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Superconductivity of La$_3$Co$_4$Sn$_{13}$ and La$_3$Rh$_4$Sn$_{13}$: A comparative study

We report the electric transport and thermodynamic properties of the skutterudite-related La$_3$Co$_4$Sn$_{13}$ and La$_3$Rh$_4$Sn$_{13}$ superconductors. Applying an external pressure to La$_3$Rh$_4$Sn$_{13}$, the resistive superconducting critical temperature Tc decreases, while the critical temperature of La$_3$Co$_4$Sn$_{13}$ is enhanced with increasing pressure. The positive pressure coefficient dTc/dP correlates with a subtle structural transition in La$_3$Co$_4$Sn$_{13}$ and is discussed in the context of lattice instabilities. Specific-heat data show that both compounds are typical BCS superconductors. However, La$_3$Rh$_4$Sn$_{13}$ also exhibits a second superconducting phase at higher temperatures, which is characteristic of inhomogeneous superconductors. We calculate the specific heat for an inhomogeneous superconducting phase, which agrees well with experimental C(T) data for La$_3$Rh$_4$Sn$_{13}$. We also found that an applied pressure reduces this second superconducting phase.