Repulsive Fermi polarons with negative effective mass

Abstract

Recent LENS experiment on a 3D Fermi gas has reported a negative effective mass (m * < 0) of Fermi polarons in the strongly repulsive regime. There naturally arise a question whether the negative m * is a precursor of the instability towards phase separation (or itinerant ferromagnetism). In this work, we make use of the exact solutions to study the ground state and excitation properties of repulsive Fermi polarons in 1D, which can also exhibit a negative m * in the super Tonks-Girardeau regime. By analyzing the total spin, quasi-momentum distribution and pair correlations, we conclude that the negative m * is irrelevant to the instability towards ferromagnetism or phase separation, but rather an intrinsic feature of collective excitations for fermions in the strongly repulsive regime. Surprisingly, for large and negative m * , such excitation is accompanied with a spin density modulation when the majority fermions move closer to the impurity rather than being repelled far away, contrary to the picture of phase separation. These results shed light on the recent observation of negative m * in the 3D repulsive Fermi polarons.

High-precision numerical solution of the Fermi polaron problem and large-order behavior of its diagrammatic series

“…Two self-energy diagrams contributing to the asymptotic large-order behavior given in Eq. (13). One topology is drawn explicitly.…”

“…It turns out that those two diagrams follow the asymptotic behavior of Eq. (13). When fitting the exponential increase in the range N = 24 − 30 for the first dominant diagram, we get R = 0.8782 (15).…”

Repulsive Fermi polarons and their induced interactions in binary mixtures of ultracold atoms

“…These metastable states-that can decay into molecules in two-and three-dimensions (3D)-are of fundamental importance since their existence and longevity offers the possibility of stabilizing repulsive Fermi gases. As a result exotic quantum phases and itinerant ferromagnetism [26][27][28][29][30][31][32][33] could be explored. While for finite impurity mass Fermi polarons constitute well-defined quasiparticles in these higher dimensional systems [34][35][36], the quasiparticle picture is shown to be ill-defined in the thermodynamic limit of one-dimensional (1D) settings [37][38][39].…”

Probing ferromagnetic order in few-fermion correlated spin-flip dynamics

“…Instead, recent pump-probe experiments [9] indicate that the formation rate of ferromagetic domains with a size comparable to the interatomic separation is larger than the corresponding molecular decay rate. Furthermore, ferromagnetic properties have been observed indirectly either by the spectroscopic study of strongly particle-imbalanced [10,11] (supplemented by [12]) and particlebalanced [9] two-component Fermi mixtures or by employing a binary Fermi gas prepared in a magnetic domain wall structure [13]. The latter experimental evidence poses the question whether stable ferromagnetism can be observed in the absence of molecule formation.…”