Colloquia / Seminars

Fall 2023

M, August 28th

From bioinspired structure formation to particle-based metamaterials

Andreas Fery Institut für Polymerforschung Dresden and Technical University Dresden, Germany

Metallic nanoparticles offer a number of interesting optical and electronic effects. A prominent example is localized surface plasmon resonance (LSPR), which is due to resonance excitations of the free electron cloud vibrations of the particles by light. Due to LSPR, plasmonic nanoparticles provide excellent opportunities for controlling electromagnetic near-fields at optical frequencies, which has led to a wide range of applications in various fields such as surface-enhanced spectroscopy, light harvesting, or photonics.

While much of the research has been devoted to understanding nanoparticle synthesis and tailoring their LSPR at the single-particle level [1-3], the ordering of particles at different length scales opens another powerful route to optical and electronic functionality due to novel collective plasmonic excitations arising from plasmonic coupling effects.

We focus on achieving such ordered particle arrays through assembly approaches. Colloidal self-assembly can indeed achieve well-defined colloidal clusters [4] and surface arrays [5] where coupling effects can be controlled. In particular, large-scale assemblies are possible in combination with biomimetic surface patterning. We discuss the underlying physicochemical principles of the assembly process and the resulting plasmonic coupling effects [6,7]. Finally, we present perspectives on how this assembly principle can be applied to metasurfaces with high field enhancement and/or ultrahigh circular dichroism

 

M, Sept 4th no class

M, Sept 11

Probing the Early Universe with Dark Matter Annihilation

Adrienne Erickcek, University of North Carolina
Observations of the oldest light in the Universe and other astronomical measurements indicate that only 5% of the current energy content of the Universe is stored in elements found on the periodic table. The other 95% is composed of dark matter and dark energy: dark matter is responsible for the growth of galaxies, while dark energy shoves galaxies apart at an accelerating rate. I will summarize the observational evidence for dark matter and dark energy, including how we infer that all galaxies are surrounded by vast halos of dark matter. These halos are thought to have formed through the mergers of smaller clumps of dark matter. As remnants of the earliest stages of structure formation, the smallest dark matter halos provide a unique probe of the expansion history prior to Big Bang nucleosynthesis. I will discuss how the evolution of the early Universe can enhance the microhalo population, thereby boosting the dark matter annihilation rate if dark matter is generated by pair production. The amplitude of this boost is highly sensitive to the size of the smallest halos, which provides an additional window into the dynamics and particle content of the early Universe. It is therefore possible to use astronomical observations to learn about the origins of dark matter and the evolution of the Universe during its first second.

M, Sept 18th

Ab Defects in Perovskites- a Key Differentiator from other Semiconductors

Jinsong Huang Department of Applied Physical Science, UNC-CH
Electronic defects within the band gap of semiconductor materials play critical roles in determining the efficiency and stability of their photovoltaic devices. Eliminating deleterious defects in semiconductors or passivating them during the fabrication process of solar cells has become one of the most fundamental tasks for the solar cell community. This scenario is also prevailing in the metal halide perovskite solar cell community which has witnessed a rapid increase of the power conversion efficiency of perovskite solar cells from 3.8% to over 26% with overwhelming reported progress on defect passivation strategies which also enhance the stability of perovskite solar cells. Any further improvement of the efficiency or stability of perovskite solar cells toward their Shockley-Queisser limitations have to rely on deeper understandings on the nature of defects in perovskite to squeeze out all non-radiative charge recombination paths by eliminating or passivating them.
Here I will present several studies of defects in perovskites, including both deep ones and shallow ones. I will show how we find out the density of defects, distribution, chemical nature, etc, of deep trapping defects. I will also report our recent discovery of unique properties of shallow defects in perovskites which enable perovskites to be much more defect tolerant.

M, Sept 25 Well Being Day- no class

M, Oct 2

Hybrid Perovskite Spintronics

Dali Sun, NCSU
Researchers have shown that hybrid organic-inorganic perovskites (or organometal trihalide perovskites) are not only aimed to be used in solar cell applications but also pursue a vast variety of fundamental research directions. One of the growing topics is the understanding of photo-physics and spin-related properties in hybrid perovskites since they play a major role in the processes of carrier photogeneration and carrier transport, which are the corner stone’s of photovoltaic applications, as well as in other optoelectronic applications. In this talk, we will discuss the spin-optoelectronic and magnetic properties of these solution-processed hybrid materials and their fundamental spin-dependent physical behavior: (i) We will talk about the observation of spintronic-Terahertz (THz) radiation in layered Pb-based hybrid perovskites interfaced with a ferromagnetic metal, produced by ultrafast spin current under femtosecond laser excitation. Due to the presence of the pronounced Rashba splitting state in Pb-based hybrid perovskites, the generated THz radiation exhibits an asymmetric intensity toward forward and backward emission direction whose directionality can be mutually controlled by the direction of the applied magnetic field and linear polarization of the laser pulse. (ii) We will show that the Dzyaloshinskii–Moriya-Interaction (DMI), a chiral antisymmetric interaction that occurs in magnetic systems with low symmetry, can be presented in layered magnetic hybrid perovskites of which the metal site, Pb is replaced by Cu. We show that layered Cu-based hybrid perovskite antiferromagnets with an interlayer DMI will lead to a strong intrinsic magnon-magnon coupling strength up to 0.24 GHz, which is four times greater than the dissipation rates of the acoustic/optical magnonic modes. Our work shows that the DMI in these hybrid antiferromagnets holds promise for leveraging magnon-magnon coupling by harnessing symmetry breaking in a highly tunable, solution-processable layered magnetic platform.

M, OCT 9th

TBA

Dr. Galen Reeves, Duke
TBA

M, Oct 16th

TBA

Regina Choe, University of Rochester
TBA

M, Oct 23rd

TBA

Steve Taylor, Vanderbilt University
TBA

M, Oct 30th

TBA

Dave Phillips, Truist Bank
TBA

M, Nov 6th

TBA

Lina Necib, MIT 
TBA

M, Nov 13th

TBA

TBA 
TBA

M, Nov 20th

TBA

Lex Kemper, NCSU
TBA

M, Nov 27th

TBA

Iman Marvian, Duke
TBA

M, Dec 4th

TBA

Smadar Naoz, UCLA
TBA

 

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

Jeffery Andrews- University of Florida
Title: It takes Two to Tango: How Binary Stars Evolve into Gravitational Wave Sources

Despite the discovery of nearly a hundred merging black holes and neutron stars by gravitational wave observatories in recent years, significant uncertainties remain regarding these systems’ formation. With the upcoming third LIGO-Virgo observing run, the study of these objects has taken on a new urgency. In this talk I will discuss how we can make progress on two fronts. First, I will describe how new approaches to modeling binary star populations are already qualitatively and quantitatively improving our understanding of how binary stars evolve in isolation. Second, I will argue that observed populations of neutron stars and black holes in the Milky Way, including recent detections of astrometric binaries by the Gaia satellite, place critical constraints on supernova physics.

M, April 17

Extreme Photonics with Nanogap Cavities: From Ultrafast Single Photon Sources to Biosensors

Maiken H. Mikkelsen
Department of Electrical and Computer Engineering, Duke University, USA

Abstract:
Nano- and quantum materials with unique optical properties hold the potential for breakthroughs in a wide range of areas from ultrafast optoelectronics and on-chip components for quantum information science to improved bio-sensing. An exciting opportunity to realize such new materials lies in controlling the local electromagnetic environment on the atomic- and molecular-scale (~1-10 nm), which enables extreme local field enhancements and drastically modified local density of states. A well-controlled platform to study the emergence of extreme photonic behavior is provided by plasmonic nanogap cavities, which consist of ultrathin dielectric gaps between metals. Here, I will provide an overview of our recent research using such structures to realize ultrafast single photon sources, high-speed thermal photodetectors with on-chip spectral filters and metasurface-enhanced biosensors.

M, April 24

Julian Munoz, Harvard University
Cosmic Dawn: The Next Frontier in Cosmology

The last decades have firmly established the existence of a dark sector of our universe. Yet, details of its content have evaded all laboratory probes. In this talk I will describe how the cosmic-dawn era, which saw the formation of the first galaxies, holds a wealth of information about dark matter and the astrophysics of the early universe. The next decade will see detailed maps of this era with both 21-cm and space telescopes like the new James Webb. I will show how to use the upcoming data to measure structure formation at smaller scales—and earlier times—than ever before. Moreover, I will introduce a new standard ruler to measure the expansion rate of our cosmos during unexplored eras. These studies pave the way to understanding dark matter and its role in the first structure formation, and provide us with a unique window to test the energy content of our universe.