symmetry. References: (1) Di Zhou, Leyou Zhang, Xiaoming Mao, “Topological Edge Floppy Modes in Disordered Fiber Networks”, Phys. Rev. Lett. 120, 068003 (2018); (2) Di Zhou, Leyou Zhang, Xiaoming Mao, “Topological Boundary Floppy Modes in Quasicrystals”, Phys. Rev. X 9, 021054 (2019).
October 1, 2020
Dr. Eric Pop, Stanford University
Electronic, Thermal, and (Some) Unusual Applications of 2D Materials
This talk will present recent highlights from our research on two-dimensional (2D) materials, including graphene, boron nitride (h-BN), and transition metal dichalcogenides (TMDs). Our results span from material growth and fundamental measurements to simulations, devices, and system-oriented applications. We have grown monolayer 2D semiconductors over large areas, including MoS2 , WSe2, and MoSe2 . We also uncovered that ZrSe2 and HfSe2 have native high-κ dielectrics ZrO2 and HfO2, which are of key technological relevance . Improved electrical contacts  led to the realization of 10 nm monolayer MoS2 transistors with high current density, near ballistic limits . We have also demonstrated new memory devices based on Mo-, Sb-, and Ge- tellurides [6,7]. These could all play a role in the 3D heterogeneous integration of nanoelectronics, which presents significant advantages for energy-efficient computation . I will also describe a few less conventional applications, where we used 2D materials as highly efficient thermal insulators  and as thermal transistors . These could enable control of heat in “thermal circuits” analogous with electrical circuits. Combined, these studies reveal fundamental limits and some unusual applications of 2D materials, which take advantage of their unique properties.
Refs:  K. Smithe et al., ACS Nano 11, 8456 (2017).  K. Smithe et al., ACS AMI 1, 572 (2018).  M. Mleczko et al., Science Adv. 3, e1700481 (2017).  C. English et al., Nano Lett. 16, 3824 (2016).  C. English et al., IEDM, Dec 2016.  I. Datye et al., Nano Lett. 20, 1461 (2020).  A. Khan et al, in prep. (2020).  M. Aly et al., Computer 48, 24 (2015).  S. Vaziri et al., Science Adv. 5, eaax1325 (2019).  A. Sood et al. Nature Comm. 9, 4510 (2018).
Eric Pop is a Professor of Electrical Engineering (EE) and Materials Science & Engineering (by courtesy) at Stanford, where he leads the SystemX Heterogeneous Integration focus area. He was previously on the faculty of UIUC (2007-13) and worked at Intel (2005-07). His research interests are at the intersection of electronics, nanomaterials, and energy. He received his Ph.D. in EE from Stanford (2005) and three degrees from MIT (MEng and BS in EE, BS in Physics). His honors include the Presidential Early Career Award (PECASE), Young Investigator Awards from the Navy, Air Force, NSF and DARPA, and several best paper and best poster awards with his students. He is an Editor of the journal 2D Materials, has served as General Chair of the Device Research Conference, and on program committees of IEDM, VLSI, APS, and MRS. In his spare time, he tries to avoid injuries while snowboarding and in a past life he was a DJ at KZSU 90.1 FM, from 2000-04. Additional information about the Pop Lab is available online at http://poplab.stanford.edu
September 24, 2020
Dr. Sarah Keller, University of Washington
Gargantuan domains in living yeast vacuole membranes (and what could make them smaller)
For decades, scientists have argued about the mechanisms of how living cell membranes acquire regions enriched in particular lipid and protein types. One highly contested theory has been that domains form via phase separation, as occurs in simple artificial membranes. This theory has been slow to gain acceptance because it contradicts the widespread assumption that inhomogeneities in living cell membranes are limited to sub-micron sizes (whereas phase separation results in much larger domains). Since the 1960s, researchers have reported tantalizing hints that membranes in yeast do indeed phase separate. However, proof of phase separation hinges on the observation of a reversible phase transition. Here, we provide that direct evidence for vacuole membranes of living yeast cells. Inversely, when might unusually small domains appear in artificial membranes? Current theories of membrane microemulsions and modulated phases present several firm hypotheses, which we test in simple vesicle systems. In our system, small, stable domains are only observed in membranes with the excess area.
January 9, 2020
Andreas Kronfeld, Fermilab
QCD is Everywhere
Quantum chromodynamics (QCD) is the modern theory of the strong nuclear force and part of the Standard Model of elementary particles. QCD is a beautiful theoretical construct that has required considerable human ingenuity to understand in detail. The beauty, ingenuity, and peculiarities can be understood via examples from particle physics and nuclear physics, of course, but also from everyday life.
January 23, 2020
David Pine, New York University
In pursuit of colloidal diamond
While suspensions of colloidal particles self-assemble into a wide variety of crystalline lattices, making them assemble into the diamond lattice has proven elusive. The desire to do so has been driven by the fact that the a dielectric diamond lattice exhibits the widest photonic band gap of any known crystalline structure. Here we report on the progress of strategies to realize a colloidal diamond lattice using DNA-coated colloids in various guises: patchy particles, particle clusters, and superlattices.
February 6, 2020
Jordan Horowitz, University of Michigan
Nonequilibrium thermodynamic limits to fluctuations and response
Thermodynamics is a remarkably successful theoretical framework, with wide ranging applications across the natural sciences. Unfortunately, thermodynamics is limited to equilibrium or near-equilibrium situations, whereas most of the natural world, especially life, operates very far from thermodynamic equilibrium. Research in nonequilibrium statistical thermodynamics is beginning to shed light on this domain. In this talk, I will present two such recent predictions. The first is a bound that quantifies how dissipation shapes fluctuations far from equilibrium. Besides its intrinsic allure as a universal relation, I will discuss how it can be used to offer energetic constraints on chemical clocks, and bound the dissipation in complex materials. The second is a collection of equalities and inequalities---akin to the Fluctuation-Dissipation theorem but valid arbitrarily far from equilibrium---that link a system’s response to the strength of nonequilibrium driving. These results open new avenues for experimentally characterizing nonequilibrium response and suggest design-principles for high-sensitivity, low-noise devices. I will also discuss how they rationalize the energetic requirements of some common biochemical motifs.
February 27, 2020
Douglas M. Hudgins, NASA
An Introduction to NASA’s Astrophysics Division and the Exoplanet Exploration Program
To many in the scientific community, NASA appears to be a “black box” containing a bewildering array of field centers, missions, projects, research programs, and who knows what else. In fact, there *is* some method to the madness, and I hope to convince you of that fact. In this talk I will provide an introduction to NASA’s Astrophysics Division—who we are, how the division is structured, the avenues by which we support the community through our missions and research programs, and the ways that we engage the scientific community. From there, I will go on to focus on the Exoplanet Exploration Program (ExEP)—the program responsible for implementing NASA's plans for discovering and characterizing exoplanets, and identifying candidates that could harbor life. We will see that the Program encompasses a wide range of projects and activities from future mission concept studies, and technology development programs to enable those missions, to precursor and follow-up ground-based science programs that enhance the science return of current NASA missions and enable the design of next generation exoplanet missions. I will feature some recent highlights from the Program’s activities and describe the ways that the Program interfaces and supports the scientific community, and is paving the way to the future.
March 5, 2020
Spencer Klein, Lawrence Berkley National Laboratory
The highest energy photons: using ultra-peripheral collisions at the LHC
and RHIC to probe nuclear structure and test the standard model
High-energy photons are a simple tool with many uses. The most energetic photons today are those that are produced in ultra-peripheral collisions (UPCs) involving protons or heavier ions. The relativistically boosted electromagnetic fields of these ions act like a flux of nearly-real photons. To the target (other) nucleus, the energies reach the PeV (10^15 eV) range. I will discuss a number of physics topics that are being studied using these photons, including photon-photon scattering, production of antihydrogen atoms, and using these photons to probe nuclear structure, particularly via vector meson photoproduction. Photoproduction is sensitive to the density and spatial distribution of gluons with very low momentum (Bjorken-x). Finally, I will conclude with a brief look forward to the recently approved ("CD-0" in Dept. of Energy parlance) electron-ion collider..
March 19, 2020
Annika Peter, Ohio State University
Testing the nature of dark matter with galaxies
The nature of dark matter is unknown. The leading paradigm for dark matter is that it consists of at least one species of non-relativistic ("cold"), weakly interacting particles, the cold dark matter (CDM) paradigm. One of the strongest predictions of CDM is the hierarchy of structure down to Earth-mass scales. However, individual self-bound clumps of dark matter--"halos"--are difficult to detect directly. Instead, we use galaxies, which grow at the centers of halos, as lampposts for these halos. By counting galaxies, we can measure the underlying population of dark matter halos, and test the nature of dark matter. In this talk, I describe two results that seem completely at odds with each other in measuring the population of small halos. I argue that the resolution to the problem is a better mapping between galaxies and halos. I will show what my group is doing so far to address the problem, and what opportunities lie ahead in the wide-field surveys of the 2020's..
March 26, 2020
David Wineland, University of Oregon
Vaden Miles Lecture, "Quantum Computers and Raising Schrödinger's Cat"
Quantum systems such as atoms can be used to store information. For example, we can store a binary bit of information in two energy levels of an atom, labeling the state with lower energy a “0" and the state with higher energy a “1.” However, quantum systems can also exist in superposition states, thereby storing both states of the bit simultaneously, a situation that makes no sense in our ordinary-day experience. This property of quantum bits or “qubits” potentially leads to an exponential increase in memory and processing capacity. It would enable a quantum computer to efficiently solve certain problems such as factorizing large numbers, a capability that could compromise the security of current encryption systems. It could also be used to simulate the action of other important quantum systems in cases where such a simulation would be intractable on a conventional computer. A quantum computer could also realize an analog of "Schrödinger's Cat," a bizarre situation where a cat could be simultaneously dead and alive. Experiments whose goal is to realize a quantum computer based on laser manipulations of atomic ions will be described but this is just one platform that many groups around the world are investigating.
April 2, 2020
Xiaoming Mao, University of Michigan
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April 9, 2020
Wei Zhang, Oakland University
From Hybrid Quantum Magnonics to Terahertz Spintronics
Hybrid magnonic systems are great candidates for making interconnects between different quantum platforms, due to their coherent and strong coupling with other quantum excitations. The ability for sensitively and reliably probing the magnon dynamics in various novel spintronic and magnonic contexts are thus crucial, for example, involving state-of-the-art approaches using spin-torques, acoustic phonons, and microwave photons. I will discuss our recent results on the detection of phase-resolved magnetization dynamics using a cw-modulated laser technique, which allows a modulation at the GHz frequencies with both amplitude- and phase-control and also phase-locking to a microwave source. We show ferromagnetic resonance measurement of both metals and insulators, as well as the quantification of spin-orbit torques in ferromagnet/heavy-metal bilayers . We also demonstrated the facile optical detection of magnon-magnon coupling in Permalloy/Y3Fe5O12 bilayers using this technique via a combination of magneto-optical Kerr and Faraday effects [2,3]. In particular, these measurements provide direct information on how the excitation of magnons in permalloy modes may suppress the excitation of magnons in YIG analogous to previously observed magnetically-induced-transparency effects in magnon-photon hybrid systems. Finally, I will make connections to a new spintronic material system for achieving strong and controllable ultrafast dynamics beyond GHz and up to THz frequency range.
 Yi Li et al, “Simultaneous Optical and Electrical Spin-Torque Magnetometry with Phase-Sensitive Detection of Spin Precession”, Phys. Rev. Applied 11, 034047 (2019).
 Yuzan Xiong et al, “Probing the Magnon-Magnon Coupling using Combinatorial Magneto-Optical Kerr and Faraday Effects”, ArXiv: 1912.13407.
 Yi Li et al, “Coherent spin pumping in a strongly coupled magnon-magnon hybrid system”, Phys. Rev. Lett. in press (2020).
September 19, 2019
James Wells, University of Michigan
Seeking clues on why matter won over antimatter
One of the most intriguing mysteries of nature is why there is more matter in the universe than anti-matter given that the basic laws of particle physics do not appear to allow for it. One promising direction of explanation attacks the conservation of baryon number, which I will argue is one of the most vulnerable principles in fundamental physics. Forthcoming proton decay and neutron oscillations experiments may reveal much about just how the universe managed to make us and not anti-us.
September 26, 2019
Aurelia Honerkamp-Smith, Lehigh University
Membrane protein transport: Balancing advection and diffusion
The fluidity of lipid membranes is essential to their biological function: cell membranes are required to be flexible, self-healing, and deformable. An additional consequence of membrane fluidity is that both lipids and proteins are highly mobile, which makes it possible for lipid-anchored proteins to travel long distances across the surface of cells. The significance of this lateral mobility for flow mechanosensing has not yet been determined. We study the mechanics of lateral membrane protein transport by flow in an in vitro model of the cell plasma membrane, lipid bilayers supported on glass. We prepare supported bilayers formed from individual GUVs inside microfluidic channels in order to study transport of membrane-anchored proteins. We observe and fit dynamic protein concentration gradients, which allow us to define protein mobility relative to a stationary lipid membrane.
October 3, 2019
Dean Lee, Michigan State University
Lattice Simulations of Nuclear Structure and Thermodynamics
In this talk I give an introduction to the method of lattice effective field theory, which is the combination of effective field theory and numerical lattice techniques. I then discuss several recent results from the Nuclear Lattice Effective Field Theory Collaboration. The first is the connection between microscopic nuclear forces and nuclear structure. This centers on a basic question of what are the truly essential features of the nuclear force needed to produce the observed properties of atomic nuclei. The second is the development of first principles simulations of the thermodynamics of nuclei and nuclear matter.
October 17, 2019
William Llope, Wayne State University
Correlations in relativistic heavy-ion collisions
In modern collider experiments, scientists collide atomic nuclei going at the speed of light in order to create large systems of deconfined quarks and gluons called the quark-gluon plasma (QGP). The scientists try to measure every particle produced in the collision in order to try to completely reconstruct the physics of the QGP, the state of our universe a few microseconds after the Big Bang. By measuring as many particles as possible per collision in a wide acceptance, scientists can explore multi-particle correlations, which indicate both prosaic reaction mechanisms (like momentum conservation) and potentially more exotic (and as yet undiscovered) mechanisms such as a critical opalescence - a beam-energy localized growth of long-range correlations signalling the existence of a critical point in the nuclear matter phase diagram. My two research groups work in two different but related directions. One group explores these correlations, and their integrals called the "fluctuations," by analyzing the experimental data collected by the STAR experiment at RHIC. My other group has been building new large-scale detector upgrades to improve the data coming from these modern experiments. The status and outlook of these efforts will be presented.
October 24, 2019
Indara Suarez, Boston University
Desperately Seeking SUSY
Questions surrounding the measured value of the Higgs mass as well as astrophysical evidence for Dark Matter suggest that new particles and/or interactions are awaiting discovery. With the significant increase in collision energy and the large datasets of the LHC Run-2, we have continued our hunt for physics beyond the Standard Model by developing new strategies and machine-learning tools. I will discuss the ongoing searches for Supersymmetric partners of the top quark, called top squarks or "stops", and how their discovery could shed light onto the nature of the lightness of the Higgs mass and Dark Matter. My talk will focus on the detector upgrades that laid the foundation for exploiting the Run-2 data, the most recent results, and possible future directions in our search for new physics.
October 31, 2019
Kate Scholberg, Duke University
Detecting the Tiny Thump of the Neutrino
Neutrinos interact only rarely with matter. Coherent elastic neutrino-nucleus scattering (CEvNS) was first predicted in 1974; it’s a process in which a neutrino scatters off an entire nucleus. By neutrino standards, CEvNS occurs frequently, but it is tremendously challenging to see. The only way to observe it is to detect the minuscule thump of the nuclear recoil. CEvNS was measured for the first time by the COHERENT collaboration using the unique, high-quality source of neutrinos from the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory. This talk will describe COHERENT's recent measurement of CEvNS, the status and plans of COHERENT's suite of detectors at the SNS, and the physics we will learn from the measurements.
November 7, 2019
Elena Gallo, University of Michigan
A census of the black hole population in nearby low mass galaxies
It is commonly accepted that a supermassive black hole – as massive as several millions or even billions Suns – sits at the center of every galaxy that is as large as (or larger than) the Milky Way galaxy. Additionally, the black hole is thought to play a critical role in shaping the large-scale properties of the galaxy it lives in. It does so by blowing galaxy-scale outflows of energy and matter, which in turn affect the fate of its host galaxy gas reservoir, and, ultimately, its ability to form stars. Whether a similar feedback mechanism takes place is not obvious when it comes to dwarf galaxies, i.e. galaxies that are one tenth of a Milky Way, or smaller. Even though dwarfs make up the overwhelming majority of the galaxy population out there, it is not clear whether all dwarf galaxies host a massive black hole at their center, and, if they do, whether the properties of those black holes, and chiefly their masses, scale with the overall galaxies’ properties the same way they do in larger galaxies. Part of the uncertainty has to do with the fact that standard “dynamical techniques”, which use the motion of gas and stars around the black hole to infer its presence, rely on being able to spatially resolve the black hole's gravitational sphere of influence. Yet the fraction of dwarf galaxies that host a massive black hole is of great interest to astronomers. The reason goes beyond simple demographics; the “black hole occupation fraction” in today’s dwarf galaxies is expected to be sensitive to the very mechanism through which these giant black holes were born when the Universe was still in its infancy. Surprisingly, this remains an open question. During this talk, I will present recent results from my group addressing these questions, in an effort to build a complete census of the population of nearby, massive black holes.
December 5, 2019
Michael Strickland, Kent State University
Bottomonium suppression in the quark-gluon plasma
The suppression of bottomonia in ultrarelativistic heavy-ion collisions is a smoking gun for the production of a long-lived strongly interacting final state. Moreover, the experimentally observed suppression is consistent with the production of a hot hydrodynamically expanding quark-gluon plasma (QGP) with initial temperatures on the order of 600-700 MeV at LHC collision energies. Theoretical models which incorporate plasma screening and in-medium bound-state breakup based on high-temperature quantum chromodynamics are in good agreement with the centrality, transverse-momentum, and rapidity dependence of the experimentally observed suppression. Importantly, these models are self-consistently coupled to the soft dynamics of the QGP using 3+1d relativistic hydrodynamics which provides tight constraints on the evolution of the QGP temperature, etc. The resulting model comparisons with LHC experimental data indicate primordial suppression of all bottomonium states, with states having the lowest binding energies suffering the most suppression. I will review recent theoretical and experimental advances in the study of in-medium heavy quarkonia suppression and discuss these advances in the larger context of the hunt for the QGP.
January 18, 2018
Shanshan Cao, Wayne State University
Probing the Quantum Chromodynamic fluid with relativistic nuclear collisions
Abstract: Nuclear matter is heated beyond two trillion degrees in relativistic heavy-ion collisions and becomes a strongly coupled plasma of quarks and gluons. This highly excited quark-gluon plasma (QGP) matter displays properties of the perfect fluid and is believed similar to the state of the early universe microseconds after the big bang. In this talk, high-energy particles and jets are utilized to probe the QGP properties. A linear Boltzmann transport coupled to hydrodynamic model is established to describe the strong interaction between energetic partons and the QGP. This includes diverse microscopic processes for both massless and massive parton scatterings, and provides a simultaneous description of the nuclear modification of heavy and light flavor hadrons observed at the RHIC and LHC experiments. To precisely extract transport coefficients of the QGP, a statistical analysis framework that includes machine learning and Bayesian methods is developed, which brings a paradigm shift in statistical comparisons between theory and experiment.
January 25, 2018
ChunNing (Jeanie) Lau, Ohio State University
Spin, Charge and Heat Transport in Low-Dimensional Materials
Abstract: Low 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 of high-quality few-layer phosphorene devices, the unprecedented current carrying capacity of carbon nanotube "hot dogs", and our recent observation of robust long distance spin transport through the antiferromagnetic state in graphene.
About the speaker: Dr. Chun Ning (Jeanie) Lau is a Professor in the Department of Physics at The Ohio State University. She received her BA in physics from the University of Chicago in 1994, and Ph.D. in physics from Harvard in 2001. She was a research associate at Hewlett Packard Labs in Palo Alto from 2002 to 2004, before joining the University of California, Riverside in 2004 as an assistant professor. She was promoted to associate professor in 2009 and full professor in 2012. Starting January 2017 she moved to The Ohio State University. Her research focuses on electronic, thermal and mechanical properties of nanoscale systems, in particular, graphene and other two-dimensional systems.
January 30, 2018
Vladimir Skokov, Brookhaven National Lab
Quantum ChromoDynamics in extreme conditions
Abstract: In my talk, I discuss two landscapes of Quantum ChromoDynamics (QCD), the theory of strong interactions, in extreme conditions. I start with hot and dense QCD, which can be probed in the collisions of heavy-ions at high energy. The goal of the heavy-ion program is to map and study the QCD phase diagram and establish the existence of a conjectured critical point in QCD. The identification of this prominent landmark in the phase diagram is possible owing to its unique signature. I argue that recent experimental measurements agree with the theoretical expectations and, if confirmed, may lead to the discovery of a QCD critical point. I then turn to the landscape of cold QCD to be probed at a future Electron-Ion Collider. I discuss one of the exciting features which is the linear polarization of strong quasi-classical gluon fields in an unpolarized nucleus.
February 1, 2018
Li Yan, McGill University
A droplet of QGP in the little bang
Abstract: One of the fundamental questions in high energy nuclear physics is how to understand the dynamics of matter systems dominated by strong interactions. Especially, in systems where temperatures are comparable to the QCD energy scale (~1012 K), such as the universe in the first microseconds after Big Bang, or in high energy heavy-ion collisions carried out at RHIC and the LHC (the Little Bang), our interest is in a novel state of matter – quark-gluon plasma (QGP). One of the significant properties of QGP is its perfect fluidity. Actually, the value of shear viscosity over entropy density ratio of QGP has been found to be very close to a theoretical lower bound. The fluidity the QGP plays an essential role in the present studies of heavy-ion experiments. QGP evolution dominates the observed correlation behaviors of the produced particles in nucleus-nucleus collisions (large colliding systems), proton-lead and even proton-proton collisions (small colliding systems). In this talk, I will demonstrate how the idea of QGP fluidity emerges from the observed phenomena in experiments. I will also explain how a "standard model" of heavy-ion collisions based on relativistic hydrodynamics is challenged by the fluid behavior in recent small colliding systems.
February 8, 2018
Chun Shen, Brookhaven National Lab
Going with the flow — the nuclear phase diagram at the highest temperatures and densities
Abstract: The nuclear matter has a complex phase structure, with a deconfined Quark-Gluon Plasma (QGP) expected to be present under conditions of extreme pressure and temperature. The hot QGP filled the universe about few microseconds after the Big Bang. This hot nuclear matter can be generated in the laboratory via the collision of heavy atomic nuclei at high energy. I will review recent theoretical progress in studying the transport properties the QGP. Recently, the Relativistic Heavy-Ion Collider (RHIC) conducted the beam energy scan experiments. It offered a unique opportunity to study the nuclear phase diagram in a hot and baryon-rich environment. I will focus on the development of a comprehensive framework that is able to connect the fundamental theory of strong interactions with the RHIC experimental observables. This dynamical framework paves the way for quantitative characterization of the QGP and for locating the critical point in the nuclear phase diagram. These studies will advance our understanding of strongly interacting many-body systems and build interconnections with other areas of physics, including string theory, cosmology, and cold atomic gases.
February 15, 2018
Mauricio Guerrero, North Carolina State University
Out of equilibrium dynamics: Lessons from Nuclear Collisions
Abstract: The current knowledge of the universe is highly constrained without understanding the formation of baryonic matter widely observed today. It is then required to know the precise details of the transition where a highly dense plasma composed by quarks, antiquarks, and gluons combine to form hadrons. Ultrarelativistic heavy ion experiments can recreate non-equilibrium extreme conditions of the early universe by colliding heavy nuclei moving nearly at the speed of light. One of the major scientific discoveries of this century is the observation of a tiny, short-lived quark-gluon plasma (QGP). This extreme state of matter behaves like a liquid with a very small viscosity. Recently, we have learned that the perfect fluidity property, observed first in nucleus-nucleus collisions, also extends to proton-nucleus and proton-proton collisions. The "nearly perfect liquid" behavior of the QGP has opened up a new avenue for studying transport properties of strongly interacting systems. Nonetheless, these experimental findings challenge the theorists to develop better models which include the non-equilibrium evolution of the expanding nuclear matter created in those collisions. In this talk, I will review the 'standard model picture' of heavy ion collisions and present some recent theoretical studies which attempt to explain the unreasonable phenomenological success of fluid dynamical models in far from local equilibrium situations. I shall pose several unanswered questions about the QGP which emerge from these new theoretical developments and discuss how in the next few years, future experimental programs at large baryon densities and energy regimes will herald a new era of discovery and unraveling of the secrets of QGP.
February 22, 2018
Hong Guo, McGill University
Electronic States of the Moiré superlattice
Abstract: Two-dimensional (2D) van der Waals (vdW) heterostructures have attracted great attention in the past five years. By stacking different 2D materials to bond via the vdW force, these artificial heterostructures provide interesting and new material phase space for exploration. In this talk, I shall focus on one aspect of the 2D vdW materials: the Moiré pattern. In visual arts, Moiré pattern is an optical perception of a new pattern formed on top of two similar stacking patterns. In 2D vdW heterostructures, the Moiré pattern is a physical superlattice which brings about novel electronic properties. To theoretically predict the physical properties of the Moiré superlattice, systems containing more than ten thousand atoms often need to be analyzed by first principles. In this talk, I shall begin by briefly discussing how one may break the "size limit" so that very large first principles simulations within the density functional theory can be carried out. Afterward, I shall present and discuss some of the calculated novel properties of the Moiré superlattice: the emergence of a secondary Dirac cone, the suppression of the carrier mobility, and the formation of multiple helical valley currents, on various 2D vdW heterostructure materials. Some of these properties can well be the basis of potential applications.
About the speaker: Dr. Hong Guo obtained B.Sc. in Physics at the Sichuan Normal University in China and Ph.D. in theoretical condensed matter physics at the University of Pittsburgh. In 1989, he joined the faculty of the Physics Department, McGill University in Montreal Canada. He is currently a James McGill Professor of Physics. His research includes quantum transport theory, nanoelectronic device physics, nonequilibrium phenomena, materials physics, density functional theory, mathematical and computational physics. He was elected to Fellow of the American Physical Society in 2004, and Fellow of the Royal Society of Canada (Academy of Sciences) in 2007. He received the Killam Research Fellowship Award from the Canadian Council for the Arts in 2004; the Brockhouse Medal for Excellence in Experimental or Theoretical Condensed Matter Physics of the Canadian Association of Physicists in 2006; and the CAP-CRM Prize in Theoretical and Mathematical Physics from Canadian Association of Physicists in 2009.
March 2, 2018
David Ceperley, University of Illinois at Urbana-Champaign, and member of the National Academy of Sciences
How can we model the hydrogen inside Jupiter and Saturn?
Abstract: Jupiter, Saturn and a host of newly discovered exoplanets are thought to be composed largely of hydrogen and helium. To understand the planets, we need properties of hydrogen and helium under the extreme conditions of temperature and pressure inside those planets, conditions hardly accessible to laboratory measurements. I will describe how we use high-performance computers to calculate those properties and thus help understand some of the most important objects in the Universe.
About the speaker: Dr. David Ceperley is the Founder and Blue Waters Professor of Physics at the University of Illinois Urbana-Champaign, and a member of the National Academy of Sciences. He received his BS in physics from the University of Michigan in 1971 and his Ph.D. in physics from Cornell University in 1976. After one year at the University of Paris and a second postdoc at Rutgers University, he worked as a staff scientist at both Lawrence Berkeley and Lawrence Livermore National Laboratories. In 1987, he joined the Department of Physics at Illinois. He was a staff scientist at the National Center for Supercomputing Applications from 1987 until 2012. Professor Ceperley is a Fellow of the American Physical Society and a member of the American Academy of Arts and Sciences and was elected to the National Academy of Sciences in 2006. He has received many honors and awards.
March 6, 2018
LongGang Pang, Lawrence Berkeley National Laboratory
Exploring the quantum chromodynamics phase transition with deep learning
Abstract: The state-of-the-art pattern recognition method in machine learning (deep convolution neural network) has been used to classify two different phase transitions between normal nuclear matter and hot-dense quark-gluon plasma. Big amount of training data is prepared by simulating heavy ion collisions with the most efficient relativistic hydrodynamic program CLVisc. High-level correlations of particle spectra in transverse momentum and azimuthal angle learned by the neural network are quite robust in deciphering the transition type in the quantum chromodynamics phase diagram. Through this study, we demonstrated that there is a traceable encoder of the phase structure that survives the dynamical evolution and exists in the final snapshot of heavy ion collisions and one can exclusively and effectively decode this information from the highly complex output using machine learning.
March 8, 2018
Christoph Naumann, Indiana University - Purdue University Indianapolis
Probing membrane protein organization and dynamics in planar model membranes using single molecule-sensitive confocal detection techniques
Abstract: The organization and distribution of proteins in the plasma membrane is widely known to influence membrane protein functionality. However, it remains challenging to decipher the underlying mechanisms that regulate membrane protein properties in the complex environment of cellular membranes. To overcome these challenges, an experimental strategy is discussed, in which the distribution, oligomerization state, and mobility of membrane proteins can be explored in a planar polymer-tethered lipid bilayer of well-defined lipid compositions using single molecule-sensitive confocal detection strategies. Results from such model membrane experiments are presented, which explore the influence of native ligands, bilayer asymmetry, and cholesterol content on the sequestration/oligomerization of urokinase plasminogen activator receptors (uPAR) and integrins [1-4]. Moreover, dual-color confocal experiments are described, which provide information about the formation and composition of uPAR-integrin complexes and the role of membrane cholesterol therein. Polymer-tethered lipid bilayer systems, comprised of phospholipids and lipopolymers, are also characterized by remarkable materials properties, which make them suitable as cell surface-mimicking substrates for the analysis of adhesion and spreading of plated cells. To illustrate the feasibility of such an application, we discuss the assembly of cadherin chimera into clusters on the surface of a polymer-tethered lipid bilayer substrate to form stable cell-substrate cadherin linkages underneath migrating C2C12 myoblasts . Cluster tracking experiments reveal the cytoskeleton-regulated long-range mobility of cell-substrate linkages, thereby displaying remarkable parallels to the dynamics of cadherin-based cell-cell junctions.
 A. P. Siegel et al. (2011) Biophys. J. 101, 1642.
 N. F. Hussain et al. (2013) Biophys. J. 104, 2212.
 Y. Ge et al. (2014) Biophys. J. 107, 2101.
 Y. Ge et al. (2018) Biophys. J. 114, 158.
 Y. Ge et al. (2016) Soft Matter 12, 8259.
March 22, 2018
W.J. Llope, Wayne State University
How to give a great talk – tips for success and traps to avoid
Abstract: Whether your future lies in academia or industry, you will have opportunities to present your recent work while standing in front of an audience of interested people. Giving a good talk is a very powerful advertisement of you and your efforts, and, if you can properly enthuse the audience during your talk, the subsequent discussions can be very helpful for your future work. Great talks come in many forms, but all share a few key positive aspects and all avoid some (unfortunately rather common) pitfalls. This colloquium will share some tips from what I've learned over the years while watching thousands of talks, and giving a few myself. This presentation is specifically aimed at our students and postdocs, although the faculty are of course welcome to share their insights as well. The atmosphere will be informal and an open discussion, especially with our younger colleagues, will be encouraged.
March 29, 2018
Vladimir Chernyak, Department of Chemistry, Wayne State University
Integrability in Non-Equilibrium Quantum Dynamics
Abstract: Nonequilibrium quantum dynamics, i.e., quantum evolution with time-dependent Hamiltonians, iħ ∂Ψ(t)/∂t = Ĥ(t)Ψ(t), as of today draws considerable attention, both in experimental and theoretical research. The simplest model with Ĥ(t)=A+Bt, and A and B being 2×2 real hermitian matrices, known as the Landau-Zener (LZ) problem has an exact solution in special functions, with the scattering matrix S, expressed in term of Gaussians and Euler Gamma-function. In the general N-dimensional case, known as Multilevel LZ (MLZ) problem, exact solutions are not available. However, for a certain class of MLZ problems that satisfy certain phenomenologically determined "integrability" conditions, including, but not limited to Demkov-Osherov (DO), Generalized Bow-Tie (GBT), and Tavis-Cummings (TC), the scattering matrix can be represented in a factorized form, with the elementary scattering events being represented in terms of the standard LZ matrix, which is the first aspect of integrability; in other words the semiclassical expressions become exact.
In this talk we reveal the reason that stands behind the aforementioned factorization: Each integrable MLZ problem can be embedded into a system of M linear first-order differential equations with respect to M-dimensional time, that satisfy the consistency conditions, the latter having a form of the zero-curvature conditions, which is the second and dynamical aspect of integrability.
We further apply our approach to obtain an exact solution for the BCS model that does not belong to the MLZ class, but rather describes decay of the initial strongly correlated state of Ns quantum spins (in the thermodynamic Ns→∞ limit the model becomes a non-trivial field theory) into an uncorrelated counterpart when the spin-spin interaction is switched off, and generally in a non-adiabatic fashion. The obtained exact solution shows an amazing property: The ground state in the strongly correlated phase "dissociates" into a Gibbs distribution in a completely integrable way, without any chaos or bath involved. We demonstrate that the aforementioned result is a third aspect of integrability, which is the natural appearance of the structures, usually associated with quantum integrability in a form of algebraic Bethe Ansatz, including the Yang-Baxter-Zamolodchikov (YBZ) equation, Artin's Braid Group, and quantum group SUq(2).
The presentation will be given in an intuitive fashion, all mathematical structures involved will be explained using the terms, which are common for a broad physics audience.
April 5, 2018
Prof. Aaron Pierce, Director of the Leinweber Center for Theoretical Physics at the University of Michigan
Dark Matter: WIMPS and Beyond
Abstract: The identity of the Dark Matter that dominates the matter density of the universe remains a mystery. Increasingly sophisticated experiments have begun to probe some of the best-motivated models of dark matter. I will review the theoretical status of one such paradigm, so-called Weakly Interacting Massive Particle (WIMP) Dark Matter, with particular emphasis on the implications of direct detection experiments. We will see that while the paradigm is alive and well, it is under non-trivial pressure, particularly in specific implementations, such as supersymmetry. This warrants searches for other types of Dark Matter. I will very briefly discuss a few such searches, including a new possibility of using the LIGO gravitational wave detector as a dark matter detector.
April 19, 2018: Vaden Miles Lecture
J. Michael Kosterlitz, Brown University (2016 Nobel Prize in Physics)
Topological Defects and Phase Transitions - A Random Walk to the Nobel Prize
Abstract: This talk is about my path to the Nobel Prize and reviews some of the applications of topology and topological defects in phase transitions in two-dimensional systems for which Kosterlitz and Thouless split half the 2016 Physics Nobel Prize. The theoretical predictions and experimental verification in two-dimensional superfluids, superconductors and crystals will be reviewed because they provide very convincing quantitative agreement with topological defect theories.
About the speaker: Dr. J. Michael Kosterlitz is a theoretical physicist recognized for his work with David J. Thouless on the application of topological ideas to the theory of phase transitions in two-dimensional systems with a continuous symmetry. The theory has been applied to thin films of superfluid 4He, superconductors and to melting of two-dimensional solids. This work was recognized by the Lars Onsager prize in 2000, membership in the AAAS 2007, and by the 2016 Nobel Prize in Physics. Dr. Kosterlitz graduated from Cambridge University earning a BSc in physics in 1965, an MA in 1966, and received a D. Phil. from Oxford in 1969. He was a postdoctoral fellow at Torino University, Italy, in 1970 and at Birmingham University, U.K., from 1970-73. There he met David Thouless and together they did their groundbreaking work on phase transitions mediated by topological defects in two dimensions. He was a postdoctoral fellow at Cornell in 1974, on the faculty at Birmingham 1974-81, Professor of Physics at Brown University 1982 – present, and elected to the National Academy of Sciences in 2017.
September 13, 2018
Lu Li, University of Michigan
Quantum Oscillations of Electrical Resistivity in an Insulator
Abstract: In metals, orbital motions of conduction electrons are quantized in magnetic fields, which is manifested by quantum oscillations in electrical resistivity. This Landau quantization is generally absent in insulators, in which all the electrons are localized. Here we report a notable exception in an insulator — ytterbium dodecaboride (YbB12). The resistivity of YbB12, despite much larger than that of usual metals, exhibits profound quantum oscillations under intense magnetic fields.
This unconventional oscillation is shown to arise from the insulating bulk, instead of conducting surface states. The large effective masses indicate strong correlation effects between electrons. Our result is the first discovery of quantum oscillations in the electrical resistivity of a strongly correlated insulator and will bring crucial insight to the understanding of the ground state in gapped Kondo systems.
September 20, 2018
Prashant Padmanabhan, Los Alamos National Lab
From plasmons to skyrmions: The ultrafast dynamics and control of novel excitations and materials
Abstract: Over the last several decades, advances in ultrafast spectroscopy have enabled us to investigate fundamental phenomena associated with the electronic, lattice, and spin degrees of freedom in condensed matter systems at their intrinsic time-scales. Moreover, intense femtosecond pulses allow us to drive materials far from equilibrium. This provides us with access to states that are often difficult, or even impossible, to probe under normal thermodynamic conditions.
As such, we can now examine the subtleties of exotic excitations, elucidate the microscopic coupling mechanisms between competing subsystems, and transiently manipulate the fundamental properties of novel materials. This talk will focus on such efforts in systems spanning prototypical semiconductors to quantum materials. Specific topics include massless collective excitations of multi-component plasmas, hole scattering dynamics in incipient ferroelectrics, and the optical control of low energy excitations in topologically protected spin textures.
September 27, 2018
Nagesh Kulkarni, Quarkonics Inc.
From Ph.D. to CEO: An Entrepreneurial Journey
Abstract: A Physics education provides general skills in problem-solving, teamwork, and knowledge in cutting--edge technologies. Add some business skills and a physicist can become an entrepreneur. I will talk about my entrepreneurial journey and share my experiences and thoughts on how scientists can play a significant role in shaping the global economy, change the game, and create tremendous value for society by leveraging their unique analytical thinking and problem-solving skills, networking, and specialized knowledge.
October 4, 2018
Yaqiong Xu, Vanderbilt University
Carbon-Based Nanomaterials for Biosensing
Abstract: Carbon-based nanomaterials, such as carbon nanotubes (CNTs) and graphene, have gained significant interest as one of the most promising materials in biological applications due to their unique physical and chemical properties. Recently we have developed an optoelectronic probing system, combining CNT/graphene transistors with scanning photocurrent measurements, fluorescence microscopy, and optical trapping techniques to investigate the molecular interface between CNTs/graphene and biological systems. We have directly measured the binding force between a single DNA molecule and a CNT in the near-equilibrium regime, where two aromatic rings spontaneously attract to each other due to the noncovalent forces between them. We have also integrated graphene-based scanning photocurrent microscopy with microfluidic platforms to investigate the electrical activities of individual synapses of primary hippocampal neurons. I will conclude by summarizing the remaining research challenges that must be surmounted in order to bring carbon-based nanomaterials into future biological applications.
October 11, 2018
Sergio E. Ulloa, Ohio University
Putting Things on Top of Other Things (Proximity effects in 2D materials)
Abstract: Proximity effects such as those produced when depositing graphene on a transition metal dichalcogenide substrate are expected to change the dynamics of the electronic states in graphene, inducing spin-orbit coupling and staggered potential effects. Putting things on top of other things is promising and going strong in 2D materials!
In this talk, I will describe some of the expectations of combining different layered materials. In particular, I will show how an effective Hamiltonian that describes different symmetry breaking terms in graphene, while preserving time reversal invariance, shows that a new topological insulator may be created by stacking "trivial" materials and applying strong electric fields. These new systems may exhibit quantum spin Hall and valley Hall effects in different conditions .
 A.M. Alsharari, M.M. Asmar and S.E. Ulloa, Phys. Rev. B 94, 241106(R) (2016); and Phys. Rev. B 97, 241104(R) (2018).
October 18, 2018
Igor Žutić, University at Buffalo
Proximity Effects in van der Waals Materials
Abstract: Advances in heterostructures and atomically thin van der Waals materials, such as graphene, suggest a novel approach to systematically design materials. A given material can be transformed through proximity effects whereby it acquires properties of its neighbors, for example, becoming superconducting, magnetic, topologically nontrivial, or with an enhanced spin-orbit coupling . Such proximity effects not only complement the conventional methods of designing materials by doping or functionalization but also can overcome their various limitations. In proximitized materials, it is possible to realize properties that are not present in any constituent region of the considered heterostructure. While the focus is on magnetic proximity effects with their applications in spintronics [2-4], the outlined principles also provide a broader framework for employing other proximity effects to tailor materials and realize unexplored phenomena.
- I. Žutić et al., Mater. Today, (2018), arxiv:1805.07942, https://doi.org/10.1016/j.mattod.2018.05.003
- P. Lazić et al., Phys. Rev. B 93, 241401(R) (2016)
- B. Scharf et al., Phys. Rev. Lett. 119, 127403 (2017)
- J. Xu et al., Nat. Commun. 9, 2869 (2018)
November 1, 2018
Weihong Qiu, Oregon State University
Kinesin-14s: Moving into a New Paradigm
Abstract: Kinesin-14s are microtubule-based motor proteins that play important roles in cell division. They were originally thought to be minus-end-directed nonprocessive motors that exhibit directional preference toward the microtubule minus ends in multi-motor ensembles but are unable to generate processive (continuous) motility on single microtubules as individual motors. During the past five years, we and others have discovered several "unconventional" kinesin-14 motors that all contain the ability to generate processive motility as individual motors on single microtubules. In this talk, I will present a series of unexpected yet exciting findings from my lab that have markedly expanded current view of the design and operation principles of kinesin-14 motors.
November 27, 2018
Ron Soltz, Lawrence Livermore National Lab
Evaluating the Iran Nuclear Deal
Abstract: The Iran Nuclear Deal evokes strong reactions. It has been called "The Worst Deal Ever" as well as "The best option for preventing Iran from obtaining a nuclear weapon. Otherwise known as the "Joint Comprehensive Plan of Action", the JCPOA has led to much debate, even if little of it has been substantive. Put into effect in 2015, the JCPOA continues to influence the behavior of six of the seven signatories, the seventh having formally withdrawn 2018, reimposing sanctions in May and November. In the history of international agreements, it is truly unique, but as it stands at the intersection of science and policy, it is also a valuable teaching tool for the role that science can play in formulating good policy, while also providing an opportunity to review a few basic concepts in nuclear physics.
November 29, 2018
Gerald Gabrielese, Northwestern University
Stringent Tabletop Tests of the Standard Model: A Tale of the Electron's Electric and Magnetic Dipole Moments
Abstract: The standard model's most precise prediction -- of the size of the electron magnetic moment -- is tested using a single electron suspended by itself for months at a time in a tabletop-sized measurement. Also, a new measurement of the electrons' other moment -- its electric dipole moment - was just completed in a very different tabletop measurement. The standard model and proposed alternatives/additions differ sharply in their predictions of the size of this moment.
December 6, 2018
Francis Halzen, Wisconsin IceCube Particle Astrophysics Center and Department of Physics, University of Wisconsin–Madison
IceCube: Opening a New Window on the Universe from the South Pole.
Abstract: The IceCube project has transformed a cubic kilometer of natural Antarctic ice into a neutrino detector. The instrument detects more than 100,000 neutrinos per year in the GeV to PeV energy range. Among those, we have isolated a flux of high-energy neutrinos of cosmic origin. We will explore the IceCube telescope and the significance of the discovery of cosmic neutrinos. We recently identified their first source: alerted by IceCube on September 22, 2017, several astronomical telescopes pinpointed a flaring galaxy powered by an active supermassive black hole, as the source of a cosmic neutrino with an energy of 290 TeV. Most importantly, the large cosmic neutrino flux observed implies that the Universe's energy density in high-energy neutrinos is close to that in gamma rays, suggesting that the sources are connected and that a multitude of astronomical objects await discovery.
Fall 2017 and prior
View archive of fall 2017 colloquium and prior.