Femtosecond time-resolved optical spectroscopy was used to systematically study photoexcited carrier relaxation dynamics in the intermediate-valence heavy-fermion system ${\text{Yb}}_{1-x}{\text{Lu}}_x{\text{Al}}_3$ $\left(0\le x\le 1\right)$. Given the demonstrated sensitivity of this experimental technique to the presence of the low-energy gaps in the charge excitation spectrum, the aim of this work was to study the effect of dilution of the Kondo lattice on its low-energy electronic structure. The results imply that in ${\text{Yb}}_{1-x}{\text{Lu}}_x{\text{Al}}_3$ the hybridization gap, resulting from hybridization of local moments and conduction electrons, persists up to 30% doping. Interestingly, below some characteristic, doping dependent temperature $T^{\ast }\left(x\right)$ the relaxation-time divergence, governed by the relaxation bottleneck due to the presence of the indirect hybridization gap, is truncated. This observation is attributed to the competing ballistic transport of hot electrons out of the probed volume at low temperatures. The derived theoretical model accounts for both the functional form of relaxation dynamics below $T^{\ast }\left(x\right)$, as well as the doping dependence of the low-temperature relaxation rate in ${\text{Yb}}_{1-x}{\text{Lu}}_x{\text{Al}}_3$.

• Received 4 August 2009

DOI:https://doi.org/10.1103/PhysRevB.80.085121