FeRMI

The thermal Hall response of the phonon gas - (Kamran Behnia / LNCMI / Seminar). - 2/07/2026, 14H
2026-07-02 14:00 - 15:30 (Europe/Paris) | Salle de seminaire du LNCMI, Champs Magnétiques Pulsés, 143 avenue de Rangueil, Toulouse, France
Seminar LNCMI
Kamran Behnia, Laboratoire de Physique et Etude de Matériaux (ESPCI, Université Paris Sciences & Lettres, CNRS)
Seminar LNCMI, 2/07/2026, 14H, LNCMI, Toulouse
Abstract
First observed two decades ago [1], the phonon Hall effect, is nowadays observed in numerous insulators, including non-magnetic ones [2-4].
The observation implies a field-induced misalignment between the heat flux and the entropy flux vectors. Phonon thermal Hall conductivity peaks when most phonon-phonon collisions become normal, indicating a link with phonon hydrodynamics and a magnetic field induced change in the balance of three phonon events.
This conjecture is backed by the case of molecular gases subject to magnetic field. In their case, the Seftleben-Beenakker effect [5] arises due to the field-induced modification of the molecular collision cross sections. The breakdown of the Born-Oppenheimer approximation in a magnetic field can offer a geometric [Berry] phase to ordinary acoustic phonons. A picture, which would explain the experimentally observed bound to the thermal Hall of elemental insulators [6].
[1] C. Strohm, G. L. J. A. Rikken and P. Wyder, Phys. Rev. Lett. 95, 155901 (2005).
[2] X. Li, B. Fauqué, Z. Zhu, and K. Behnia, Phys. Rev. Lett. 124, 105901 (2020).
[3] X. Li, Y. Machida, A. Subedi, Z. Zhu, L. Li & K. Behnia, Nature Comm. 14, 1027 (2023).
[4] X. B. Jin et al., Phys. Rev. Lett. 135, 196302 (2025).
[5] L. Hermans, A. Schutte, H. Knaap, and J. Beenakker, Physica 46, 491 (1970).
[6] K. Behnia, Phonon thermal Hall as a lattice Aharonov-Bohm effect, SciPost Phys. Core 8, 061 (2025).
Lien Zoom : https://univ-grenoble-alpes-fr.zoom.us/j/94228207316?pwd=[REDACTED]

Prediction and Control of Evolving Complex Communities. - (Derviş Can Vural / LPT / Seminar). -7/07/2026, 14H
2026-07-07 14:00 - 15:00 | Salle de conférence, Bâtiment 3R4
Séminaire LPT
Derviş Can Vural (University of Notre Dame)
Seminar LPT, 7/07/2026, 14H, 3R4, salle de conférence
Summary :
The interactions between species in a complex ecological community can be described in terms of simple reaction kinetics: Just like an enzyme turns a substrate into a product, a goat transforms plant mass into goat mass. However, unlike ordinary chemical reactions, the stoichiometry and reaction rates between species can change over time. In this talk I will present the mathematical description of three interesting cases where this happens [1-4]. (1) Evolution: Species adapt to each other: for example, some plants will evolve to become bitter or poisonous, in which case a goat may not carry out its transformation as effectively. In turn, the goat may evolve in response to this change, say, producing enzymes that will neutralize the poison. This “evolutionary arms race” will often finalize with an extinction (2) Migration: In some other cases, one or more new species might migrate and invade a community, thereby transforming the reaction network. This could happen naturally, through unintentional human activities, or intentionally, via ecological engineering. (3) Fluctuations: The mass transfer stoichiometry between species are often modulated by external factors such as sunlight, pH, and temperature: For example, in a cold year, a goat has to eat more plant mass before producing a new goat, since it must spend some of the energy to produce heat. In all three cases, I will focus on the statistics of species abundances, and the “first passage statistics” to extinction or some other biologically relevant target.
[1] V. Nguyen, D.C. Vural, Extinction in complex communities as driven by adaptive dynamics. Journal of Evolutionary Biology, January 1 (2021)
[2] V. Nguyen, D.C. Vural, Theoretical guidelines for editing ecological communities. Journal of Theoretical Biology, 534, 110945 (2021)
[3] M. Heidelman, D.C. Vural, Coexistence, crossover and extirpation in coalescent communities and ecotones, Oikos, e11952 (2026 )
[4] Eskin C, Nguyen V, Vural DC. Feasibility as a moving target: Fluctuating species interactions lead to universal power law in equilibrium abundances. arXiv preprint arXiv:2602.18942. 2026 Feb 21.

CRYSTAL, an ongoing project for a theoretical chemical approach to solids. - (Silvia Casassa / LCPQ / Seminar). - 9 / 07 /2026, 15H
2026-07-09 15:00 - 16:30 | Salle de conférence, Bâtiment 3R4
Séminaire du LCPQ
Silvia Casassa CRYSTAL group in the University of Turin
Seminar LCPQ, 9/07/2026, 15H

Topological phases of the bilinear-biquadratic spin-1 chain with single-ion anisotropy. - (Zakaria Jouini / LPT / Seminar). -16/07/2026, 11H
2026-07-16 11:00 - 12:00 | Salle de conférence, Bâtiment 3R4
Séminaire LPT
Zakaria Jouini (EPFL)
Seminar LPT, 16/07/2026, 11H, 3R4, salle de conférence
Summary
Since Haldane’s prediction of a finite spin gap in integer-spin chains, the spin-1 chain has served as a paradigmatic model of quantum magnetism. Beyond the Heisenberg interaction, the most general SU(2)-invariant model includes a biquadratic interaction, often completed by an on-site anisotropy, typical of spin-1 ions in solids.
In this talk, I will present an approach combining bosonization and Bethe ansatz to show that dominant biquadratic interactions in the presence of an on-site anisotropy stabilize a gapped topological phase, in a region of the phase diagram previously identified by numerical studies as critical with central charge c=2. I will argue that this phase is a new realization of the Haldane phase in the spin-1 chain, characterized by hidden quadrupolar order, and I will discuss the prospects and implications for its experimental realization.
