Kondo Lattice Behavior in the Ordered Dilute Magnetic Semiconductor Yb_{14-x}La_xMnSb_{11}

We report Hall, magnetic, heat capacity, and doping studies from single crystals of ${\mathrm{Yb}}_{14}{\mathrm{MnSb}}_{11}$ and ${\mathrm{Yb}}_{13.3}{\mathrm{La}}_{0.7}{\mathrm{MnSb}}_{11}$. These heavily doped semiconducting compounds are ferromagnetic below 53 and 39 K, respectively. The renormalization of the carrier mass from $2{m}_{e}$ near room temperature to $20{m}_{e}$ at 5 K, plus the magnetic evidence for partial screening of the Mn magnetic moments suggest that these compounds represent rare examples of an underscreened Kondo lattice with ${T}_{K}\ensuremath{\approx}\phantom{\rule{0.3em}{0ex}}285\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.