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arXiv:1806.09805 · Physical Review A

Enhancement of the thermal-transport figure of merit and breakdown of the Wiedemann-Franz law in unitary Fermi gases

Xinloong Han, Boyang Liu, Jiangping Hu

Published Oct 9, 2019• 9 citations
arXivHTMLPDF

Abstract

We theoretically investigate the thermal-transport properties of a unitary Fermi gas between two reservoirs connected by a quantum point contact. We find several distinguished properties that are qualitatively different from those of weak or noninteracting gas systems. The particle transport figure of merit is drastically enhanced in the unitary regime. The Lorentz number violates the Wiedemann-Franz law, demonstrating the breakdown of Fermi liquid. These transport properties are the hallmarks of the unitary Fermi gas and are attributed to the existence of preformed Cooper pairs. The variation of the thermal-transport coefficients due to the changing of the transmission coefficient is also studied.

DOI
10.1103/physreva.100.043604

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We theoretically investigate the thermal-transport properties of a unitary Fermi gas between two reservoirs connected by a quantum point contact. We find several distinguished properties that are qualitatively different from those of weak or noninteracting gas systems. The particle transport figure of merit is drastically enhanced in the unitary regime. The Lorentz number violates the Wiedemann-Franz law, demonstrating the breakdown of Fermi liquid. These transport properties are the hallmarks of the unitary Fermi gas and are attributed to the existence of preformed Cooper pairs. The variation of the thermal-transport coefficients due to the changing of the transmission coefficient is also studied.
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.
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We study the electrical and thermal transport through the SU(N)(N)(N) Wolff model with the use of bosonization. The Wilson ratio reaches unity as NNN grows to infinity. The electric conductance is dominated by the charge channel, and decreases monotonically with increasing interaction. The thermal conductivity enhances in the presence of a local Hubbard UUU. The Wiedemann-Franz law is violated; the Lorentz number depends strongly on the interaction parameter, which can be regarded as a manifestation of spin-charge separation.