Eur. Phys. J. D 65, 83-89 (2011)
https://doi.org/10.1140/epjd/e2011-20084-5

Regular Article

Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases

¹ ICFO-Institut de Ciencies Fotoniques, 08860 Castelldefels (Barcelona), Spain
² Física Teorica: Informació i Processos Quàntics, Universitat Autònoma de Barcelona, 08193 Bellaterra, Spain
³ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark

^a e-mail: pietro.massignan@icfo.es

Received: 31 January 2011
Received in final form: 18 May 2011
Published online: 27 July 2011

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.