Repulsive polarons and itinerant ferromagnetism in strongly polarized Fermi gases

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.

Metastability and coherence of repulsive polarons in a strongly interacting Fermi mixture

“…The theoretical curves are based on an extension of an approach presented in Refs. [12,23] to our case of a relatively narrow Feshbach resonance with κ F R * = 0.95 and thus a considerable effective range of the interaction (see Supplementary Information).…”

“…The properties of the repulsive polaron, which is intrinsically unstable due to the presence of the molecule-hole continuum (MHC) and of the attractive polaron, are obtained from the self energy. In particular, the interaction induced energy shift and the decay rate are given by the real part and twice the imaginary part of the self energy, respectively [12].…”

“…Previous treatments [12,23,24,27,[35][36][37] were based on a universal scattering amplitude, describing broad Feshbach resonances. To include effects of the finite effective range we employ a many-body T-matrix given by [38,39] an energy-dependent length parameter, with a bg , ∆B, B 0 , and δ µ being the background scattering length, the width, the center, and the relative magnetic moment of the Feshbach resonance.…”

“…In the population-imbalanced case, quasiparticles coined Fermi polarons are the essential building blocks and have been studied in detail experimentally [16] for attractive interactions. Recent theoretical work [9,12,13] has suggested a novel quasiparticle associated with repulsive interactions. The properties of this repulsive polaron are of fundamental importance for the prospects of repulsive many-body states.…”

Theory of excitation of Rydberg polarons in an atomic quantum gas

“…In fact, wave functions of the type in Eq. (7) were found to yield a rather accurate description of impurities coupled to a fermionic bath of cold atoms via Feshbach resonances [29][30][31][32][33] and also describe exciton impurities in two-dimensional semiconductors [34]. When truncated at the one-excitation level, such an ansatz leads to limited agreement with experimental observations for impurities in a Bose gas [35,36].…”