Attractive Polaron Research Articles

Repulsive polarons found

Quasiparticles known as repulsive polarons are predicted to occur when 'impurity' fermionic particles interact repulsively with a fermionic environment. They have now been detected in two widely differing systems. See Letters p.615 & p.619 Metastable states in Fermi gases with strong repulsive interactions are of fundamental interest, but the realization of such systems is challenging because they are intrinsically unstable against decay. Two groups have overcome this obstacle and report the detection of the theoretically predicted repulsive Fermi polarons. Kohstall et al. study a three-dimensional system of ultracold potassium impurities resonantly interacting with a Fermi sea of lithium atoms. The character of the interaction stabilizes the repulsive regime, enabling the authors to detect long-lived, metastable repulsive polarons. Koschorreck et al. study both attractive and repulsive Fermi polarons in a two-dimensional, spin-imbalanced Fermi gas of potassium atoms, and find evidence for a pairing transition. The results from these two studies hold promise for the creation of exotic states with ultracold fermionic atoms, such as ferromagnetic quantum phases.

Fermi polarons in two dimensions

We theoretically analyze inverse radio-frequency (rf) spectroscopy experiments in two-component Fermi gases. We consider a small number of impurity atoms interacting strongly with a bath of majority atoms. In two-dimensional geometries we find that the main features of the rf spectrum correspond to an attractive polaron and a metastable repulsive polaron. Our results suggest that the attractive polaron has been observed in a recent experiment [B. Fr\"ohlich et al., Phys. Rev. Lett. 106, 105301 (2011)].

Excitation spectra and rf response near the polaron-to-molecule transition from the functional renormalization group

A light impurity in a Fermi sea undergoes a transition from a polaron to a molecule for increasing interaction. We develop a new method to compute the spectral functions of the polaron and molecule in a unified framework based on the functional renormalization group with full self-energy feedback. We discuss the energy spectra and decay widths of the attractive and repulsive polaron branches as well as the molecular bound state and confirm the scaling of the excited state decay rate near the transition. The quasi-particle weight of the polaron shifts from the attractive to the repulsive branch across the transition, while the molecular bound state has a very small residue characteristic for a composite particle. We propose an experimental procedure to measure the repulsive branch in a Li6 Fermi gas using rf-spectroscopy and calculate the corresponding spectra.