Kondo Behavior of U in CaB$_6$

TL;DR

This study reports the observation of Kondo behavior in uranium-doped CaB$_6$, showing metallicity and characteristic low-temperature phenomena, unlike in LaB$_6$, highlighting the unique impurity-host interactions.

Contribution

It demonstrates for the first time Kondo phenomena induced by U impurities in a semiconducting host, CaB$_6$, and compares it with LaB$_6$ to show the specificity of the effect.

Findings

Resistance minimum at 17 K in U-doped CaB$_6$

Logarithmic increase of resistivity below 17 K

Kondo behavior absent in U-doped LaB$_6$ above 0.4 K

Abstract

Replacing U for Ca in semiconducting CaB$_6$ at the few at.% level induces metallic behaviour and Kondo-type phenomena at low temperatures, a rather unusual feature for U impurities in metallic hosts. For Ca$_{0.992}$U$_{0.008}$B$_6$, the resistance minimum occurs at $T$ = 17 K. The subsequent characteristic logarithmic increase of the resistivity with decreasing temperature merges into the expected $T^2$ dependence below 0.8 K. Data of the low-temperature specific heat and the magnetization are analyzed by employing a simple resonance-level model. Analogous measurements on LaB$_6$ with a small amount of U revealed no traces of Kondo behavior, above 0.4 K.