Superconductivity in the presence of disorder in skutterudite-related La${}_3$Co${}_4$Sn${}_{13}$ and La${}_3$Ru${}_4$Sn${}_{13}$ compounds; electrical transport and magnetic studies

La${}_3$Co${}_4$Sn${}_{13}$ and La${}_3$Ru${}_4$Sn${}_{13}$ were categorized as BCS superconductors. In a plot of the critical field $H_{c2}$ vs $T$, La${}_3$Ru${}_4$Sn${}_{13}$ displays a second superconducting phase at the higher critical temperature $T_c^{\star }$, characteristic of inhomogeneous superconductors, while La${}_3$Co${}_4$Sn${}_{13}$ shows bulk superconductivity below $T_c$. We observe a decrease in critical temperatures with external pressure and magnetic field for both compounds with $\frac{dT_c^{\star}}{dP} > \frac{dT_c}{dP}$\mathrm{.}Thepressuredependencesof$T_c$ are interpreted according to the McMillan theory and understood to be a consequence of lattice stiffening. The investigation of the superconducting state of La${}_3$Co${}_x$Ru${}_{4-x}$Sn${}_{13}$ shows a $T_c^{\star }$ that is larger then $T_c$ for $x<4$. This unique and unexpected observation is discussed as a result of the local disorder and/or the effect of chemical pressure when Ru atoms are partially replaced by smaller Co atoms.