A comprehensive study of chemical substitution effects on superconductivity in LaPt4Ge12*

The compound PrPt$_4$Ge$_{12}$ has attracted significant attention following observations of signatures of unconventional superconductivity such as time reversal symmetry breaking from $\mu$SR measurements. In contrast, LaPt$_4$Ge$_{12}$ is a conventional BCS-type superconductor, interestingly, with the same superconducting transition temperature, $T_c$, as PrPt$_4$Ge$_{12}$ ($T_c$ = 8 K). To elucidate the properties of superconductivity in PrPt$_4$Ge$_{12}$, the system La$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$ was investigated and the results are compared to our previous work on Pr$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$. Measurements of magnetic susceptibility, electrical resistivity, and specific heat were performed demonstrating that $T_c$ is suppressed more rapidly in La$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$ than in Pr$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$. Specific heat measurements reveal a crossover in the temperature dependence of the superconducting state of La$_{1-x}$Ce$_x$Pt$_4$Ge$_{12}$, changing from a power law for $x$ = 0 to an exponential for the Ce-substituted samples, possible evidence of a transition from a multiband to a single-band superconducting energy gap. Th substitution for La did not produce the crossover.