Symmetry changes at the Néel point in the high-Tc superconductor YbBa2Cu3O7

A Landau-type theory has been applied to the description of the TN=330±20 mK magnetic phase transition in the high temperature superconductor YbBa2Cu3O7. It has been demonstrated that the magnetic order parameter is one-dimensional and that the magnetic ordering has a three-dimensional Ising character. All possible (symmetry allowable) low-temperature spin structures were subsequently determined. They are antiferromagnetic, with the magnetic moments directed along the basis vectors of the Bravais lattice and they exactly fulfil the non-linear differential equations which follow from the variational problem for the thermodynamic potential density. One of the obtained spin structures is identical to that for which B. Roessli et al. obtained the best fit to the experimental diffraction pattern. The domain structure of the superconductor was subsequently determined on the grounds of Landau symmetry considerations.