Collinear and helical phases of the monoclinic and rhombohedral antiferromagnet. Application to the α- and β-phases of solid oxygen

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Abstract
Classical spins localised on the sites of a rhombohedral lattice and coupled by nearest-neighbour Heisenberg exchange show infinite degeneracy of the ground state; there are infinite inequivalent isoenergetic helices which minimise the classical ground-state energy. This configuration is referred to as a 'degenerate helix' (DH). A genuine DH configuration provides unusual elastic neutron scattering peaks, owing to the infinite inequivalent helices of the ground state. A DH configuration provides the first example of an elastic neutron scattering peak width not determined by the limited instrumental resolution. The rhombohedral Heisenberg antiferromagnet (RAF) model is suitable for describing the beta -phase of solid oxygen which is stable at intermediate temperatures (24-44 K) while in the low-temperature alpha -phase the lattice is stretched and becomes monoclinic. Recent experimental data for elastic and inelastic neutron scattering by polycrystalline samples of solid oxygen in both the alpha - and the beta -phases are satisfactorily interpreted on the basis of the authors' theoretical results.