Events at Physics
Events on Thursday, April 13th, 2023
- R. G. Herb Condensed Matter Seminar
- Valley and Momentum Polarization in Multilayer Graphene
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Jia Leo Li, Brown University
- Abstract: Within the flat energy bands of multilayer graphene, the enhanced influence of Coulomb interaction is reflected by the emergence of exchange-driven instabilities, which stabilize isospin-ordered phases by lifting the spin and valley degeneracy. These isospin-ordered phases are ubiquitous in the low-temperature phase space of multilayer graphene, having been observed in Bernal bilayer, rhombohedral trilayer, and a number of twisted graphene heterostructures near the magic-angle. Here, I will discuss a new type of Coulomb-driven instability that is driven by the exchange interaction between trigonal-warping-induced Fermi pockets. It is recently proposed that such instability could induce a spontaneous condensation of charge carriers into one of the Fermi pockets, giving rise to a net polarization in the momentum space. Such exchange-driven instability can be directly probed and characterized using angle-resolved nonlinear transport measurement at the second-harmonic frequency. Beyond the experimental identification of momentum polarization, I will discuss its implication on our understanding of electronic orders in the flat energy band of multilayer graphene.
- Host: Alex Levchenko
- NPAC (Nuclear/Particle/Astro/Cosmo) Forum
- LDMX: The Light Dark Matter eXperiment
- Time: 2:30 pm
- Place: CH 4274
- Speaker: Matthew Solt, The University of Virginia
- Abstract: The constituents of dark matter are still unknown, and the viable possibilities span a very large mass range. Specific scenarios for the origin of dark matter sharpen the focus on a narrower range of masses: the natural scenario where dark matter originates from thermal contact with familiar matter in the early Universe requires the DM mass to lie within about an MeV to 100 TeV. Considerable experimental attention has been given to exploring Weakly Interacting Massive Particles in the upper end of this range (few GeV – ~TeV), while the region ~MeV to ~GeV is largely unexplored. Most of the stable constituents of known matter have masses in this lower range, tantalizing hints for physics beyond the Standard Model have been found here, and a thermal origin for dark matter works in a simple and predictive manner in this mass range as well. It is therefore a priority to explore. If there is an interaction between light DM and ordinary matter, as there must be in the case of a thermal origin, then there necessarily is a production mechanism in accelerator-based experiments. The most sensitive way, (if the interaction is not electron-phobic) to search for this production is to use a primary electron beam to produce DM in ﬁxed-target collisions. The Light Dark Matter eXperiment (LDMX) is a planned electron-beam fixed-target missing-momentum experiment that has unique sensitivity to light DM in the sub-GeV range. This seminar will give an overview of the theoretical motivation, the main experimental challenges and how they are addressed, as well as projected sensitivities in comparison to other experiments.
- Host: Matthew F Herndon
- Astronomy Colloquium
- Unveiling Luminous Obscured Black Hole Growth with the Wide-Area Multi-Wavelength Stripe 82X Survey
- Time: 3:30 pm
- Place: 4421 Sterling Hall
- Speaker: Dr. Stephanie LaMassa, Space Telescope Science Institute
- Abstract: Active galactic nuclei (AGN) serve as signposts of growing supermassive black holes, allowing us to study how these objects evolve from the early Universe to the present day. Surveying patches of the sky uncover representative samples of AGN, with different survey strategies favoring different populations. Rare objects that have a low space density, like high luminosity AGN, can only be adequately sampled via wide-area surveys that probe a large volume of the Universe. "Stripe 82X”, an X-ray survey covering ~30 deg^2 of the rich multi-wavelength Sloan Digital Sky Survey Stripe 82 field, is designed to uncover obscured, luminous black hole growth. In this talk, I will summarize the AGN demographics discovered in Stripe 82X and how they compare with AGN identified from wide-area optical and mid-infrared surveys. I will discuss what we have learned about the host galaxies of these luminous AGN and how AGN properties evolve with redshift and luminosity. Finally, I will mention exciting future prospects for learning more about the cosmic evolution of rare (yet important!) high-redshift, high luminosity obscured AGN.
- Host: Ke Zhang