Electron spin resonance in antiferro-quadrupolar-ordered CeB6

CeB6 is a cubic heavy-fermion compound with a Gamma(8) ground quartet for which an ESR signal was observed. All other Ce or Yb compounds displaying an ESR signal have strong magnetic anisotropy and ferromagnetic correlations among the spins. The role of the ferromagnetic correlations is to narrow the resonance width rendering the signal observable. In CeB6 the orbital content of the Gamma(8) quartet gives rise to an antiferro-quadrupolar-ordered phase below 4 K. Single ions with a Gamma(8) ground multiplet are expected to display four transitions, however, only one has been observed. The following questions are addressed in this paper: (1) why is only one transition seen, (2) why is this transition observed if the Kondo temperature is comparable to the linewidth and the resonance frequency, and (3) are there ferromagnetic correlations between the Ce ions? The answer to these questions is associated with the antiferro-quadrupolar order. While for other Ce and Yb compounds with ESR signal it is difficult to distinguish if the resonance is due to localized spins or conducting heavy electron spins, an itinerant picture within the antiferro-quadrupolar phase is necessary for CeB6.