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

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 nonequilibrium 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 noninteracting 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 challenges for microscopic modeling of the finite temperature dynamics of the unitary Fermi gas.

Thermopower and thermal conductance of a superconducting quantum point contact

“…Here S is the normal-state Seebeck coefficient which satisfies the Mott law, see Eqs. (22) and (29). At small…”

“…Next, taking similar steps, we evaluate the Seebeck coefficient. It is defined as the ratio of the thermoelectric coefficient (29) and conductance (33),…”

“…It requires inelastic relaxation length be much shorter than the distance between the NS and SN interfaces. This is not a stringent condition for a cold atoms gas close to the unitary limit [29], but may require further analysis in the case of electron transport [22][23][24][25]42]. We also assume a sufficiently short BCS coherence length which simplifies [43] consideration of the conductance of NSN junction.…”

“…These kinds of contacts are currently studied in several different experimental settings. These include proximized semiconductor quantum wires [22,23], atomic point contacts [24,25], and trapped cold atoms [26][27][28][29].…”

Plasmon localization, plasmon relaxation, and thermal transport in one-dimensional conductors

“…A research direction that has attracted a considerable amount of interest in recent years is the thermal transport and energy relaxation in various 1D setups, including systems of photons 2 and of cold atoms 3,4 as well as quantum Hall edges with counterpropagating modes [5][6][7][8][9][10][11][12][13] . The quantum Hall structures provide a particularly suitable experimental platform for the exploration of this class of phenomena.…”