Events During the Week of September 13th through September 20th, 2026
Monday, September 14th, 2026
- Plasma Physics (Physics/ECE/NE 922) Seminar
- Painleve property: from dynamical accessibility of phase-space holes and gravity cusps in 1D Vlasov-Poisson dynamics to stellarator optimization for quasisymmetry
- Time: 12:00 pm - 1:00 pm
- Place: 2241 Chamberlin Hall
- Speaker: Wrick Sengupta, Princeton University
- Abstract: We address the dynamical accessibility of a quasistationary state from the infinitely many formal equilibria of the Vlasov-Poisson system. We give a first-principles asymptotic selection theory for Bernstein-Greene-Kruskal (BGK) holes produced by two-stream relaxation and cold gravitational clumps produced by collisionless collapse based on two central ideas. First, repeated shell crossing leads to particle bunching and phase-space granulation through caustic formations. Nonlinear phase mixing erases the angle information but preserves the action dependence of the distribution function (DF). Following Berry and O'Dell's work on caustic whorls, and Jarzynski's least-biased information-theoretic interpretation, we show that the resulting coarse-grained DF is a circus-tent DF, made self-consistent for Vlasov-Poisson dynamics. Second, the selected potential is constrained by the Painleve property (PP) of Poisson's equation written in Sagdeev form. PP implies that the once-integrated Poisson equation must belong to an algebraic class reducible to Riccati or Weierstrass form up to suitable variable transformations, which must be determined by the underlying physical processes. The adiabatic theory describes the regular coherent self-organized state in both BGK and gravity. The excluded regions where action-angle variables fail: the O-point caustic sheet in gravity and the X-point separatrix sheet in BGK, require a phase-space-turbulence analysis, which we leave for subsequent work. Finally, we show that PP is also directly relevant to a very different area of plasma physics: the neoclassical optimization of stellarators. PP enables the construction of a reduced phase space for quasisymmetry where the well known quasisymmetric configurations such as the Landreman-Paul are constrained to live. Analytical predictions are thoroughly benchmarked against existing configurations such as those from the QUASR database.
- Theory Seminar (High Energy/Cosmology)
- De Sitter shockwave and static patch holography
- Time: 1:00 pm - 2:30 pm
- Place: Chamberlin 5280
- Speaker: Yiming Chen, Stanford University
- Abstract: We study a version of de Sitter static patch holography in which the Euclidean gravitational path integral, with an observer worldline included, is conjectured to compute a trace. Motivated by recent evidence for this conjecture from the sphere path integral, we test it further by inserting operators along the observer worldline and computing two-point functions and out-of-time-ordered four-point correlators (OTOCs). Building on earlier work, we compute the OTOC using the shockwave formalism, incorporating both observer recoil and gravitational backreaction. I will explain that the OTOC conflicts with two basic properties of a trace in a Hilbert space: cyclicity and positivity. This talk is based on arXiv:2607.14042.
- Host: Jakob Moritz
Tuesday, September 15th, 2026
- No events scheduled
Wednesday, September 16th, 2026
- No events scheduled
Thursday, September 17th, 2026
- R. G. Herb Condensed Matter Seminar
- The Atomic Single Electron Transistor
- Time: 10:00 am - 11:00 am
- Place: 5310 Chamberlin Hall
- Speaker: Dahlia Klein, University of Chicago
- Abstract: Electrons in solids owe their properties to the periodic potential landscapes they experience. The advent of moiré lattices has revolutionized our ability to engineer such landscapes on nanometer scales, leading to numerous groundbreaking discoveries. Despite this progress, direct imaging of these electrostatic potential landscapes remains elusive. Here, we introduce the Atomic Single Electron Transistor (SET), a novel scanning probe that uses a single atomic defect in a van der Waals (vdW) material as an ultrasensitive, high-resolution potential sensor. Built upon the quantum twisting microscope (QTM) platform, this probe leverages the QTM’s capability to form a pristine, scannable 2D interface between vdW heterostructures. Using the Atomic SET, we present the first direct images of the electrostatic potential in a canonical moiré interface: graphene aligned to hexagonal boron nitride. This potential exhibits an approximate C_6 symmetry, minimal dependence on carrier density, and a substantial magnitude of ~60 mV even in the absence of carriers. Theory indicates that this symmetry arises from a delicate interplay of physical mechanisms with competing symmetries. Intriguingly, the measured magnitude significantly exceeds theoretical predictions, suggesting that current understanding may be incomplete. With 1 nm spatial resolution and sensitivity to potentials generated by only a few millionths of an electron’s charge, the Atomic SET enables ultrasensitive imaging of charge order and thermodynamic properties across a wide range of quantum phenomena, including symmetry-broken phases, quantum crystals, vortex charges, and fractionalized quasiparticles.
- Host: Tiancheng Song
- Astronomy Colloquium
- Tracking the Grist and Flakes of Planetesimals with Dust Polarization
- Time: 3:30 pm - 4:30 pm
- Place: 4421 Sterling Hall
- Speaker: Daniel Lin, Jansky Postdoctoral Fellow, NRAO
- Abstract: How planetesimals form and evolve in circumstellar disks is one of the most important questions in astrophysics. In this talk, I will demonstrate that polarization is a unique tool for studying the properties of dust grains throughout the planet formation process. In protoplanetary disks, ALMA has revolutionized the field of disk-scale polarization by detecting and resolving percent-level polarization. I will first present observational evidence of large, scattering, aligned grains. I will then introduce a new grain alignment mechanism that can trace the relative motion between dust and gas offering a connection to how planetesimals form. In debris disks, where planetesimals have fully formed, I will demonstrate how dust polarization, aided by neural networks, can constrain dust composition.
- Host: Nicholas Stone
Friday, September 18th, 2026
- Physics Department Colloquium
- Exploring the Extreme Universe with Gamma-ray Observatories
- Time: 3:30 pm - 4:30 pm
- Place: Chamberlin 2241
- Speaker: Reshimi Mukherjee, Columbia/Barnard College
- Abstract: Very-high-energy (VHE) gamma-ray astrophysics has emerged as an exciting and vital field, with major discoveries made through experiments in space and on the ground. In space, the Fermi gamma-ray telescope studies some of the most violent processes in the Universe, and explores nature's highest energy accelerators. At even higher energies, gamma-ray astronomy can be carried out using ground-based telescopes, which detect the flashes of blue light from air-showers caused by gamma rays impacting the upper atmosphere. Some of the most exciting sources detected at very high energies are blazars, with relativistic jets, and Galactic sources including supernova remnants, pulsar wind nebulae, and binary systems. Gamma-ray production in all these sources occurs due to particle acceleration in extreme conditions of gravitational or magnetic fields, implying the existence of shocks and cataclysmic explosions. Gamma-ray astronomy works synergistically with the study of ultra high energy cosmic rays and observation of the Universe with the elusive high-energy neutrino. Multi-messenger astronomy has emerged as one of the most effective ways to observe the Universe. This talk will outline the scientific motivation for high energy gamma-ray astronomy, describe the techniques involved, and survey the astrophysics of the extreme Universe, as revealed by observations made with gamma rays, in particular with the VERITAS observatory in southern Arizona. In addition, the talk will cover future prospects and new directions with the GRAMS project, next-generation mission for gamma-ray and antimatter studies using a liquid Argon (LAr) Time Projection Chamber (TPC), that will provide unprecedented sensitivity to gamma rays in the under-explored “MeV-gap” region in high energy astrophysics.
- Host: Ke Fang