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Science

Probing a quantum emulsion

Jelena Stajic

Published Jan 30, 2020• 0 citations
Publisher

Abstract

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).

DOI
10.1126/science.367.6477.523-f

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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).
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.
Physics Pump-probe techniques, in which a short light pulse knocks the system out of equilibrium and another pulse is then used to monitor the dynamics, are commonplace in solid-state physics. Amico et al. used such a technique in a fermionic atomic gas to study what happens when the gas is plunged into a regime where the atoms strongly repel one another. The nature of this regime in an even mixture of two spin states has been debated—a ferromagnetic state was expected, but instead the atoms had a tendency to form pairs. Here, the researchers disentangled the two pathways and found that correlations consistent with ferromagnetism initially increased faster than the pairs formed, but neither clearly dominated over the other. Phys. Rev. Lett. 121 , 253602 (2018).
In the 1930s, two main paradigms for the theoretical description of ferromagnetism were developed: Heisenberg ferromagnetism of localized fermions (e.g., in a lattice), and Bloch or Stoner ferromagnetism of nonlocalized fermions (i.e., in a gas), also called itinerant ferromagnetism. Despite many theoretical predictions, itinerant ferromagnetism has remained elusive in experiments. This ferromagnetic state is predicted to occur for strong repulsive interactions, corresponding to a regime that is very challenging to describe theoretically because there are multiple competing physical effects, including superfluid pairing. In this paper, we point out that the problem of itinerant ferromagnetism for atomic Fermi gases is different from that of electron gases in metals due to the short-range nature of the interatomic interactions. We also show that the standard saddle point used to describe itinerant ferromagnetism of the electron gas in metals does not apply, because in the short-range limit of this approximation the Pauli exclusion principle is violated. As a remedy, we introduce a modified interaction pseudopotential for ultracold gases which includes both local (Hartree) and nonlocal (Fock) terms while preserving the Pauli exclusion principle in the short-range regime. Furthermore, we demonstrate the usefulness of this method to study the existence and stability of itinerant ferromagnetism in ultracold atomic gases. Lastly, we obtain the critical temperature for the ferromagnetic transition as a function of the opposite-spin interaction strength and find a rather good agreement with recent experimental results.
Cold Atom Magnetism Magnetic ordering arises from the strong interactions between atoms, with its origins deeply rooted in quantum mechanics. How the ordering comes about, however, has long been a topic of debate because most condensed-matter systems are limited by a somewhat fixed parameter space. Cold atom systems, by comparison, provide the ability to tune the magnitude and sign of the atom-atom interaction, as well as the density. Jo et al. (p. 1521 ; see the Perspective by Zwerger ) exploit this flexibility to use an ensemble of ultracold fermionic atoms as a “quantum simulator” to explore the possibility of magnetic ordering. As the repulsive interaction between atoms is increased, an instability occurs in the free two-component Fermi gas (or jellium), which results in a phase transition and the ferromagnetic ordering of the atoms.
Regular pattern formation is ubiquitous in nature; it occurs in biological, physical, and materials science systems. Here we propose a set of experiments with ultracold atoms that show how to examine different types of pattern formation. In particular, we show how one can see the analog of labyrinthine patterns (so-called quantum emulsions) in mixtures of light and heavy atoms (that tend to phase separate) by tuning the trap potential and we show how complex geometrically ordered patterns emerge (when the mixtures do not phase separate), which could be employed for low-temperature thermometry. The complex physical mechanisms for the pattern formation at zero temperature are understood within a theoretical analysis called the local density approximation.