Colloquia / Seminars
The UNC Physics colloquium takes place on Mondays 3:30-4:30pm ET in Phillips 265 unless otherwise stated.
Campus Life Experience (CLE) :Throughout their academic career, a student will need to complete 16 CLEs as part of the requirements for graduation. CLE events in Heel Life will count for a student. Students should attend at least 2 CLE events per semester. Attendance at colloquiums will qualify as CLE credit.
F, Apr 8
The Missing Physicists
This special colloquium, “The Missing Physicists,” will feature a panel discussion of the March 2022 “Missing Physicists” series in Science about the barriers Black physicists face and potential models for change.
- Mohammad Ahmed [he/him], NC Central University
- Roy Clarke [he/him], University of Michigan
- Zack Hall [he/him], UNC Chapel Hill
- Sheila Kannappan [she/her], UNC Chapel Hill
- Kent Wallace [he/him], Fisk University
- Jennifer Weinberg-Wolf [she/her], UNC Chapel Hill
The discussion will be moderated by Akaa Daniel Ayangeakaa [he/him] of UNC Chapel Hill.
3:30pm ET, Chapman 125 or via Zoom
M, Jan 24
Emergent hydrodynamics in a strongly interacting dipolar spin ensemble in diamond
Chong Zu, Washington University
Conventional wisdom holds that macroscopic classical phenomena naturally emerge from microscopic quantum laws. However, building direct connections between these two descriptions has remained an enduring scientific challenge. In particular, it is difficult to quantitatively predict the emergent ‘classical’ properties of a system (for example, diffusivity, viscosity, and compressibility) from a generic microscopic quantum Hamiltonian. Here we introduce a hybrid solid-state spin platform in diamond, where the underlying disordered, dipolar quantum Hamiltonian gives rise to the emergence of unconventional spin diffusion at nanometre length scales . In particular, the combination of positional disorder and on-site random fields leads to diffusive dynamics that are Fickian yet non-Gaussian. Finally, by tuning the underlying parameters within the spin Hamiltonian via a combination of static and driven fields, we demonstrate direct control over the emergent spin diffusion coefficient. If time permits, I will end by describing our recent efforts to realize a quantum simulation platform based upon spin defects in 2D . C. Zu, et al., Nature 597, 45-50 (2021)
 E. Davis, et al., arXiv:2103.12742 (2021)
3:30pm ET, via Zoom here
Meeting ID: 959 6088 9513
M, Jan 31
Radium ions and radioactive molecules
Andrew Jayich, University of California, Santa Barbara
The bottom row of the periodic table is famous for its radioactive elements, which compared to stable isotopes are little-explored. Many heavy radioisotopes have exotic nuclei which grant them enhanced discovery potential. Radioactive elements also hold promise for advancing technology. Modern atomic physics techniques, such as laser cooling and ion trapping, allow for efficient use of unstable elements and their study in highly-controlled environments. In this context we will discuss our recent work with laser-cooled radium ions. This heavy species is promising for controlling other radioactive atoms and molecules at the level of single quantum states and studying them with high precision spectroscopy. To date with trapped radium ions we have produced a number of radioactive molecules which are good candidates for studying time symmetry violation to address open questions in physics. We have also developed a new technique for rapidly identifying molecular ions. In addition to producing and controlling molecules, the radium ion holds promise in its own right, for example as a transportable optical clock candidate.
3:30pm ET, Phillips 265
M, Feb 7
Nanophotonic Interfaces to Control Plasmons and Spins
Laura Kim, Massachusetts Institute of Technology
Light-matter interactions enabled by photonic quasiparticles play a crucial role in observing ultrafast phenomena as well as enabling next-generation nanophotonic devices and quantum technologies. In the first part of the presentation, I will present the first experimental demonstration of a mid-infrared light-emitting mechanism, originating from an ultrafast coupling of optically excited carriers into hot plasmon excitations in graphene. Such emission processes produce gate-tunable, non-Planckian emission behavior that is not dictated by the free-space photonic density of states. This work provides a platform for achieving ultrafast, ultrabright, on-chip mid-infrared light sources. In the second part of the presentation, I will present a diamond resonant metasurface that can mediate efficient spin-photon interactions and enable a new type of quantum imaging system. This quantum metasurface containing nitrogen-vacancy (NV) spin ensembles achieves local field concentration over a micron-scale NV layer, and it coherently encodes information about the local magnetic field on spin-dependent phase and amplitude changes of near-telecom light. The projected performance makes the studied quantum imaging metasurface appealing for the most demanding applications such as imaging through scattering tissues and spatially resolved chemical NMR detection.
3:30pm ET, Phillips 265
W, Feb 9
Probing Quantum Materials with Scanning Probe Microscopy
Yonglong Xie, Harvard University
Electrons inside matter can behave as complex particles that do not exist in the Standard Model. These seemingly impossible effects are examples of emergent phenomena—that is, unexpected collective behavior—of electrons in quantum materials. The discovery and characterization of new emergent phenomena in quantum materials not only expand the boundary of our knowledge, but also provide unique opportunities for future quantum technologies. However, these effects often manifest in subtle ways, and thus detecting them requires developing new, more sophisticated measurement tools.
In this talk, I will demonstrate how a class of experimental techniques called scanning probe microscopy can be a general tool for unlocking new phenomena in quantum materials. To illustrate the power of this approach, I will focus on our recent experimental observation of novel topological quantum states in magic-angle graphene, enabled by scanning single-electron-transistor microscopy. In addition, I will highlight other examples in which scanning probe microscopy permits the discovery of novel phases in other quantum materials. Finally, I will conclude by outlining how pushing the boundaries of existing scanning probe microscopy will enable the discovery and characterization of new emergent phenomena and functionalities in quantum materials, devices, and circuits.
3:30pm ET, Phillips 215
M, Feb 14
The elusive lightness of neutrinos
John F. Wilkerson, UNC/TUNL/ORNL
Neutrinos, enigmatic fundamental particles, were long assumed to be massless until a series of revolutionary experiments over the past two decades revealed that they actually exhibit complex behavior and must possess non-zero mass. From these and other recent measurements we know that neutrinos have minuscule masses, at least 500,000 times lighter than the electron. Yet we still do not know the neutrino’s actual mass nor why it is so light. Determining this absolute neutrino-mass scale is vital to our understanding of fundamental interactions, cosmology, astrophysics and ultimately to answering the underlying question on the origin of particle masses. This talk will review our current understanding of neutrinos and then address the question of how one “weighs” a neutrino? The techniques and latest results from cosmology, double beta decay and direct kinematical methods will be presented, with a focus on the Karlsruhe Tritium Neutrino experiment (KATRIN), which today (Feb. 14) published a new limit, reaching for the first time sub-eV sensitivity from a direct neutrino mass experiment.
3:30pm ET, Phillips 265
W, Feb 16
Quantum point contacts in an oxide superconductor
Evgeny Mikheev, Stanford University
Superconductors and semiconductors are typically thought of as distinct material categories. Each has fascinating and technologically useful electronic properties. What if one could combine them in one material? This is rare but possible: for example, the oxide SrTiO3 superconducts at carrier densities so low that its superconductivity can be tuned by applying electric fields. I will explain how this can be done locally by applying voltages to nanopatterned gate electrodes. This presents a unique opportunity to create superconductor/normal state junctions without the complexity of interfacing two dissimilar materials.
I will present my recent work [1, 2] on defining narrow constrictions between larger superconducting regions in SrTiO3. I will demonstrate how the combination of quantum confinement and local gate tuning results in quantized staircase shapes in normal state conductance. This is a hallmark of clean ballistic behavior that typically requires working with pristine high-mobility semiconductors. An even more distinctive quantization signature is observed in the superconducting state: each ballistic mode can only carry a finite quantum of supercurrent, and a staircase shape is seen in the critical current. This work inches us closer than ever to experimentally realizing superconducting junctions coupled by one or few perfectly transmitting ballistic modes. This a difficult but important technological goal: such junctions are key enablers for several approaches to protect quantum information from dephasing and bit-flip errors. My future target is to integrate oxide nanostructures into gate-tunable transmon qubits (gatemons), Andreev qubits, and nanowires with topological superconductivity. E. Mikheev, I. T. Rosen, D. Goldhaber-Gordon, Science Advance 7, eabi6520 (2021)
 E. Mikheev, I. T. Rosen, M. A. Kastner, D. Goldhaber-Gordon, arXiv:2110.11535
3:30pm ET, Phillips 215
M, Feb 21
For the new material’s era beyond Silicon age: discovering, understanding, and manipulating quantum materials.
Na-Hyun Jo, Lawrence Berkeley National Laboratory
We are currently living on the edge of the silicon age, characterized by the rapid growth of the semiconductor industry. Yet, a new class of materials destined to become as familiar as silicon is underway. These materials are so-called quantum materials. In quantum materials, quantum effects manifest over a wide range of energy and length scales and give rise to exotic properties such as superconductivity, non-trivial topology, and many more. For the last decade, topological materials have been of great interest in the materials science community. With the steady rise of topological materials, the community’s attention is now shifting to strongly correlated topological materials that host a largely unexplored territory from both a theoretical and experimental perspective. In this colloquium, recent efforts to discover, understand, and manipulate the topological state of matter via strong correlation will be reviewed. First, I will describe how the spin degree of freedom can manifest a novel quantum state of matter. Second, the effect of lattice degree of freedom on manipulating quantum materials will be discussed. Finally, I will also briefly state the exciting future of the field.
3:30pm ET, Phillips 265
W, Feb 23
Quantum information processing based on spins in semiconductor quantum dots
Yinyu Liu, Harvard University
The field of Quantum Information is of great excitement in both fundamental physics and industry. One promising platform for quantum computing is gate-defined quantum dots in semiconductors. The greatest limiting factor currently is that delicate quantum states can lose their quantum nature due to interactions with their environment. Other open challenges are to coherently control large-scale spin qubits and develop methods to entangle quantum bits that are separated by significant distances.
Silicon-based materials are promising due to the long lifetimes of electrons’ quantum states, but also challenging due to the difficulty in fabrication and valley degeneracy. I will report a singlet-triplet qubit with a qubit gate that is assisted by the valley states. This work would potentially relax the design and fabrication requirement for scaling. Moreover, strong coupling between electron spins and photons in hybrid circuit-QED architecture has been achieved in this research field. Quantum optics, long-distance quantum entanglement, and communication via photons are promised. To address that, I will present my project on indium arsenate (InAs) double quantum dots (DQD) that are embedded in circuit-QED architecture. We demonstrated the direct evidence of photon emission from a DQD in the microwave regime. By achieving stimulated emission from one DQD in these works, we invented a semiconductor single atom maser that can be tuned in situ. I will demonstrate that a semiconductor-based quantum dot is a promising platform for quantum information as well as for fundamental physics.
3:30pm ET, Phillips 215
M, Feb 28
Quantum transduction and spin-orbitronics using hybrid magnonic systems
Wei Zhang, Oakland University
Hybrid magnonic systems have gained increased interest due to their potential impact in quantum transduction and coherent information processing. Magnons, as the fundamental excitation of magnetically ordered materials, exhibit promising features that can be tuned and made to coherently couple to other excitations, such as microwave photons, light, phonons, and other magnons. I will introduce our recent work in the coherent magnon-photon coupling in a permalloy-superconducting resonator device and the magnon-magnon coupling of a permalloy-yttrium iron garnet device. By controlling the coupling strength and the quantized spin waves, the hybrid magnon spectra can also exhibit magnetically induced transparency, which can be readily detected via our home-built magneto-optic spectroscopy. Lastly, I will show how spin-orbit effects can be used as an effective means to both tune and detect the dynamic magnetic properties in magnonic systems.
3:30pm ET, Phillips 265
M, Mar 7
How do you Merge two Black Holes? The Present and Future of Gravitational-wave Astrophysics
Since 2015, The Laser Interferometer Gravitational-wave Observatory (LIGO) has detected ~100 gravitational waves from merging black holes and neutron stars, inaugurating a new era of observational astronomy. But how are these systems formed in the first place, and what can that tell us about the lives and deaths of massive stars and the star clusters and galaxies that make them? In this talk, I will attempt to answer these questions by describing how massive and old star clusters, such as the globular clusters in the Milky Way, are the ideal site for the production of binary black holes. I will show how the dynamical assembly of binaries in these dense stellar environments imprints detectable features in the gravitational waves themselves, and how multiple mergers in star clusters can produce black holes with masses that cannot be formed from single or binary stars. Finally, I will place these results within the broader context of galaxy formation and assembly, describing a new project to model star clusters self-consistently from collapsing giant molecular clouds in an MHD simulation of a Milky Way-mass galaxy. These results can provide a direct link between the study of globular cluster formation, the assembly of galaxies, massive black holes, and the future of gravitational-wave astronomy.
3:30pm ET, Phillips 265
M, Mar 21
A Song of Ice and Fire: the Fate of Planetary Systems After Stellar Death
Andrew Vanderburg, MIT
In the past 30 years, astronomers have discovered thousands of planets orbiting
stars outside the solar system. Most of the exoplanets we know of today orbit stars
that will eventually exhaust their nuclear fuel, expand into red giants, shed their
outer layers, and contract into dense remnants called white dwarfs. How does the
process of stellar death affect any orbiting planets in the system? I will review
our knowledge of planets beyond the main sequence and discuss new insights gleaned
from our discoveries of two very different systems: a disintegrating minor planet
around WD 1145+017 and an intact giant planet candidate around WD 1856+534. I will
conclude by discussing the prospects for habitability in white dwarf systems long
after the host star’s death and how with some luck, we may be able to test these
ideas in the next decade.
3:30pm ET, Phillips 265
M, Mar 28
Changes in the physics higher ed landscape
Andy Rundquist, Hamline University
As the provost for a small, private, liberal arts school in the midwest, where I’ve been on the physics faculty for 20 years, I am happy to share our approaches to the upcoming demographic changes we sometimes call the impending “demographic cliff.” I’ll also share some curricular and pedagogical approaches that we think will work well with those demographic changes, including ways to include student voices in assessment.
3:30pm ET, Phillips 265
M, Apr 4
Departments with high-use of active learning in introductory STEM courses: How did they get there?
There is now a convincing body of research showing that a wide variety of active learning instructional strategies consistently improve student learning and other desired outcomes when compared to traditional instruction. Like most fields, there is a substantial gap between the research-based knowledge about effective teaching and the actual practices of instructors. In 2019 we conducted a web-based survey of 3,769 instructors who taught introductory chemistry, mathematics, and physics courses. Survey responses suggest that the instructional practices are more strongly influenced by departmental factors than individual or institutional factors. To understand how these departments got there, we conducted interviews with 27 instructors in 16 departments that were in the top quartile in terms of the use of active learning in their introductory courses. We developed a model that highlights the relevant characteristics of departments that have high use of active learning instruction in their introductory courses. According to this model, there are four main characteristics of such departments (motivated people, knowledge about teaching, opportunities, and cultures and structures that support active learning) and two positive feedback loops. There are two main take-away messages for those interested in promoting the use of active learning. The first is that all four components are important. A weak or missing component will limit the desired outcome. The second is that desired outcomes are obtained and strengthened over time through the two positive feedback loops. It is not realistic to expect meaningful sustainable change to occur in less than three years.
3:30pm ET, Phillips 265
M, Apr 11
3:30pm ET, Phillips 265
M, Apr 18
Increasing Accuracy in Measurements of the Hubble Constant: Is There Evidence for New Physics?
Wendy Freedman, University of Chicago
An important and unresolved question in cosmology today is whether there is new physics that is missing from our current standard Lambda Cold Dark Matter (LCDM) model. Recent measurements of the Hubble constant, Ho — based on Cepheids and Type Ia supernovae (SNe) — are discrepant at the 4-5-sigma level with values of Ho inferred from measurements of fluctuations in the cosmic microwave background (CMB). The latter assumes LCDM, and the former assumes that systematics have been fully accounted for. If real, the current discrepancy could be signaling a new physical property of the universe. I will present new results based on an independent calibration of SNe Ho based on measurements of the Tip of the Red Giant Branch (TRGB). The TRGB marks the luminosity at which the core helium flash in low-mass stars occurs, and provides an excellent standard candle. Moreover, the TRGB method is less susceptible to extinction by dust, to metallicity effects, and to crowding/blending effects than Cepheid variable stars. I will address the current uncertainties in both the TRGB and Cepheid distance scales, the promise of upcoming James Webb Space Telescope data, as well as discuss the current tension in Ho and whether there is a need for additional physics beyond the standard LCDM model.
Meeting ID: 960 5170 9190
3:30pm ET, Zoom only
M, Apr 25
The dark side of string theory
Ulf Danielson, Uppsala University Sweden
Cosmological horizons, as well as black hole horizons, pose great challenges to fundamental physics. This is true also in string theory, where the black hole information paradox is still not resolved, and it remains unclear if a cosmological constant can even exist. In this lecture I will give an introduction to the main problems, review the status of the field, and speculate about the future.
Meeting ID: 923 8228 8816
3:30pm ET, Zoom only
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
Meeting ID: 960 5054 5723
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
M, Oct 24
James Vesenka, University of New England
Via Zoom only
Time: Oct 24, 2022 03:30 PM Eastern Time (US and Canada)
Meeting ID: 963 5980 7048
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.