Oscillatory-like expansion of a Fermionic superfluid

Area of Science:

• Quantum physics
• Ultracold atomic gases
• Condensed matter physics

Background:

• Ultracold Fermi gases exhibit complex behaviors across the BEC-BCS crossover.
• Understanding their dynamic properties is crucial for quantum simulation and condensed matter studies.

Purpose of the Study:

• To investigate the expansion dynamics of a Fermi superfluid in a cigar-shaped optical dipole trap.
• To explore these dynamics across the entire BEC-BCS crossover and at various temperatures.
• To extract the effective polytropic index (γ¯) of the Equation-of-State (EoS) from expansion data.

Main Methods:

• Utilizing a cigar-shaped optical dipole trap to confine the Fermi gas.
• Observing anisotropic hydrodynamic expansion of the atom cloud over 30 ms at low temperature (0.06(1)T_F).
• Analyzing expansion dynamics using superfluid hydrodynamic equations to determine the effective polytropic index (γ¯).
• Measuring normalized quasi-frequencies of the oscillatory expansion.

Main Results:

• The atom cloud exhibited anisotropic hydrodynamic expansion with horizontal plane oscillations.
• The extracted γ¯ values showed non-monotonic behavior across the BEC-BCS crossover, agreeing with theory on the unitarity and BEC sides.
• Normalized quasi-frequencies dropped significantly from BEC to BCS sides, reaching a minimum of 1.73 around 1/k_Fa=-0.25.

Conclusions:

• The study provides a method to extract the EoS of a Fermi superfluid from expansion dynamics.
• The non-monotonic behavior of γ¯ and the measured quasi-frequencies offer new insights into strongly interacting Fermi gases.
• This work enhances the understanding of dynamic properties in ultracold Fermi gases across the BEC-BCS crossover.