Theory on Crystal-Field Excitation of Paramagnetic Pr-Ion Systems with Degenerate Ground Levels and Multipolar Exchange Interactions: Origin of Two Spectral Components with Large and Small Dispersions

Abstract

The spectral line shape of inelastic neutron excitation was theoretically studied for paramagnetic Pr ion systems with crystal-field splitting and intersite exchange interactions. It was assumed that the ground level is the doublet E (Γ₃) and the excited level is the triplet T1 (Γ₄) of the cubic point group. The excitation spectra are given by the superposition of two components: One has a relatively strong intensity and a sharp peak with a large dispersion; the other shows a weak and broad peak with a small dispersion. Similar features that have been observed recently in PrMg₃ were explained by the calculation. The former component is ascribed to the usual E→T1 excitation with the dispersion due to the dipole-type exchange interaction, and the latter to a transition accompanied by the simultaneous excitation of fluctuation modes in the manifold at the ground level. Multipolar-type exchange interactions increase the intensity of the latter. The theory was developed based on Mori’s memory function formalism, and a symmetrized form of a standard operator was introduced to treat the multipolar interaction systematically.