June 2025

Abstracts of the Quantum Center Lunch Seminar

Date: Thursday, June 5, 2025
Place: ETH Zurich, Hönggerberg, HPF G 6
Time: 12:00 - 13:30

Critical aging and relaxation of O(N) models with long-range interactions

Valerio Pagni - Quantum Statistical Physics Group (Defenu Group), ETH Zurich

A sudden quench of O(N) models with long-range interactions from a disordered state to the critical point triggers an interesting dynamical evolution that is universal not only in the long-time relaxation to equilibrium, but also at short times, as it exhibits aging behavior characterized by an independent critical exponent. We show how this problem, naturally formulated in terms of a stochastic Langevin equation, can be mapped to an effective field theory. We then present our renormalization group approach, which enables a non-perturbative calculation of critical exponents, and briefly compare it to more traditional methods.

Entropy spectroscopy of a bilayer graphene quantum dot

Christoph Adam - The Ensslin Nanophysics Group (Ihn Group), ETH Zurich

We measure the entropy change of charge transitions in an electrostatically defined quantum dot in bilayer graphene. Entropy provides insights into the equilibrium thermodynamic properties of both ground and excited states beyond transport measurements. For the one-carrier regime, the obtained entropy shows that the ground state has a two-fold degeneracy lifted by an out-of-plane magnetic field. This observation is in agreement with previous direct transport measurements and confirms the applicability of this novel method. For the two-carrier regime, the extracted entropy indicates a non-degenerate ground state at zero magnetic field, contrary to previous studies suggesting a three-fold degeneracy. We attribute the degeneracy lifting to the effect of Kane-Mele type spin--orbit interaction on the two-carrier ground state, which has not been observed before. Our work demonstrates the validity and efficacy of entropy measurements as a unique, supplementary experimental tool to investigate the degeneracy of the ground state in quantum devices build in materials such as graphene. This technique, applied to exotic systems with fractional ground state entropies, will be a powerful tool in the study of quantum matter.