Spring 2026 Colloquium Schedule
Colloquia are Wednesdays at 4:00 p.m. in the JILA Auditorium.
Coffee, tea and cookies will be available in G1B31 (across from G1B20) from 3:30 - 3:50 p.m.
January 14 — Joint Physics-APS Colloquium, "String Theory Reborn"
- Presenter: Andrew Hamilton, CU Boulder
- Host: Mike Litos
- Abstract: String theory offers a viable theory of quantum gravity, with spin 2 gravitons encoded in closed strings. But the failure to find evidence for supersymmetry at the LHC has left string theory in an uncertain state. A solution to the problem is in plain sight: revert to classic nonsupersymmetric, bosonic string theory, reenvisaged as a theory of all the forces, not just the strong force. The classic theory correctly reproduces the Brauer-Weyl (1935) algebraic relation between fermions and bosons seen in the standard model, whereas supersymmetry does not.
Sages rejected the classic theory on the grounds that (1) it does not admit fermions, and (2) its ground state is tachyonic. But rejection (1) assumes that fermions are strings, whereas the fermions of bosonic string theory are the endpoints of strings, and are not themselves strings; in modern parlance, the fermions are excitations of the D-brane boundary of strings. As to rejection (2), the properties of the tachyon are precisely those of a Higgs field: it is a multiplet of the unbroken symmetry; the "vacuum" state where the Higgs field vanishes identically is tachyonically unstable; and it has spin zero. The gauge group of bosonic string theory is tightly constrained. I show that a 26-dimensional bosonic string theory that fits the standard model emerges without contrivance. Unburdened by supersymmetry, bosonic string theory has the potential to bring string theory back into the realm of testable physics accessible to present-day observation and experiment.
January 21 — "Quantum Simulation of Correlated Exciton Phases via Ultrafast Optical Microscopy"
- Presenter: Libai Huang, Purdue University
- Host: Markus Raschke
- Abstract: Moiré superlattices formed from transition metal dichalcogenide (TMDC) heterostructures have emerged as a compelling platform for exploring quantum many-body physics. These systems are viewed as a solid-state counterpart to ultracold atomic gases in optical lattices for quantum simulation. A central open question concerns the coherence and dynamics of quantum phases arising from photoexcited moiré excitons, especially under dissipative conditions.
To address this, we employed transient photoluminescence and ultrafast reflectance microscopy to directly image non-equilibrium exciton phase transitions in twisted WS2/WSe2 heterobilayers. Surprisingly, both experimental data and theoretical modeling reveal that strong long-range dipolar repulsion between moiré excitons leads to a freezing of exciton motion in the Mott insulator phase, persisting for over 80 ns. This result defies the conventional expectation that repulsive interactions delocalize particles, while attractive ones promote binding. The observed phenomenon of frozen dynamics due to strong repulsive interactions is characteristic of highly coherent systems, a feature previously realized exclusively in ultracold gases.
We further investigated the interplay between exciton and charge orders in Bose-Fermi mixture, as well as ballistic exciton flow driven by generalized electron Wigner crystals, revealing rich and tunable excitonic correlations in moiré systems.
January 28 — "Adventures in the Ferroelectric Nematic Realm"
- Presenter: Noel Clark, CU Boulder
- Host: Leo Radzihovsky
- Abstract: In 2017-2018 liquid crystal research groups working independently in the UK and Japan, exploring two distinct families of rod-shaped organic molecules, each reported an unknown nematic-like liquid crystal phases in their materials. In 2020 we showed that the unknown phase in the UK compound, RM734, was a ferroelectric nematic: a 3D liquid phase with a fluid spontaneous polarization field, P. This was a notable event in LC science because ferroelectricity was put forth in the 1910’s, by Peter Debye and Max Born, as a possible stabilizing mechanism for the nematic phase. Nematic polar ordering was revisited extensively experimentally since that time, in systems ranging from colloidal suspensions of rods or discs, to main chain polymers, and melts of polar molecules, and claimed but has never established with certainty. Following on we found, in mixtures of RM734 with the Japanese compound DIO, that the unusual phase in DIO was actually the same ferroelectric nematic as in RM734. This convergence moved us to coin the notion “Ferroelectric Nematic Realm,” for what saw for potential as a new soft matter subfield. With two molecules and one phase this wasn’t much of a Realm, but now it has grown to ~500 molecules, making ~15 new phases, and exhibiting a growing body of exotic LC phenomenology. I will present some of the highlights of the chemical physics and applications of this fundamentally new kind of fluid.
February 4
- Presenter: Andrew Gettelman, PNNL
- Host: Ivy Tan
- Abstract:
February 11
- Presenter: Eric Bittner, University of Houston
- Host: Sean Shaheen
- Abstract:
February 18
- Presenter: Terry Wallace, Lawrence Livermore National Laboratory
- Host: Markus Raschke
- Abstract:
February 25
- Presenter: Adam Koberinski, Rotman Institute of Philosophy, Western University
- Host: Heather Demarest
- Abstract:
March 4
- Presenter: Hamish Gordon, Carnegie Mellon University
- Host: Ivy Tan
- Abstract:
March 11
- Presenter: Ben Lev, Stanford University
- Host: Ana Maria Rey
- Abstract:
No Colloquium March 18 — Spring Break
March 25
- Presenter: Mehran Kardar, Massachusetts Institute of Technology
- Host: Leo Radzihovsky
- Abstract:
April 1
- Presenter: Scott Pratt, Michigan State University
- Host: Jamie Nagle
- Abstract:
April 8
- Presenter: Rachel Henderson, Michigan State University
- Host: Bethany Wilcox
- Abstract:
April 15
- Presenter: Phil Nelson, University of Pennsylvania
- Host: Leo Radzihovsky
- Abstract:
April 22
- Presenter: Long Ju, Massachusetts Institute of Technology
- Host: Victor Gurarie
- Abstract:
For more information about colloquia this semester, contact: Mike Litos