Eric Sembrat's Test Bonanza

Abstract:

Do we live in a simulation?  The School of Physics and the Society of Physics Students will host a public debate between faculty from the College of Science and the College of Computing to answer this question.  This event is free and open to the all.  There will be time at the conclusion of the debate for audience members to direct questions towards the faculty panel.

Abstract

Electrons in solids have been shown to support new non-interacting topological phases of matter such as topological insulators and semimetals. In this talk I will discuss three aspects of how topology plays a role in magnetic solids, which intrinsically involve strong electronic correlations. We will begin with a discussion of simple magnetically ordered states, and show how momentum space topology plays an important role in understanding fluctuations around such ordered states.

I will illustrate this via the emergence of Dirac magnons and associated edge states in a new cobaltate material we have studied in collaboration with neutron scattering experimentalists, and also discuss related work from other groups. Turning to more complex magnetic orders, we find that real space topology plays a key role in understanding the physics of skyrmions, tiny magnetic swirls which are of great interest in the context of dense information storage. I will discuss how this real space topology gets imprinted onto electronic states, leading to various types of unconventional responses such as quantum oscillations and measured anomalous optical Kerr effects.

Such responses can be used to probe or potentially manipulate skyrmions. Similar to soft-matter systems, skyrmions can exist as isolated particles, form thermally fluctuating liquids, or assemble into crystals. Starting from the crystalline state of skyrmions, we show that if quantum fluctuations become very strong, such crystals can undergo T=0 melting into an exotic chiral quantum liquid. This spin liquid state is found to exhibit unusual many-body topological properties, bearing a striking resemblance to the fractional quantum Hall state of matter.

Abstract

Synthetic rubber and biological hydrogels are dramatically different, but their properties and functions are largely determined by the structure and architecture of their common components polymers. Such a deterministic correlation poses opportunities in the field of polymer physics. In this talk, I will discuss how concepts and knowledge in polymer physics help understand biological questions, which, in turn, inspires the design of new soft materials.

First, I will discuss the biophysical roles of mucus hydrogel in human lung defense. It will be discussed how pathologically relevant biophysical parameters of mucus help understand interactions among mucus, extracellular matrix, and epithelial cells – the three major components of lung defense. Second, inspired by the structure of constituent molecules of mucus, I will show the development of a soft, thermo-reversible, solvent-free rubber with stiffness on the order of 1 kPa. The temperature triggered solid-to-liquid transition enables the elastomers a new class of soft materials for direct-ink-write 3D printing. I will also discuss immediate applications and emerging challenges stimulated by these discoveries.

Bio:

Liheng Cai is an Assistant Professor at the University of Virginia, where he currently holds joint appointments to the Department of Materials Science and the Department of Chemical Engineering, and a courtesy appointment to the Department of Biomedical Engineering. He received his Ph.D. in Materials Science from the University of North Carolina, where he researched with Prof. Michael Rubinstein on theoretical polymer physics and with Prof. Richard C. Boucher on experimental biophysics. During his postdoctoral training with Prof. David Weitz at Harvard, he switched to experiments. Since 2018, he has been leading Soft Biomatter Laboratory at UVa, where his group focuses on understanding and controlling the interactions between active soft materials and living systems with the mission to solve challenges in energy, health, and environmental science. He received North Carolina Impact Award, Harvard Postdoctoral Award for Professional Development, and NSF CAREER Award.

Understanding the mechanisms which trigger an Active Galactic Nuclei (AGN or accreting supermassive black hole) and the role of these AGN in galaxy evolution, specifically in regulating or quenching star formation in galaxies, is a highly debated area of study. Theoretically, it is expected that mergers of galaxies should lead to the triggering of an AGN. In fact, this triggering is required by simulations to reproduce the observed properties of galaxies. Unfortunately, confirming that mergers trigger AGN is observationally challenging. The large observed variation in the frequency of AGN in different merger phases (wise/close pairs, post-mergers) are due to a number of factors including obscuration, time delays, AGN luminosities and AGN lifetimes. The overall interdependence of AGN luminosities and lifetimes impacts any correlation that should be seen between merger signatures and AGN frequency. Here I present a volume limited catalog of visually identified close pairs and merger remnants from the Sloan Digital Sky Survey (SDSS). Using this sample, I will present results based on an ongoing NASA Chandra and related NSF project to constrain the AGN frequency and multi-wavelength properties of systems whose merger signature lifetimes are theoretically expected to be similar to low luminosity AGN lifetimes.  I will describe the role of the SDSS MaNGA survey in characterizing the galaxy properties of these low luminosity AGN and the cold gas outflows from these systems. Time permitting, I will also describe a recent project to constrain the dual AGN fraction in a complete sample of dual galactic nuclei with separations between 1 and 10 kpc.

To be determined.

Abstract

Coming Soon.

Abstract

Microbes live in complex communities which play a central role in global nutrient cycles. The flow of nutrients around these cycles are defined by, and define, microbial community structure and metabolic function. What are the organizing principles of metabolically functional microbial communities that sustain life on Earth? How have ecological and evolutionary forces given rise to these organizing principles and can we use them to design and control functional microbial consortia? To address this problem we present work interrogating the flow of metabolites through microbial communities in two contexts.

Our first study uses denitrification as a model process whereby bacterial communities convert nitrate to di-nitrogen gas. Using natural isolates, sequencing and metabolite measurements we develop a statistical approach to mapping community level denitrification rates to genomic composition. Remarkably, we show that the gene content of a community can be used to predict the flux of metabolites through the community.

Second, we present new work on self-sustaining closed microbial biospheres — these complex communities sustain themselves indefinitely when provided with only light by cycling nutrients. We present a new technique to quantify carbon cycling in closed ecosystems non-invasively. We show that an algae-bacteria closed ecosystem persistently cycles carbon on the timescale of months. These microbial biospheres provide a powerful new tool for understanding how nutrient fluxes structure microbial communities to permit the persistence of life on Earth.
 

Abstract

I will talk about the following topics: firstly, we found that small animal fliers (body size <10 cm) flying directly into an artificial waterfall have a degraded flight performance. We hypothesized that those effects can prevent blood-sucking insects crossing through waterfalls. This can explain why medium-sized birds risks themselves building their nests behind waterfalls. Secondly, we investigated the 3D kinematics and swimming performance of the black ghost knifefish in response to the complex wake of a free oscillating cylinder. We found that vortex-induced vibrations in bluff bodies create an unsteady flow environment that is more challenging for swimming animals, than Kármán vortex streets