Critical Vortex Shedding in a Strongly Interacting Fermionic Superfluid.

Area of Science:

• Atomic, Molecular, and Optical Physics
• Condensed Matter Physics
• Quantum Fluids

Background:

• Superfluidity in strongly interacting Fermi gases is a key area of condensed matter physics.
• Understanding vortex shedding dynamics is crucial for characterizing superfluid behavior.
• The BEC-BCS crossover provides a unique regime to study transitions in quantum fluids.

Purpose of the Study:

• To investigate the critical velocity for vortex shedding in a fermionic superfluid across the BEC-BCS crossover.
• To measure the dependence of critical velocity on obstacle travel distance and system parameters.
• To explore the role of fermionic degrees of freedom in vortex dynamics.

Main Methods:

• Utilized a strongly interacting fermionic superfluid of Lithium-6 ($^6$Li).
• Employed an optical obstacle moved through the superfluid to induce vortex shedding.
• Directly imaged vortices post-time-of-flight to measure critical velocity ($u_{vor}$) as a function of obstacle travel distance (L).

Main Results:

• Observed that $u_{vor}$ increases as obstacle travel distance L decreases, with the fastest rate in the unitary regime.
• In the Bose-Einstein Condensation (BEC) regime, an empirical dissipation model with constant $\eta$ described $u_{vor}$(L).
• Across the resonance, a bimodal behavior emerged with a step increase in $\eta$ at a characteristic distance $L_c$, strengthening towards the BCS regime.

Conclusions:

• The observed bimodal behavior in vortex shedding is attributed to the emergence of fermionic degrees of freedom.
• This study provides new insights into the complex dynamics of vortex shedding in Fermi condensates.
• The findings highlight the importance of underlying fermionic nature in superfluid phenomena across the BEC-BCS crossover.