Probing itinerant ferromagnetism with ultracold quantum gases

Nearly Ferromagnetic Phases in Ultracold Fermi Gases: Simulations of $^3$ HeliumNearly Ferromagnetic Phases in Ultracold Fermi Gases: Simulations of $^3$ Helium

Kathryn Levin, Chih-Chun Chien, Hao Guo

Bulletin of the American Physical SocietyMar 19, 20100 citations

Phase-ordering dynamics in itinerant quantum ferromagnets

Ronojoy Saha, D. Belitz, T. R. Kirkpatrick

Bulletin of the American Physical SocietyMar 8, 20070 citations

Exchange interactions and itinerant ferromagnetism in ultracold Fermi gases

E. Vermeyen, Carlos A. R. Sá de Melo, Jacques Tempere

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.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.

arXiv:1511.04502Physical Review AAug 31, 20188 citations

Probing a quantum emulsion

Jelena Stajic

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).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).

ScienceJan 30, 20200 citations

Nearly Ferromagnetic Phases in Ultracold Fermi Gases: Simulations of

^3

Helium