Stoner ferromagnetism in a thermal pseudospin-1/2 Bose gas

TL;DR

This paper analyzes the finite-temperature phase diagram of a pseudospin-1/2 Bose gas, revealing conditions for ferromagnetic order and exploring the potential for bosonic analogs of electronic phases like Cooper pairing.

Contribution

It introduces a theoretical framework combining RPA and Hartree-Fock methods to predict magnetic phases in spinor Bose gases, including bosonic analogs of Stoner ferromagnetism and Cooper pairing.

Findings

Normal ferromagnetic phases appear above superfluid transition.

Easy-axis/easy-plane ferromagnetism depends on inter-species interaction strength.

Stripe order is not favored in the normal phase within the approximations used.

Abstract

We compute the finite-temperature phase diagram of a pseudospin-$1/2$ Bose gas with contact interactions, using two complementary methods: the random phase approximation (RPA) and self-consistent Hartree-Fock theory. We show that the inter-spin interactions, which break the (pseudo) spin-rotational symmetry of the Hamiltonian, generally lead to the appearance of a magnetically ordered phase at temperatures above the superfluid transition. In three dimensions, we predict a normal easy-axis/easy-plane ferromagnet for sufficiently strong repulsive/attractive inter-species interactions respectively. The normal easy-axis ferromagnet is the bosonic analog of Stoner ferromagnetism known in electronic systems. For the case of inter-spin attraction, we also discuss the possibility of a \textit{bosonic} analog of the Cooper paired phase. This state is shown to significantly lose in energy to the transverse ferromagnet in three dimensions, but is more energetically competitive in lower dimensions. Extending our calculations to a spin-orbit-coupled Bose gas with equal Rashba and Dresselhaus-type couplings (as recently realized in experiment), we investigate the possibility of stripe ordering in the normal phase. Within our approximations however, we do not find an instability towards stripe formation, suggesting that the stripe order melts below the condensation temperature, which is consistent with the experimental observations of Ji \textit{et al.} [Ji \textit{et al.}, Nature Physics \textbf{10}, 314 (2014)].