A generalized molecule approach capturing the Feshbach-induced pairing physics in the BEC-BCS crossover.

Area of Science:

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

Background:

• Feshbach resonances are crucial for controlling interactions in ultracold atomic gases.
• Understanding the BEC-BCS crossover and BCS regime requires accurate theoretical models.
• Experimental data on closed-channel fractions (Z) and Fermi temperature (T_F) provide benchmarks for theoretical predictions.

Purpose of the Study:

• To develop a refined two-body, two-channel model incorporating trap effects for ultracold Fermi gases.
• To accurately reproduce experimental measurements of molecular binding energies and closed-channel fractions.
• To investigate the relationship between Z and T_F across different regimes, including unitarity and the BCS side.

Main Methods:

• Inclusion of a trap effect with characteristic energy defined by the Fermi temperature (T_F).
• Application of a two-body, two-channel model to analyze Feshbach resonances.
• Comparison of model predictions with experimental data for 40K and 6Li atomic Fermi gases.

Main Results:

• The model successfully reproduces measured binding energies for ultracold molecules in 40K Fermi gas.
• Experimental closed-channel fraction (Z) is accurately reproduced across the BEC-BCS crossover and BCS regime for 6Li Fermi gas.
• The predicted behavior Z ∝ T_F at unitarity matches recent experimental findings, including the proportionality constant.

Conclusions:

• The refined model provides a quantitatively accurate description of ultracold Fermi gases, including the BCS regime.
• The model resolves previously reported discrepancies between theory and experiment regarding the Z dependency on T_F.
• The study reports predicted proportionality constants for future experimental comparison.