Events at Physics |
Events on Thursday, November 6th, 2025
- R. G. Herb Condensed Matter Seminar
- Searching for Experimental Signatures of Fractionalization
- Time: 10:00 am
- Place: 5310 Chamberlin
- Speaker: Pietro Bonetti, Harvard
- Abstract: In this talk, I will present some of my recent work which aims at understanding experimental observations in several strongly correlated platforms—cuprates, cold atomic simulators and organic compounds—in terms of exotic fractionalized states exhibiting topological order.
In the cuprates, the pseudogap phase shows multiple signatures of reconstruction of the large Fermi surface into four small hole pockets. In the absence of spontaneous symmetry breaking, this appears to contradict Luttinger’s theorem, which predicts that the volume enclosed by the Fermi surface remains constant. The resolution of this apparent contradiction lies in assuming that the pseudogap phase hosts fractionalized excitations with emergent gauge fields. I will show that these excitations are crucial not only for preserving Luttinger’s theorem but also for reproducing the Fermi surface topology observed in quantum oscillation measurements at low temperatures, where charge density wave order develops and further reconstructs the four hole pockets.
In the second part of the talk, I will discuss a recent experiment realizing a Hubbard model on a Lieb lattice in a cold atomic simulator [Lebrat et al., arXiv:2404.17555 (2024)]. At half-filling, a flat band was observed for zero Hubbard U, and a ferrimagnetic state for nonzero U, in agreement with theoretical predictions. In the presence of strong interactions, a new flat band was reported to emerge at quarter filling without symmetry breaking. Employing parton theories, we consider quantum fluctuations of several magnetic ground states identified using Hartree-Fock theory. We find a state with a flat band at the Fermi level coexisting with fractionalized bosonic spinon excitations carrying Z2 gauge charges. This state represents a striking realization of doping-induced fractionalization.
If time permits, I will briefly review how ultrafast spectroscopic experiments in the cuprates and in the organic compound κ-(BEDT-TTF)2Cu2(CN)3 can be interpreted in terms of fractionalized excitations. - Host: Elio König
- Astronomy Colloquium
- Dynamics and Energetic Transients in Galactic Nuclei
- Time: 3:30 pm - 4:30 pm
- Place: 4421 Sterling Hall
- Speaker: Dr. Itai Linial, Hubble Fellow at NYU
- Abstract: Centers of galaxies host a variety of dynamical processes, owing to the high density of stars and the presence of a central Supermassive Black Hole (SMBH). Orbital relaxation brings stellar objects into close encounters with the SMBH, driving a plethora of energetic phenomena. These include tidal disruption events, stellar collisions, the formation of X-ray binaries, compact object mergers detectable by LIGO, and gravitational wave (GW) inspirals of stellar objects towards the SMBH. In this talk, I will review some of these processes and their observable signatures, with an emphasis on newly discovered classes of repeating flares associated with SMBHs. I will discuss theoretical models, key open questions, and how these phenomena inform our understanding of accretion physics, SMBH growth and evolution, and the connection to their host galaxies. A powerful array of current and upcoming time-domain surveys and instruments will uncover thousands of high-energy sources in galactic nuclei in the coming years. I will highlight how theoretical interpretation of these discoveries can address fundamental questions in astrophysics, and conclude with broader implications for multi-messenger observations in the era of space-based GW detectors such as LISA.
- Host: Nicholas Stone