Breakdown of the Wiedemann-Franz law in a unitary Fermi gas

TL;DR

This study investigates heat and particle transport in a resonantly interacting Fermi gas, revealing a significant violation of the Wiedemann-Franz law and providing insights into non-equilibrium steady states in quantum point contacts.

Contribution

It demonstrates the breakdown of the Wiedemann-Franz law in a unitary Fermi gas and measures transport coefficients in a non-equilibrium state, advancing understanding of strongly interacting fermionic systems.

Findings

Lorenz number violates Wiedemann-Franz law by an order of magnitude

System reaches a non-equilibrium steady state with finite temperature and chemical potential differences

Seebeck coefficient aligns with non-interacting Fermi gas expectations

Abstract

We report on coupled heat and particle transport measurements through a quantum point contact (QPC) connecting two reservoirs of resonantly interacting, finite temperature Fermi gases. After heating one of them, we observe a particle current flowing from cold to hot. We monitor the temperature evolution of the reservoirs and find that the system evolves after an initial response into a non-equilibrium steady state with finite temperature and chemical potential differences across the QPC. In this state any relaxation in the form of heat and particle currents vanishes. From our measurements we extract the transport coefficients of the QPC and deduce a Lorenz number violating the Wiedemann-Franz law by one order of magnitude, a characteristic persisting even for a wide contact. In contrast, the Seebeck coefficient takes a value close to that expected for a non-interacting Fermi gas and shows a smooth decrease as the atom density close to the QPC is increased beyond the superfluid transition. Our work represents a fermionic analog of the fountain effect observed with superfluid helium and poses new challenges for microscopic modeling of the finite temperature dynamics of the unitary Fermi gas.