Colloquia / Seminars

Spring 2023

M, Jan 9

Measurements of Fission Product Yields Using Monoenergetic Neutron and Gamma-ray Beams

Jack Silano, LLNL

Fission is the most complex and violent process in nuclear physics, splitting an atomic nucleus into two or more fragments and releasing enormous amounts of energy. Fission product yields (FPYs), the nuclei produced by the division of a fissioning nucleus, contain a wealth of information about the nuclear fission process. FPY distributions are sensitive to the species of nucleus undergoing fission, its excitation energy, spin, and the underlying microscopic physics that drive fission. In addition, FPYs are relevant to fundamental and applied science such as heavy element nucleosynthesis, the reactor antineutrino anomaly, nuclear forensics, reactor decay heat calculations, and stockpile stewardship. In this colloquium, I will give an overview of a joint LLNL-LANL-TUNL research program aimed at performing high-precision FPY measurements using quasimonoenergetic neutron and gamma-ray beams at Triangle Universities Nuclear Laboratory. Over the past decade, our efforts to modernize our experimental process and analysis – as well as the development of a state-of-the-art rapid target transfer system (RABITTS) – have enabled us to measure FPYs with half lives ranging from several months to less than one second. I will present a selection of our FPY data and demonstrate how they can be used to study fundamental science in the fission mechanism. Finally, I will discuss ongoing work to prepare for FPY measurements using radioactive isotope beams at facilities such as the Facility for Rare Isotope Beams (FRIB).

(nuclear physics; dedicated to Prof. Karwowski’s retirement)

M, Jan 16th MLK day no class

M, Jan 23

The extremes of galaxy and black hole formation

Ryan Hickox, Dartmouth College
We have recently seen a remarkable convergence in our overall understanding of how galaxies in the Universe form, through the collapse of gas in dark matter halos, and growth through star formation and merging over cosmic time. However, many of the key physical processes, such as gas dynamics, feedback, and the growth and impact of supermassive black holes, are complex and challenging to understand in detail. One observational technique is to study the extremes of galaxy and black hole formation where these effects are most pronounced. I will present observational studies of a few of these extreme cases, including compact starbursts driving powerful winds, rapidly growing supermassive black holes that are heavily obscured by gas and dust, and supermassive black holes in low-mass dwarf galaxies, Finally, l will point toward some exciting possibilities with new observing facilities. This work has been generously supported by NASA and the National Science Foundation.

M, Jan 30

Ab Initio Simulation of Thouless Pumping of Electrons in Floquet Topological Phase

Yosuke Kanai, Department of Chemistry, UNC-CH
I will discuss nonadiabatic Thouless pumping of electrons in trans-polyacetylene and related systems in the framework of topological Floquet engineering using first-principles theory. By employing maximally localized Wannier function gauge in real-time time-dependent density functional theory, we connect the winding number, a topological invariant, to a molecular-level understanding of the quantized pumping via valence bond model. Exploiting the gauge-invariant nature of the quantum dynamics with respect to single-particle orbitals, we show that so-called dynamical transition orbitals give a minimal description of the dynamics in terms of the particle-hole excitation. The quantum dynamics responsible for the Thouless pumping is largely characterized by the dynamics of a single orbital as it undergoes changes from its π bonding orbital character at equilibrium to acquiring resonance and antibonding characters in the driving cycle. This allows us to explain the presence/absence of the Floquet topological phase in the polymer systems with seemingly minor chemical changes on the basis of the valence bond model. I will conclude by discussing existing challenges in first-principles theory for modeling electron dynamics in increasingly complex systems and phenomena.

M, Feb 6

Using Science for Good

James H. Dickerson, Chief Scientific Officer, Consumer Reports

We expect that the products we use every day will be safe, reliable, and effective.  However, that does not always occur. A computer battery can unexpectedly catch fire, bedroom furniture can be unstable and topple, and food can be contaminated.  Consumer Reports (CR) is a non-profit organization that is committed to revealing the truth and raising the bar for safety and fairness, and empowering consumers with trusted information. Learn how CR uses science for good, applying its scientific findings for diverse audiences—from consumers to rule makers, industry to government, all with the goal of driving marketplace change that benefits everyone.
This presentation describes the underpinning endeavors of my organization, Consumer Reports.  Thus, the topic will attempt to address the skepticism that many people have about “ratings” or “reviews” of consumer products.  Part of that skepticism is that Consumer Reports provides perspectives that are no different from those often seen in online user reviews, “comparative testing” sites, and internet consumer forums.  My intent is to demonstrate that the robust scientific investigations that we undertake at CR are, indeed, robust, data-driven, and superior to those offered by others.

M, Feb 13 well-being day, no class

M, Feb 20

Classical wave-particle duality

Pedro J. Sáenz, Applied Mathematics, UNC-Chapel Hill

A millimetric liquid droplet may walk across the surface of a vibrating fluid bath, self-propelled through a resonant interaction with its own guiding wave field. By virtue of the coupling with their wave fields, these walking droplets, or ‘walkers’, extend the range of classical mechanics to include certain features previously thought to be exclusive to the quantum realm. In this talk, we will combine experiments, simulations and theory to discuss a number of hydrodynamic quantum analogs, including orbital quantization in a rotating frame, hydrodynamic spin lattices, and the absence of diffusion over random topographies.

M, Feb 27

Dark Matter in the Universe

Katherine Freese, UT Austin
Abstract: The nature of the dark matter in the Universe is among the longest and most important outstanding problems in all of modern physics. The ordinary atoms that make up the known universe, from our bodies and the air we breathe to the planets and stars, constitute only 5% of all matter and energy in the cosmos. The remaining 95% is made up of a recipe of 25% dark matter and 70% dark energy, both nonluminous components whose nature remains a mystery. I’ll begin by discussing the evidence that dark matter is the bulk of the mass in the Universe, and then turn to the hunt to understand its nature. Leading candidates are fundamental particles including Weakly Interacting Massive Particles (WIMPs), axions, sterile neutrinos, as well as primordial black holes. I will discuss multiple experimental searches: at CERN in Geneva; in underground laboratories; with space telescopes; with gravitational wave detectors; and even with DNA. I’ll tell you about our novel idea of Dark Stars, early stars powered by dark matter heating, and the possibility that the James Webb Space Telescope could find them. At the end of the talk, I’ll turn to dark energy and its effect on the future of the Universe.

M, March 6

Igor Andreoni, University of Maryland / NASA Goddard Space Flight Center
Title: Seize the night in the era of wide-field optical surveys

We are living in a golden era for optical time-domain astronomy. Wide-field surveys such as the Zwicky Transient Facility (ZTF) image most of the observable sky every night, opening a discovery space historically difficult to explore in the optical. The ability to crunch big data efficiently has become key to discovery. I will present results for two science cases in particular that we have directly addressed with ZTF real-time searches. These include binary neutron star mergers, which are great multi-messenger sources, and a rare class of tidal disruption events. Finally, I will present prospects for the upcoming LSST at Vera C. Rubin Observatory, which will generate millions of transient alerts every night and give us an unprecedented view of the dynamic sky.

 

M, March 13 Spring Break

M, March 20

Jorge Piekarewicz, Florida State University

Title: Heaven and Earth: Nuclear astrophysics in the multimessenger era

The historic detection of gravitational waves from the binary neutron star merger GW170817 by the LIGO-Virgo collaboration is providing fundamental new insights into the astrophysical site for the creation of the heaviest elements in the cosmos and the equation of state on neutron-rich matter. Since then, electromagnetic observations of neutron stars together with measurements of the properties of neutron-rich nuclei at terrestrial facilities are placing stringent constraints on the nature of neutron-rich matter. It is this unique synergy between heaven and earth that will be the focus of this presentation.

M, March 27

Rocky Kolb, University of Chicago

“Schrödinger’s Alarming Phenomenon”

The big bang is a laboratory to explore the properties of particles that cannot be created in terrestrial laboratories. In addition to thermal processes, there is another source of cosmological particle production. In 1939 Edwin Schrödinger pointed out that particle-antiparticle pairs could be created merely by the violent expansion of space. The spontaneous appearance of particles from the vacuum so disturbed Schrödinger that he referred to it as an “alarming” phenomenon.  The phenomenon is now thought to be the origin of density fluctuations produced in inflation as well as a background of gravitational waves. Gravitational particle production is a rich phenomenon, which continues to be explored.

M, April 3

Yong P. Chen, Purdue University

Stories of Magnetism: Some New Twists

Magnetism, important in many everyday technologies, is a well-studied quantum phenomenon (in fact it is one of the best examples of many-body and correlated quantum phenomenon, which can exist even above room temperatures). The past few years has witnessed the rise of a variety of van der Waals (vdW) layered magnetic “2D materials”.  These materials, including ferromagnets, antiferromagnets, and even candidate spin liquids, have challenged our fundamental understanding of magnetism, where the very existence of 2D magnetism has been somewhat surprising. Meanwhile, these magnetic 2D materials, which can be made down to atomic thickness, are relatively easy to transfer, deform and tune their properties, have presented exciting new opportunities for realizing novel physics and functionalities, and potential applications in spintronics and quantum technologies.

In this talk, I will discuss our recent experimental work on such magnetic 2D materials and related heterostructures.  I will highlight some intriguing findings such as: how stretching a 2D magnet can enhance its magnetism; and how stacking two antiferromagnets (each with zero measured magnetization) can produce an exotic ferromagnet whose magnetization can increase when heated — revealing a new type of magnetism known as “Moire magnetism” with rich phases and behaviors tunable by a voltage and by the relative “twisting” angle when the two magnets are stacked together. Finally, I will explore the interplay and interface between magnetism and topology in these materials and heterostructures, which may pave the way for probing exotic quasiparticles, such as Majorana fermions or even non-Abelian anyons still awaiting to be clearly demonstrated experimentally.

M, April 10

TBA

Chiara Mingarelli, University of Connecticut
Abstract: TBA

M, April 17

TBA

Maiken H. Mikkelsen, Duke University
Abstract: TBA

M, April 24

TBA

Julian Munoz, Harvard University
Abstract: TBA

Fall 2022

M, Sep 12

It’s about time: manipulating energy and information flows far away from thermal equilibrium.

Zhiyue Lu, UNC Chemistry
Thermodynamics provides a successful theoretical framework to describe the equilibrium properties of substances and near-equilibrium processes within the linear response regime. However, our daily-life experiences, industrial processes, and almost all aspects of biology are ubiquitously far away from thermal equilibrium. First, we will briefly review the modern theory of non-equilibrium statistical mechanics and stochastic thermodynamics. Then, this framework is applied to resolve a 2000-year-old myth named the Mpemba effect. This counter-intuitive effect claimed that hot water can freeze faster than cold water. Finally, inspired by the Mpemba effect, we will briefly demonstrate the non-equilibrium design principle behind life-like intelligent materials with surprising information and energy controllability.

M, Sep 19

Eleonora Di Valentino,
The University of Sheffield
(via zoom only)

The scenario that has been selected as the standard cosmological model
is the Lambda Cold Dark Matter (ΛCDM), which provides a remarkable fit
to the bulk of available cosmological data. However, discrepancies among
key cosmological parameters of the model have emerged with different
statistical significance. While some portion of these discrepancies may
be due to systematic errors, their persistence across probes can
indicate a failure of the canonical ΛCDM model. I will review these
tensions, showing some interesting extended cosmological scenarios that
can alleviate them.

Join Zoom Meeting

https://unc.zoom.us/j/96050545723?pwd=RnRSaW96OTgzMkUvWk5ubUNxZHQwQT09

Meeting ID: 960 5054 5723

Passcode: 017017

M, Oct 3

Time Domain and High Frequency Dynamic Nuclear Polarization

Robert Griffin, MIT
Dynamic nuclear polarization (DNP) has become an invaluable tool to enhance sensitivity of magic angle spinning (MAS) NMR, enabling the study of biomolecules and materials which are otherwise intractable. In this presentation we explore some new aspects of time domain DNP experiments and their applications. One of the main thrusts of DNP was to provide increased sensitivity for MAS spectroscopy of
membrane and amyloid protein experiments. A problem frequently encountered in these experiments is the broadened resonances that occur at low temperatures when motion is quenched. In some cases it is clear that the proteins are homogeneously broadened, and therefore that higher Zeeman fields and faster spinning is required to recall the resolution. We show this is the case for MAS DNP spectra of Ab1-42 amyloid fibrils where the resolution at 100 K is identical to that at room temperature. Furthermore, we compare the sensitivity of DNP and 1H detected experiments and find that DNP, even with a modest ℇ=22, is ~x6.5 times more sensitive. We have also investigated the frequency swept-integrated solid effect (FS-ISE) and two recently discovered variants – the stretched solid effect (SSE) and the adiabatic solid effect (ASE). We find that the latter two experiments can give up to a factor of ~2 larger enhancement than the FS-ISE. The SSE and ASE experiments should function well at high fields. Finally, we discuss two new instrumental advances. First, a frequency swept microwave source that permits facile investigation of field profiles. It circumvents the need for a B0 sweep coil and the compromise of field homogeneity and loss of helium associated with such studies. This instrumentation has permitted us to elucidate the polarization transfer mechanism of the Overhauser effect, and also revealed interesting additional couplings (ripples) in field profiles of cross effect polarizing agents. Second, to improve the spinning frequency in DNP experiments, we have developed MAS rotors laser machined from single crystal diamonds. Diamond rotors should permit higher spinning frequencies, improved microwave penetration, and sample cooling.

M, Oct 10

Stellar Rotation in Young Clusters using K2 and TESS

Luisa Rebull, Caltech/IPAC

K2 provided a phenomenal opportunity to study properties of stars in clusters, particularly young low-mass stars, far beyond the expectations of the original Kepler mission. The high-precision photometry provided by K2 allows us to probe stellar variability to lower masses and lower amplitudes than has ever been done before. Younger stars are generally more rapidly rotating and have larger star spots than older stars of similar masses, so spots rotating into and out of view reveal the (surface) rotation rate of these stars. K2 has monitored stars from several clusters, most notably Rho Oph (~1 Myr), Taurus (~5 Myr), USco (~20 Myr), the Pleiades (~125 Myr), and Praesepe (~700 Myr). The light curves have yielded thousands of rotation rates, and revealed far greater diversity in light curves than was anticipated. Now that we have TESS data as well, we can add stars from many more clusters, including the Upper Centaurus-Lupus (UCL) and Lower Centaurus-Crux (LCC) young moving groups (~15 Myr). In this talk, I will review my K2 results including new TESS results from UCL/LCC.

M, Oct 17

TBA

TBA
Abstract TBA

M, Oct 24

James Vesenka, University of New England

Via Zoom only

Time: Oct 24, 2022 03:30 PM Eastern Time (US and Canada)

https://unc.zoom.us/j/96359807048?pwd=TzZYT0JscVFISDEzTHJ1UUZKNlRFQT09

Meeting ID: 963 5980 7048

Passcode: 596830

The conversion of a traditional lecture and lab physics curriculum at a small liberal arts college into a dynamic, studio-based physics program anchored by modeling physics instruction is described.  Modeling instruction is a student-centered, hands-on, guided-inquiry approach to construct science understanding from the ground up, based on simple models that are at the heart of mechanics, electricity, fluids and waves.  The instruction has evolved to embrace introductory physics for the life sciences (IPLS) models that better prepare the student population for instruction in other life sciences courses.  Modeling physics instruction helps students build essential critical thinking and communication skills that extend beyond the reach of the course.  Examples using the iOLab as a tool to explore energy and momentum conservation from the perspective of modeling instruction will be presented.

 

M, Oct 31

Christian Iliadis (in honor of Art Champagne’s retirement)

This special colloquium will celebrate the distinguished research career of our friend and colleague, Art Champagne. Most of you remember Art as our former department chair and TUNL director, and as a distinguished teacher. Outside the nuclear physics group, few of you will have an impression of the world-class quality of his achievements in nuclear astrophysics research. I will discuss Art’s years at Yale, Princeton, and UNC. My story will start with primitive meteorites, and continue with galactic radioactivity, stellar evolution, and cosmology. You are strongly encouraged to attend the colloquium to celebrate an exceptional research career.

330pm Phillips 265

M, Nov 7

Nuclei from Scratch: Ab Initio Emergence of Rotation and Dynamical Symmetry

Mark Caprio, University of Notre Dame.
A fundamental goal in nuclear theory is to obtain an ab initio ( “from the beginning”) description of the nucleus, that is, directly from the forces between the nucleons within the nucleus and from the many-body Schrödinger equation. Despite formidable computational challenges, such an approach is now feasible, at least for the lightest nuclei. Beyond simply providing quantitative predictions, a successful ab initio description can also provide qualitative insight into the physical nature of the phenomena arising in these nuclei. Perhaps most intriguing is the question of how collective correlations, which involve cooperative motion of all the nucleons, arise out of the complex interactions of these nucleons. In this colloquium, we will focus on collective deformation and rotation in light nuclei (in particular, the beryllium isotopes), and on how ab initio calculations confirm the importance of dynamical symmetries in understanding the nature of the many-body correlations underlying these phenomena.

M, Nov 14

New Opportunities for Dark Matter Searches in Cosmology

Kimberly Boddy, UT Austin.

Understanding the fundamental nature of dark matter is one of the major challenges facing the physics community today. There are dedicated experimental efforts to search for dark matter interactions with the Standard Model of particle physics, but no concrete evidence of such interactions has been observed. In this talk, I will demonstrate how cosmological and astrophysical observations offer exciting, new possibilities for understanding dark matter beyond its gravitational impact. I will describe the effects of dark matter scattering and annihilation processes in the early Universe and show how observational data constrain broad classes of dark matter models.

M, Nov 21

Tom Kephart, Vanderbilt University (in honor of Jack Ng’s retirement)

“Jack Ng and a Journey to Holographic Foam Cosmology”

We celebrate Kenan Professor Yee Jack Ng, who has had a very influential and successful research career as a theoretical physicist. His works span particle physics from fundamental theory to phenomenology, gravity from space time foam to black holes to cosmology and more. There is far too much to cover in a single lecture, but we can give a nice representative example. We will follow one extended thread in the tapestry of Jack’s research that takes us from space-time foam to Holographic Foam Cosmology.