Magnetic excitations in the quadrupolar ordered CeB₆.

Cerium hexaboride is a textbook example of an f-electron system hosting an exotic antiferroquadrupolar order. Even after decades of intense studies, it still lacks a complete theoretical description, and experiments continue to present puzzles in the form of unexpected observations. In recent years, we have scrutinized the magnetic excitation spectrum of CeB₆ and its doped derivatives such as Ce_1-xLa_xB₆ and Ce_1-xNd_xB₆. In this symposium on f-electron materials, I would like to present our most recent results with a focus on magnetic-field and doping dependence of spin correlations and collective magnon excitations in these systems. We find clear signatures of long-range RKKY interactions between the f-electron multipoles in the diffuse quasielastic response in zero magnetic field that sensitively depends on charge doping. Further, field-induced collective magnon modes have been mapped out in fields up to 14.5 T applied along different crystallographic directions, revealing new high-field magnetic excitations that were not previously detected in electron spin resonance (ESR) or any other probes. I will also discuss the current progress towards the theoretical understanding of these results.