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arXiv:1910.14279 · Physical Review A

Exploring emergent heterogeneous phases in strongly repulsive Fermi gases

Francesco Scazza, Giacomo Valtolina, Andrea Amico, P. E. S. Tavares, M. Inguscio, Wolfgang Ketterle, G. Roati, Matteo Zaccanti

Published Jan 8, 2020• 13 citations
arXivHTMLPDF

Abstract

An experimental study of a Fermi gas of ultracold atoms with strong repulsive interactions leads to the discovery of an emulsion state of anticorrelated fermions and pairs. This finding allows exploration of quantum emulsions and also possibly of inhomogeneous superfluid regimes.
DOI
10.1103/physreva.101.013603

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Ultracold atomic Fermi gases close to Feshbach resonances represent the ideal platform to study strongly correlated fermionic states [1]. Up to now, most of the experimental efforts have been dedicated to the study of the famous BEC-BCS crossover, i.e. the transition from a Bose-Einstein condensate of tightly bound molecules to a BCS superfluid of long-range fermion pairs. However, such fermionic gases are the ideal environment to explore also the effects of strong, repulsive interactions. The interest in studying repulsive Fermi gases is manifold, since many phase transitions are driven by the strong repulsion between electrons in solids. One notable example is represented by the onset of itinerant ferromagnetism in metals, as envisioned by Stoner almost ninety years ago.
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Quantum Gases Ultracold atomic gases with strongly repulsive interactions can be a useful model system for studying the emergence of itinerant ferromagnetism. However, the same atoms that can order magnetically are also prone to pairing into loosely bound molecules. If an atomic system is suddenly placed in a strongly interacting regime, both processes occur and an inhomogeneous, phase-separated state consisting of unpaired and paired atoms can form. Scazza et al. studied this state with a variety of probes. The data were consistent with the formation of a quantum emulsion consisting of polarized domains coexisting with paired atoms. The state was stabilized by low temperatures and strong interactions. Phys. Rev. A 101 , 013603 (2020).
We exploit a time-resolved pump-probe spectroscopic technique to study the out-of-equilibrium dynamics of an ultracold two-component Fermi gas, selectively quenched to strong repulsion along the upper branch of a broad Feshbach resonance. For critical interactions, we find the rapid growth of short-range anticorrelations between repulsive fermions to initially overcome concurrent pairing processes. At longer evolution times, these two competing mechanisms appear to macroscopically coexist in a short-range correlated state of fermions and pairs, unforeseen thus far. Our work provides fundamental insights into the fate of a repulsive Fermi gas, and offers new perspectives towards the exploration of complex dynamical regimes of fermionic matter.