Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases

TL;DR

This paper investigates the properties of repulsive polarons in polarized Fermi gases, their stability, decay channels, and implications for itinerant ferromagnetism, providing theoretical insights into quasiparticle behavior and phase separation.

Contribution

It introduces a detailed analysis of repulsive polarons, including their energy, lifetime, and role in ferromagnetic phase formation, highlighting effects of mass imbalance and decay processes.

Findings

Repulsive polarons have well-defined energies and finite lifetimes.

Mass imbalance lowers the critical interaction strength for phase separation.

Short quasiparticle decay times hinder experimental observation of ferromagnetism.

Abstract

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.