We report the electric transport and thermodynamic properties of the skutterudite-related La3Co4Sn13 and La3Rh4Sn13 superconductors. Applying an external pressure to La3Rh4Sn13, the resistive superconducting critical temperature Tc decreases, while the critical temperature of La3Co4Sn13 is enhanced with increasing pressure. The positive pressure coefficient dTc/dP correlates with a subtle structural transition in La3Co4Sn13 and is discussed in the context of lattice instabilities. Specific heat data show that both compounds are typical BCS superconductors. However, La3Rh4Sn13 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 La3Rh4Sn13. We also found that an applied pressure reduces this second superconducting phase.

a b s t r a c tLa 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 , 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. Additionally, for La 3 Ru 4 Sn 13 we find that dT + c =dP > dT c =dP. The pressure dependences of 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 4Àx Sn 13 shows a T + c 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.

We report the observation of a superconducting state below ∼ 8 K coexistent with a spin-glass state caused by atomic disorder in Ce substituted Ca3Rh4Sn13. Measurements of specific heat, resistivity, and magnetism reveal the existence of inhomogeneous superconductivity in samples doped with Ce with superconducting critical temperatures Tc higher than those observed in the parent compound. For Ca3Rh4Sn13, the negative value of the change in resistivity ρ with pressure P , dρ dP correlates well with the calculated decrease in the density of states (DOS) at the Fermi energy with P . Based on band structure calculations performed under pressure, we demonstrate how the change in DOS would affect Tc of Ca3Rh4Sn13 under negative lattice pressure in samples that are strongly defected by quenching.