Magnons in a Model with Antiferromagnetic Properties.
The symmetry properties of the photon are different from those expected at first sight for a Goldstone particle belonging to a degeneracy of the ground state with respect to the isospin group. But the violation of this group in elementary-particle physics is of a very special nature; the remaining invariances are a rotation around an axis and the C-conjugation (replacing the G-conjugation). In order to study the symmetry of the Goldstone particles belonging to this special form of symmetry breaking, a model of solid-state physics with antiferromagnetic properties is investigated. The violation of symmetry in such a model resembles that in elementary-particle physics, while in a conventional ferromagnet there is no analogue to the G- or C-conjugation. The resulting Goldstone particles of the model do have the same symmetry as the photons in isospace. The properties of the photons with respect to the homogeneous part of the Lorentz group have no counterpart in the model and are discussed in connexion with earlier investigations of Bjørken, Higgs and Kibble.
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