Superfluid vs Ferromagnetic Behaviour in a Bose Gas of Spin-1/2 Atoms

TL;DR

This paper investigates the phase transitions in a spin-1/2 Bose gas, revealing how inter-component interactions influence the coexistence and order of superfluid and ferromagnetic phases.

Contribution

It demonstrates the conditions under which superfluid and ferromagnetic transitions coincide or differ in temperature, and discusses the impact of a condensate acting as an effective magnetic field.

Findings

Superfluid and ferromagnetic transitions occur simultaneously at certain interaction strengths.

Strongly attractive interactions cause ferromagnetic transition to precede superfluid transition.

Condensate presence polarizes the normal cloud, acting as an effective magnetic field.

Abstract

We study the thermodynamic phases of a gas of spin-1/2 atoms in the Hartree-Fock approximation. Our main result is that, for repulsive or weakly-attractive inter-component interaction strength, the superfluid and ferromagnetic phase transitions occur at the same temperature. For strongly-attractive inter-component interaction strength, however, the ferromagnetic phase transition occurs at a higher temperature than the superfluid phase transition. We also find that the presence of a condensate acts as an effective magnetic field that polarizes the normal cloud. We finally comment on the validity of the Hartree-Fock approximation in describing different phenomena in this system.