Repulsive Fermi polarons with negative effective mass

TL;DR

This paper investigates the negative effective mass of Fermi polarons in 1D, showing it is an intrinsic excitation feature unrelated to phase separation or ferromagnetism, and offers an alternative interpretation of recent experimental observations.

Contribution

The study uses exact solutions to demonstrate that negative effective mass in Fermi polarons is an intrinsic collective excitation property, not a precursor to phase separation or ferromagnetism.

Findings

Negative m* is an intrinsic excitation feature, not linked to instability.

Large negative m* correlates with spin density modulation.

Results provide an alternative interpretation of recent experimental data.

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 suggest an alternative interpretation of negative $m^*$ as observed in recent LENS experiment.