Singlet-triplet model of cuprate magnetism

A model for describing the spin subsystem of cuprates within a model of polar singlet-triplet Jahn-Teller centers is proposed. In this model spin ordering is described by two vector order parameters S (the total spin of a CuO4 cluster) and V (the operator of the change in spin multiplicity per cluster). It is shown within a modified mean-field approximation that the formation of a noncollinear magnetic structure characterized by an antiferromagnetic wave vector, which, however, has a nontrivial temperature dependence of the order parameters, is possible along with the formation of ordinary collinear structures of the (anti)ferromagnetic type. The temperature dependences of the order parameters and the principal equilibrium thermodynamic characteristics, viz., the static susceptibility and the specific heat, are obtained.