LPTMC Seminars

A Josephson junction in a multimode environment: emergent quantum phase transition and exact low-energy duality

The physics of a single Josephson junction coupled to a resistive environment is a long-standing fundamental problem at the center of an intense debate about the existence and properties of the superconducting-to-insulating Schmid-Bulgadaev transition. To circumvent the potential subtleties in the original derivation, we investigate the emergent criticality of a junction coupled to a multimode resonator when the number of modes is increased [1]. By solving the system via exact diagonalization, we show that at the transition point the spectrum displays universality (scale invariance) not only at low frequencies. This reflects in finite-frequency spectral signatures of the phase transition, in agreement with recent experiments. The spectrum at the critical point is successfully compared with analytical and numerical results obtained in the past [2]. Finally, we prove a low-energy exact self-duality of the model, that emerges from two different finite-size circuits with different conserved quantities [3]. This confirms and generalizes the approximate self duality that is usually invoked, and proves the independence of the transition point on the ratio of Josephson to charging energy.

[1] Giacomelli L., Ciuti C., Nature Communications, 15(1), 5455 (2024)

[2] Paris, Giacomelli, Daviet, Ciuti, Dupuis, Mora, Phys. Rev. B, 111(6), 064509 (2025)

[3] Giacomelli L., Devoret M. H., Ciuti C., Phys. Rev. Lett., 136(13), 130401 (2026)

Slimming down through frustration

In many disease, proteins aggregate into fibers. Why? One could think of molecular reasons, but here we try something more general. We propose that when particles with complex shapes aggregate, geometrical frustration builds up and fibers generically appear. Such a rule could be very useful in designing artificial self-assembling systems.