Kondo behavior of U in CaB6

Replacing U for Ca in semiconducting CaB6 at a few atomic percent level induces metallic behavior and Kondo-type phenomena at low temperatures, a rather unusual feature for U impurities in metallic hosts. For Ca0.992U0.008B6, the resistance minimum occurs at T≈17K. The subsequent characteristic logarithmic increase of the resistivity with decreasing temperature merges into a T2 dependence below 0.8 K. Correspondingly, the low-temperature specific heat indicates the formation of a ground state with a strongly enhanced electronic specific heat parameter. © 2005 Elsevier B.V. All rights reserved.