LaOs4Sb12 and LaRu4Sb12, a contrast with the unconventional behavior of PrOs4Sb12

Recent measurements of the penetration depth $\lambda ({\rm T})$ in single crystals of PrOs$_{4}$Sb$_{12}$ have revealed unconventional behavior in the order symmetry of the superconducting energy gap$^{[1]}$. We present, for contrast, high precision measurements undertaken of LaOs$_{4}$Sb$_{12}$ and LaRu$_{4}$Sb$_{12}$, under analogous conditions, where we observe fully gapped behavior. These results show that the substitution of La for Pr yields similar behavior as that seen in the fully gapped superconductor PrRu$_{4}$Sb$_{12} \quad ^{[2] }$These materials appear to have SC electron densities, $\rho $, characteristic of s-wave, BCS superconductors with the data exhibiting a fit to $\rho $ of exponential at low temperature. All measurements were done at temperatures from 0.1K to 0.9K for LaOs$_{4}$Sb$_{12}$, and to 3.8K for LaRu$_{4}$Sb$_{12}$ using a self-inductive oscillator technique. Fits of $\rho $ were made assuming specific heat jumps of 1.74$\gamma $T$_{c}$ and 1.81$\gamma $T$_{c}$ respectively with T$_{c}$'s of 0.76K and 3.6K respectively. Measurements have been performed on Pr(Os$_{1-x}$Ru$_{x})_{4}$Sb$_{12} \quad ^{[3]}$ to map it's transition from fully gapped to unconventional behavior, and so we propose future work of mapping a similar transition in a series of Pr$_{1-x}$La$_{x}$Os$_{4}$Sb$_{12}$ materials. [1]Elbert E.M. Chia \underline {\textit{et al.}}, Phys. Rev. Lett. 91, 247003 (2003) [2] Elbert E.M. Chia \underline {\textit{et al.}}, Phys. Rev. B 69 180509(R) (2004) [3] Elbert E.M. Chia \underline {\textit{et al.}}, condmat/0411395