Exploring emergent heterogeneous phases in strongly repulsive Fermi gases

TL;DR

This paper investigates the complex behavior of strongly repulsive ultracold Fermi gases, revealing a heterogeneous phase with coexisting paired and unpaired fermions, and explores the properties of this emergent quantum emulsion state.

Contribution

It provides a detailed analysis of the low-temperature heterogeneous phase in strongly repulsive Fermi gases, highlighting the coexistence and micro-scale phase separation of pairs and unpaired fermions.

Findings

Identification of a low-temperature heterogeneous phase

Observation of macroscopic coexistence of paired and unpaired fermions

Evidence of micro-scale phase separation

Abstract

Recent experiments have revitalized the interest in a Fermi gas of ultracold atoms with strong repulsive interactions. In spite of its seeming simplicity, this system exhibits a complex behavior, resulting from the competing action of two distinct instabilities: ferromagnetism, which promotes spin anticorrelations and domain formation; and pairing, that renders the repulsive fermionic atoms unstable towards forming weakly bound bosonic molecules. The breakdown of the homogeneous repulsive Fermi liquid arising from such concurrent mechanisms has been recently observed in real time through pump-probe spectroscopic techniques [A. Amico et al., Phys. Rev. Lett. 121, 253602 (2018)]. These studies also lead to the discovery of an emergent metastable many-body state, an unpredicted quantum emulsion of anticorrelated fermions and pairs. Here, we investigate in detail the properties of such an exotic regime by studying the evolution of kinetic and release energies, the spectral response and coherence of the unpaired fermionic population, and its spin-density noise correlations. All our observations consistently point to a low-temperature heterogeneous phase, where paired and unpaired fermions macroscopically coexist while featuring micro-scale phase separation. Our findings open new appealing avenues for the exploration of quantum emulsions and also possibly of inhomogeneous superfluid regimes, where pair condensation may coexist with magnetic order.