Vanishing RKKY interactions in Ce-based cage compounds.

We report the results of thermodynamic measurements in external magnetic field of the cubic Ce-based cage compounds CeCd(= Ni,Pd). Our analysis of the heat-capacity data shows that the Γdoublet is the ground state multiplet of the Ceions. Consequently, for the Γdoublet it can be theoretically shown that the Ruderman-Kittel-Kasuya-Yosida interaction between the localized Ce moments mediated by the conduction electrons, must vanish at temperatures much lower than the energy separating the ground state doublet from the first excited Γquartet. Our findings provide an insight as to why no long range order has been observed in these compounds down to temperatures in the milliKelvin range.