Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms (0902.3021v2)

Abstract: We have observed Fermi polarons, dressed spin down impurities in a spin up Fermi sea of ultracold atoms. The polaron manifests itself as a narrow peak in the impurities' rf spectrum that emerges from a broad incoherent background. We determine the polaron energy and the quasiparticle residue for various interaction strengths around a Feshbach resonance. At a critical interaction, we observe the transition from polaronic to molecular binding. Here, the imbalanced Fermi liquid undergoes a phase transition into a Bose liquid coexisting with a Fermi sea.

• The paper reports the observation of Fermi polarons in a spin-imbalanced lithium-6 Fermi gas using rf spectroscopy.
• It quantifies key parameters, measuring a polaron energy of -0.64εF and a quasiparticle residue of 0.39 in the critical regime.
• These findings reveal the transition from polaronic states to molecular binding, advancing our understanding of many-body quantum physics.

Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms

The paper "Observation of Fermi Polarons in a Tunable Fermi Liquid of Ultracold Atoms" by Schirotzek et al. presents a detailed experimental investigation into the formation and characteristics of Fermi polarons in a spin-imbalanced Fermi gas of ultracold lithium-6 atoms. This research is crucial for understanding many-body quantum systems with impurities and opens a pathway to exploring intricate phases of matter in cold atom experiments.

Key Findings

The paper successfully observes Fermi polarons—quasiparticles formed when a spin down impurity interacts with a surrounding spin up Fermi sea. This interaction creates a localized distortion around the impurity, much akin to traditional lattice polarons in solid-state physics but occurring here in a fermionic medium. The identification of these polarons is marked by a distinctive narrow peak within the rf spectrum of the spin down impurities, which emerges from a broader incoherent spectrum.

Methodology

The experiment utilizes an ultracold mixture of lithium-6 atoms to create a spin-imbalanced Fermi sea. Through rf spectroscopy, the energies and integral properties of the polarons, such as their quasiparticle residue, are measured as the interaction strength is varied near a Feshbach resonance. The ability to control interactions in this system allows for a precise characterization of the transition from a polaronic state to molecular binding as attraction increases.

Numerical Results

In the critical regime where $1/k_F a \sim 1$, the authors observe a transition from the polaronic regime to a molecular binding state, a signature of phase change from a Fermi liquid to a Bose liquid of bound pairs. Experimentally, the polaron energy was measured to be $E_{\downarrow} = -0.64 \epsilon_F$, closely matching theoretical predictions. The quasiparticle residue $Z$ was determined to be 0.39 at resonance, reflecting a robust polaronic dressing of the impurity.

Moreover, the paper varies the impurity concentration and detects weak interactions between polarons, consistent with the properties of a landau Fermi liquid. Despite strong interactions between the bare impurities and the environment, the polarons exhibited weak mutual interaction, indicating stability in this phase.

Implications

This observation of Fermi polarons not only corroborates theoretical predictions but also provides a quantitative analysis linking polaron physics to broader many-body phenomena. Practically, such insights could inform the design of materials with specific electronic properties and enhance our comprehension of high-$T_C$ superconductors, where polaron dynamics play a significant role.

For future developments, the transition dynamics studied here could inspire explorations into quantum simulators predicated on cold atom platforms. A promising direction would be to examine polarons as components in coherent matter wave systems, potentially contributing to the understanding of not only current many-body problems but also forming a foundational basis for quantum computing mechanisms involving impurity states in ultracold gases.

Conclusion

In summary, the paper presents an impressive experimental achievement of measuring Fermi polarons with tunable interaction strengths and quantifying their physical parameters. The results have substantial implications both theoretically, by affirming conceptual models of quasiparticle dynamics, and practically, suggesting paths for future explorations in quantum simulation and condensed matter physics.