Repulsive Fermi Polarons in a Resonant Mixture of Ultracold ^{6}Li Atoms

Area of Science:

• Ultracold atom physics
• Quantum many-body systems
• Feshbach resonance physics

Background:

• Investigating strongly interacting quantum systems is crucial for understanding emergent phenomena.
• Fermi polarons, quasiparticles formed by a mobile impurity in a Fermi sea, are key to studying many-body physics.
• Ultracold atomic gases provide a tunable platform to explore fundamental quantum interactions.

Purpose of the Study:

• To investigate the properties of repulsive Fermi polarons in ultracold lithium-6 atoms.
• To characterize the behavior of these quasiparticles under strong repulsive interactions near a Feshbach resonance.
• To determine the stability and thermodynamic limits of the repulsive Fermi liquid state.

Main Methods:

• Radio-frequency spectroscopy was employed to probe a polarized spin mixture of ultracold lithium-6 atoms.
• Experiments focused on the regime of strong repulsive interactions near a broad Feshbach scattering resonance.
• Key properties including energy (E_{+}), effective mass (m^{*}), residue (Z), and decay rate (Γ) were extracted.

Main Results:

• Well-defined coherent quasiparticles were observed even at unitarity-limited interactions.
• Repulsive Fermi polarons exhibited an energy (E_{+}) exceeding the bath's Fermi energy above a critical interaction strength.
• The effective mass (m^{*}) diverged and became negative, indicating instability.

Conclusions:

• The repulsive Fermi liquid state becomes energetically and thermodynamically unstable beyond a critical interaction strength.
• The observed negative effective mass is a signature of this instability in the strongly interacting many-body system.
• This study provides insights into the limits of Fermi liquid theory in strongly correlated quantum gases.