Nature of heavy-fermion states arising in the vicinity of an isolated cerium or holmium magnetic impurity in LaB$_6$

The behavior of the electrical resistivity and magnetoresistance of lanthanum hexaboride with isolated (${\sim}\,1\,\%$) Ce or Ho magnetic impurities has been studied. It has been shown that the low-temperature growth of the resistivity is characteristic of the weak localization regime for charge carriers rather than of the Kondo effect. The negative magnetoresistance observed in Сe$_x$La$_{1-x}$B$_6$ и Ho$_x$La$_{1-x}$B$_6$ at liquid helium temperatures also cannot be interpreted in terms of the Kondo model and corresponds to the formation of many-body states of a spin-polaron type near magnetic rare-earth ions in LaB$_6$.