Temperature Dependence of Magnetic Excitons in Singlet‐Triplet Systems

Recent observations1 indicate that the magnetic exciton energies in the singlet ground state systems fcc Pr and Pr3Tl are nearly temperature independent up to several times the ferromagnetic ordering temperature. Since the lowest excited crystal field state is a triplet, assuming that higher levels can be ignored one can describe these systems in terms of two pseudospins 1/2 on each site2. We have investigated the temperature dependence of the exciton energies using a decoupling scheme for the pseudospin equations of motion which consistently retains pair correlations, including those between pseudospin operators on adjacent sites. Numerical calculations have been performed using this scheme and a boson formalism for the correlation functions. The results are in agreement with experiment in that the exciton energies do not show any appreciable temperature dependence up to about twice the ordering temperature. The physical reason for this behavior is that kBTc is small compared to a typical exciton energy,...