Theoretical model for the low-temperature ordering of Pr in PrBa2Cu3O6+x

Theoretical model is presented to describe the anomalous ordered phase of Pr ions in PrBa₂Cu₃O_6+x below T_Pr≈12-17K . The model considers the Pr multipole degrees of freedom and coupling between the Cu and Pr subsystems. We identify the symmetry allowed coupling of Cu and Pr ions and conclude that only an ab -plane Pr dipole ordering can explain the Cu spin rotation observed at T_Pr by neutron diffraction by Boothroyd [Phys. Rev. Lett. 78, 130 (1997)]10.1103/PhysRevLett.78.130. A substantial enhancement of the Pr ordering temperature is shown to arise from the Cu-Pr coupling which is the key for the anomalous magnetic behavior in PrBa₂Cu₃O_6+x .