Dissipation Mechanisms in Fermionic Josephson Junction

Superfluids are generally known as fluids whose flow is unaffected by the container they move in, exhibiting no viscosity. This is true as long as the velocity of the flow is small enough. At higher speeds, flows tend to create whirls that are known as quantum vortices. In the paper, we have investigated the flow occurring in the so-called Josephson junction, where two superfluids are separated by a thin barrier that allows for the tunneling between condensates. The difference in chemical potential across the junction controls the flow rate. We consider Fermi superfluid formed when fermions group into correlated pairs, called Cooper pairs. We show a substantial difference in dissipation as one goes from the weakly interacting (BCS regime) to strongly interacting (unitary Fermi gas) superfluid. For the latter, the superfluid flow generates vortices at the barrier region, which subsequently carries the excitation energy away. The dissipative process in strongly interacting Fermi gas turns out to be similar to the one observed in Josephson junctions in weakly interacting bosonic systems. In contrast, for weakly interacting cases, the dissipation occurs predominantly through breaking Cooper pairs and depletion of the superfluid condensate. Surprisingly, while the origin of the dissipation has changed, the overall global dynamics of the Josephson junction are unaffected. Indeed, available experimental results do not show a significant change in the junction dynamics between regimes. Numerical simulations, like those presented in the paper, are presently the only method to probe the microscopic dynamics of the system across the whole spectrum of interaction strengths in detail.

Details of the article:

G. Wlazłowski, K. Xhani, M. Tylutki, N.P. Proukakis, P. Magierski,
Dissipation Mechanisms in Fermionic Josephson Junction,
Phys. Rev. Lett. 130, 023003 (2023) [arXiv:2207.06059].