Freiburg 2024 – scientific programme

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A: Fachverband Atomphysik

A 24: Fermionic Quantum Gases II (joint session Q/A)

A 24.2: Talk

Wednesday, March 13, 2024, 14:45–15:00, Aula

Universal Entropy Transport in Fermionic Superfluids across the BEC-BCS Crossover — Jeffrey Mohan, •Simon Wili, Philipp Fabritius, Mohsen Talebi, Meng-Zi Huang, and Tilman Esslinger — ETH Zürich, Otto-Stern-Weg 1, 8093 Zürich, Switzerland

Particle transport between two superfluids is often associated with reversible, entropy-free supercurrents, such as in the Josephson and fountain effects. However, this only applies to weakly-coupled superfluids in the linear response regime. Here, we experimentally investigate particle and entropy flow within a ballistic channel, strongly coupling two superfluids across the BEC-BCS crossover. Our observations reveal large currents of both particles and entropy. While these currents depend on the channel's geometry, the entropy transported per particle appears constant across different geometries. Instead, it is influenced by the interaction strength and reservoir degeneracy. This suggests that the non-equilibrium currents flowing through the channel inherit the universal equilibrium properties from the reservoirs. Moreover, when distinguishing advective and diffusive entropy currents, we find that the Wiedemann Franz law, which describes the relation of these currents in Fermi liquids, is strongly violated at unitarity but partially restored on the BCS side. The present observations raise fundamental questions about transport in strongly interacting, non-equilibrium Fermi systems.

Keywords: Transport; BEC-BCS; Non-equilibrium; Universality; Entropy