Soft Modes and Mode Splitting in Paramagnetic TbP

In contrast to spin-only systems the paramagnetic phases of systems with crystal field split ground multiplets exhibit magnetic excitations of finite energy. In the presence of a magnetic exchange coupling the excitons will propagate through the lattice. A measurement of the dispersion allows in principle an insight into the exchange interaction, which is of fundamental importance for the understanding of the magnetic phenomena.

Unfortunately the evolution of the dispersion at low temperatures is in most cases interrupted by the onset of magnetic order due to an alignment of the magnetic moments of the crystal field ground state. A noticeable dispersion of well-defined excitons is only expected in systems with non-magnetic ground states if in addition the exchange is about equal to the crystal field energies1.

This criterion is very restrictive and only a few systems are known in the literature which meet the condition.