Hybridization-mediated anisotropic coupling and excitation behavior in cerium monopnictides

The neutron scattering measurements of Rossat-Mignod et al. on CeBi in the type I antiferromagnetic phase give unusual excitation behavior. The dispersion is anisotropic, being very flat for q∥k (the propagation vector of the structure), and showing a minimum at the zone boundary for q⊥k. Previously, the equilibrium behavior of CeBi has been explained using an interionic interaction arising from band-f electron hybridization (Coqblin–Schrieffer interaction) as mediated by the band electrons. The resulting interaction is very anisotropic, giving strong coupling within planes, with a much weaker coupling between planes transverse to a saturated moment. Using the same theory, we find that calculations of the excitation spectrum in the RPA at T=0 for a set of parameters giving equilibrium behavior similar to experiment, give results that compare well with the experimental excitation behavior of CeBi. The dispersionless behavior for q∥k and the minimum in energy at the zone boundary for q⊥k are both reproduced by our theory.