Suppression of anharmonic phonons and s-wave superconductivity by defects in the filled skutterudite LaRu4As12

In filled-skutterudite compounds, guest atoms are centered at highly symmetric cages essentially formed by pnictogens. The weak bonding of the guest atoms to the cage-forming elements is expected to give rise to local atomic vibrations in anharmonic potential which can influence electronic properties and may even promote superconductivity. However, the relation between the unusual low-energy phonon excitations and the electronic properties is still elusive, and evidence establishing a direct link between them is a long-standing issue. Here, we investigate the impact of artificial atomic defects introduced by electron irradiation in the filled-skutterudite superconductor LaRu4As12. Our high-resolution heat capacity measurements reveal that the electronic specific heat is substantially reduced with increasing concentration of atomic defects. Moreover, the irradiation suppresses the fully gapped s-wave superconductivity as well as the contribution of anharmonic phonons in the specific heat in a correlated fashion. Our findings imply that the anharmonic phonons play an important role in enhancing electronic specific heat and superconductivity.