Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases

We analyze the properties of a single impurity immersed in a Fermi sea. At positive energy and scattering lengths, we show that the system possesses a well-defined but metastable excitation, the repulsive polaron, and we calculate its energy, quasiparticle residue and effective mass. From a thermodynamic argument we obtain the number of particles in the dressing cloud, illustrating the repulsive character of the polaron. Identifying the important 2- and 3-body decay channels, we furthermore calculate the lifetime of the repulsive polaron. The stability conditions for the formation of fully spin polarized (ferromagnetic) domains are then examined for a binary mixture of atoms with a general mass ratio. Our results indicate that mass imbalance lowers the critical interaction strength for phase-separation, but that very short quasiparticle decay times will complicate the experimental observation of itinerant ferromagnetism. Finally, we present the spectral function of the impurity for various coupling strengths and momenta.Comment: Substantial improvements to the section describing quasiparticle decays (included a discussion of two-body and three-body processes), and to the criteria for the stability of the itinerant ferromagnetic phas

Fermi-polaron-like effects in a one-dimensional (1D) optical lattice

We study a highly imbalanced Fermi gas in a one-dimensional (1D) optical lattice from the polaronic point of view. The time-evolving block decimation algorithm is used to calculate the ground state and dynamics of the system. We find polaronic behaviour qualitatively similar to that in the recent experiment by Schirotzek et al (2009 Phys. Rev. Lett. 102 230402), where radio-frequency (rf) spectroscopy was used to observe polarons in 3D space. In the weakly interacting limit, our exact results are in excellent agreement with a polaron ansatz, and in the strongly interacting limit, the results match with an approximative solution of the Bethe ansatz (BA), suggesting crossover from a quasiparticle to a charge-density excitation regime.Peer reviewe