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 Ce$T_{2}$Cd$_{20}$ ($T$ = Ni,Pd). Our analysis of the heat-capacity data shows that the $\Gamma_7$ doublet is the ground state multiplet of the Ce$^{3+}$ ions. Consequently, for the $\Gamma_7$ doublet it can be theoretically shown that the Ruderman-Kittel-Kasuya-Yosida (RKKY) 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 $\Gamma_8$ quartet. 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.