Polarization of interacting bosons with spin

TL;DR

This paper proves that for interacting bosons with spin, the ground state is always fully polarized without spin-dependent forces, impacting understanding of spinor Bose-Einstein condensates and related magnetic phenomena.

Contribution

It provides a rigorous proof that the ground state of spinful interacting bosons is always fully polarized in the absence of spin-dependent forces, clarifying theoretical debates.

Findings

Ground state is always fully polarized without spin-dependent forces

Degeneracy of polarized ground states depends on particle spin

Magnetization and susceptibility exceed paramagnetic values at T>0

Abstract

We demonstrate rigorously that in the absence of explicit spin-dependent forces one of the ground states of interacting bosons with spin is always fully polarized -- however complicated the many-body interaction potential might be. Depending on the particle spin, the polarized ground state will generally be degenerate with other states, but one can specify the exact degeneracy. For T>0 the magnetization and susceptibility necessarily exceed that of a pure paramagnet. These results are relevant to recent experiments exploring the relation between triplet superconductivity and ferromagnetism, and the Bose-Einstein condensation of atoms with spin. They eliminate the possibility, raised in some theoretical speculations, that the ground state or positive temperature state might be antiferromagnetic.