Physics Colloquium, Fall 2021

Thursdays 4:00 p.m. 104 Physics or online via Zoom.

Contact colloquium organizer Dr. Shun Saito at for the Zoom link.

(Link to main colloquium page)

Quantum Electrodynamics: A Mongraph with a Textbook Component

The development of quantum electrodynamics started in the late 1940s,
when Feynman and Schwinger, and Tomonaga, developed the concept
of renormalized quantum field theory, to deal with the infinities
that arose in perturbative calculations of scattering processes.
The characteristic element of the calculations was the emergence
of so-called loop corrections, which describe the self-interaction
of the quantum fields. The application of the formalism to bound
states is marred with additional difficulties, captured in the
so-called Bethe-Salpeter formalism. Nevertheless, the theory has
enabled theorists to calculate transition energies in simple
atomic systems like hydrogen and helium to unprecedented accuracy,
approaching 13 or 14 decimals. Also, fundamental physical constants
have been determined to unprecedented accuracy. This talk describes
a book, a mixture of a textbook and a monograph, which has been
developed over the last 15 years and which summarizes the latest
developments in the field, but also serves as an advanced textbook
on they quantum mechanics of bound states. It should appeal to
students as well as experienced researchers.

Title: Photographing Black Holes and Magnetic Fields with the Event Horizon Telescope

Two years ago, in April 2019, the Event Horizon Telescope (EHT) released the first images of a black hole, resolving the shadow of the supermassive black hole M87* at the center of the nearby active galaxy M87. Furthermore, the EHT recently captured the polarization of the photon ring enclosing the shadow, resolving the magnetic field near the event horizon. The EHT uses very long baseline interferometry at 1.3 mm wavelength with a global network of radio telescopes, providing the sharpest view of the universe that resolves black hole accretion and jet base at the edge of the event horizon. In this talk, I will present an overview of the past, present, and future of black hole imaging with the EHT. I will first discuss the major breakthroughs enabling to provide these results and also the major outcomes from these horizon-scale images. Then, I will give our next decadal forecast of the forthcoming exciting era to study black holes through direct imaging.

Title: Weyl semimetals: the case of CeAlGe


Single crystals have played an important role in technological advances. One notable example is silicon which is widely used nowadays in transistors, solar cells, semiconductor detectors, and most importantly integrated circuits (chips) used in the computer. Up until now, the scaled size, capacity, and speed of those chips have progressed immensely due to technological advances and have been roughly following Moore’s law. In order to achieve a further continued technology scaling of integrated circuits or replace them with new devices, new materials are necessary. New materials are especially important for the next generation of computers-quantum computers.

Weyl semimetals are among the materials proposed to have significant potential in informational technologies[1] and to harbor the necessary elements for quantum computing [2]. They host Weyl nodes at specific points in their Brillouin zone, a pair of relativistic fermions with different chirality, Weyl fermions. The nontrivial momentum-space topology due to the Weyl nodes leads to various fascinating phenomena, such as the chiral anomaly, chiral magnetic effect, anomalous magnetoresistance and Hall effect,[3,4] large nonsaturating thermopower [6] and ultrafast photocurrents [7] just to name a few. The essential ingredients for the realization of the Weyl semimetal are the absence of inversion symmetry and or time-reversal symmetry. The RAlX (whereR=Rare Earth and X=Ge, Si) family has been recently identified as a large class of Weyl semimetal based on systematic first-principles band structure calculations.[8] In this respect, I will present details and importance of crystal growth of non-centrosymmetric CeAlGe single crystals, their physical properties, anomalous magnetotransport, and discuss the future implications of our findings and the tunability of RAlGe and RAlSi families.
[1] B. Zhao et al., Phys. Rev. Research 2, 013286 (2020)
[2] N. P. Armitage, E. J. Mele, and A. Vishwanath, Rev. Mod. Phys. 90, 015001 (2018).
[3] L. Wollmann, A. K. Nayak, S.S.P Parkin, and C. Felser, Book Series: Annual Review of Materials Research 47, 247 (2017)
[4] D. Li et al., Nature 572, 624 (2019).
[5] B. Skinner et al., Sci. Adv. 4, 1 (2019)
[6] N. Sirica et al., Phys. Rev. Lett. 122, 197401 (2019)
[7] G. Chang et al., Phys. Rev. B 97, 041104 (2018).

Probing the Cosmic Energy Density Inventory with Tomographic Intensity Mapping
The formation of stars, galaxies, and the large-scale structure in the Universe drives complex energy density flows over a wide range of scales from atomic nuclei to the Hubble length. The net effect could be summarized by a census of the density parameters, Ω, for different entries of the cosmic inventory over time. I will talk about my ongoing effort to probe the history of some key cosmic constituents, including stars, dust, thermal and gravitational energy associated with the large-scale structure. These are constrained via intensity mapping of the cosmic UV, IR, and the Sunyaev-Zeldovich effect backgrounds tomographically as functions of redshift via a new clustering-based technique. While these measurements are already pushing our understanding of the Universe, they represent only the beginning of a new chapter of observational cosmology using the entire radiation field as opposed to only detectable bright galaxies. I will conclude by sharing my excitement on the future prospects of extragalactic background intensity mapping.

Title: Physics on the Brain

Abstract: Understanding how the brain works requires that we study not only specialized circuits and functionally-specific brain areas, but how activity is organized across the brain as a whole. From this perspective, the brain can be viewed as a complex system, similar to the climate or the local ecology. Two major challenges to successful characterization of whole-brain activity are the measurement of activity throughout the brain with high spatial and temporal resolution, and extraction of features that summarize the enormous amount of data that results. In this talk, I will discuss the physics behind functional magnetic resonance imaging, the most common tool for data acquisition, and state- and trajectory-based descriptions of the resulting data. The findings obtained with these tools have the potential to change how we think about the brain, guide personalized interventions for the treatment of neurological and psychiatric disorders, and inspire new approaches in artificial intelligence.

Excitons: from basic concepts to first-principles theories

Excitons are ubiquitous in light-matter interactions: in insulators and semiconductors they dominate the optical processes close to the gap. Excitons are a key factor determining the performance of optoelectronic devices such as LEDs or solar cells, and they are candidates for qubits in quantum information. While excitons can be modeled relatively easily as bound electron-hole pairs, quantitatively accurate first-principles theories are computationally quite demanding. In this talk I will give an overview of recent progress using time-dependent density-functional theory (TDDFT) for excitonic properties in solids. I will show that TDDFT can yield accurate optical spectra and exciton binding energies in a variety of materials, including perovskites. Furthermore, I will show how real-time exciton dynamics in the short-pulse and nonlinear regime can be simulated and visualized using TDDFT.

Ultrafast electron, phonon and infrared polaritonic coupled thermal transport across thin films and interfaces


The fundamental interactions among photons, electrons and phonons at interfaces dictate heat transport in a wide array of materials and interfaces used in computing memory storage, energy conversion technologies, and thermal barriers in extreme environments.  In this talk, I discuss our work using advances in pump-probe thermometry to realize new extremes and novel regimes of electron and phonon thermal transport in novel materials across interfaces.  These studies are enabled by pump-probe thermometry techniques, such as time-domain and steady-state thermoreflectance (TDTR and SSTR, respectively), that enable thermal conductivity and thermal boundary conductance measurements of thin films, interfaces and composite systems.1

Through a series of thermoreflectance measurements that measure the heat flow in a range of metal and non-metal systems with picosecond and nanometer resolution, I will discuss our results novel metal and non-metal alloys, non-metal superlattices, and heterointerfaces that elucidate novel regimes and control of electron and phonon thermal conductivity, including the thermal coupling between electrons, phonons and photons via polaritonic mechanisms.  Specifically, I will discuss:

-Unprecedented control of electron and phonon thermal conductivity in high entropy alloy thin films, including the ability to achieve ultralow phonon thermal conductivities in entropy stabilized oxides2 and tunability of electron thermal conductivity in high entropy carbides3

-The wave particle duality of phonon thermal conductivity realized in oxide superlattices, which gives rise to coherent phonon transport.4,5

-Coupled electron-phonon transport across interfaces, and how ballistic electron transport can be used to control mid-infrared plasmon polaritons and photonic absorption through a novel interfacial heat transfer mechanism: “ballistic thermal injection” (BTI).6 We demonstrate how this BTI mechanisms can be used to control plasmon lifetimes with heat.

1. Journal of Applied Physics 126, 150901 (2019).
2. Advanced Materials 30, 1805004 (2018).
3. Acta Materialia 196, 231 (2020).
4. Nature Materials 13, 168 (2014).
5. to appear in Nature (preprint: arXiv cond-mat.mtrl-sci:2105.10030 (2021)).
6. Nature Nanotechnology 16, 47 (2021).


Are supercooled liquids and glasses related to conventional fluids and solids?

Supercooled liquids and glasses are often described in terms of quenched metastable states in complex high dimensional landscapes that have little to do with their ordinary equilibrium counterparts. This latter assumption seems natural in view of the singular behaviors of glass formers. Indeed, unlike typical phase transitions from liquids to solids, as supercooled liquids transform into glasses, their dynamics become exceedingly slow at much lower temperatures while showing only far fainter thermodynamic and normal structural signatures (if any). Here, we ask whether we can still, nonetheless, relate glassy and conventional equilibrium dynamics by noting that the equations of motion in both systems are the same (since they are both governed by the same disorder free Hamiltonian). These identical equations of motion suggest various connections. Amongst other things, a predicted collapse directly relating the viscosity and dielectric response times of supercooled liquids with those in the regular equilibrated solid and fluid states is seen to experimentally hold over 16 decades of relaxation times for all known types of glass formers. 

The finalists are:

Xuecheng Ye
Title: Stripe order, impurities, and symmetry breaking in a diluted frustrated magnet

Sujan Bastola
Title: Fully Differential Investigation of Two-Center Interference in Dissociative Capture in p + H2 Collisions

Jack Crewse
Title: Localization of the Higgs mode near the superfluid-Mott glass quantum phase transition

Title: FRB science results from CHIME

Abstract: Fast radio bursts (FRB's) are a recently discovered, poorly understood class of transient event, and understanding their origin has become a central problem in astrophysics. I will present FRB science results from CHIME, a new interferometric telescope at radio frequencies 400-800 MHz. In the ~3 years since first light, CHIME has found ~20 times more FRB's than all other telescopes combined, including ~60 new repeating FRB's, the first repeating FRB with periodic activity, a giant pulse from a Galactic magnetar which may be an FRB in our own galaxy, and millisecond periodicity in FRB sub-pulses. These results were made possible by new algorithms which can be used to build radio telescopes orders of magnitude more powerful than CHIME. I will briefly describe two upcoming projects: outrigger telescopes for CHIME (starting 2022) and CHORD, a new telescope with ~10 times the CHIME mapping speed (starting 2024).