Eli Eisenberg, Elliott H. Lieb
Polarization of interacting bosons with spin
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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.