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.
Highlights
In strongly correlated electron systems, such as transitionmetal oxides and heavy fermion compounds, the simultaneous presence of multiple interaction mechanisms and the concurrence of distinct competing instabilities can lead to the spontaneous emergence of spatially inhomogeneous states
Convincing signatures for a ferromagnetic instability within the repulsive Fermi liquid have been obtained through studies of spin dynamics [14] and time-resolved quasiparticle spectroscopy [15,16], already early experiments [13,17] found the system dynamics to be fundamentally affected by another type of instability, antithetical to ferromagnetism and associated with the tendency of repulsive fermions to combine into weakly bound pairs
Rabi oscillation measurements unveiled in real time the breakdown of the homogeneous repulsive Fermi liquid, in favor of a rapidly developed heterogeneous phase composed of molecular and spin-polarized atomic microdomains [16]
Summary
In strongly correlated electron systems, such as transitionmetal oxides and heavy fermion compounds, the simultaneous presence of multiple interaction mechanisms and the concurrence of distinct competing instabilities can lead to the spontaneous emergence of spatially inhomogeneous states These are characterized by nanometer-scale structures hosting different phases and order parameters [1,2]. A natural framework to understand this state is a quantum emulsion, where pairs and unpaired fermions macroscopically coexist while featuring phase segregation at the microscale of few interparticle spacings [16] While this complex behavior undermines the prospects for the realization of the basic Stoner model with ultracold atoms, it connects the fate of repulsive Fermi gases to other intriguing instances of highly correlated fermionic matter, and it makes their exploration highly relevant per se.
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