Kondo Physics in a Rare Earth Ion with well localized {\it 4f } electrons*

We observe a rise in the low temperature resistivity of dilute La$_{1-x}$Nd$_x$B$_6$ single crystals. The specific heat data for the same samples show that there is an entropy of $R$ln4 per mole Nd involved in the excess specific heat above that of LaB$_6$. All these features are consistent with a Kondo scale of $T_K$ = 1 K. However, Nd has a well localized {\it 4f }$^{ 3}$ {\it J} = 9/2 Hund's Rule configuration which is not expected to be Kondo coupled to the conduction electrons in LaB$_6$. We conjecture that the unexpected Kondo effect arises via participation of {\it 4f} quadrupolar degrees of freedom of the Nd crystal field ground state quartet. Raman experiments as well as detailed theoretical studies do indicate that one expects strong quadrupolar influence in the properties of NdB$_6$.