Kondo Physics in a Rare Earth Ion with Well Localized 4f Electrons

TL;DR

This study reports unexpected Kondo behavior in Nd-doped LaB$_6$, likely due to quadrupolar degrees of freedom of Nd's 4f electrons, despite initial expectations of no Kondo coupling.

Contribution

It reveals a novel Kondo effect mediated by 4f quadrupolar degrees of freedom in a well-localized rare earth ion, challenging conventional understanding.

Findings

Kondo features observed at T_K ≲ 0.3 K in Nd-doped LaB$_6$

Unexpected Kondo effect linked to 4f quadrupolar degrees of freedom

Contradicts prior assumptions about non-Kondo coupling in such systems

Abstract

Dilute Nd in simple cubic LaB$_6$ shows electrical resistance and specific heat features at low temperature consistent with a Kondo scale of $T_K \lesssim$ 0.3 K. Nd has a well localized {\it 4f}$^{3}$ {\it J} = 9/2 Hund's Rule configuration which is not anticipated 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.