Strong enhancement of s-wave superconductivity near a quantum critical point of Ca₃Ir₄Sn₁₃

We report microscopic studies by muon spin rotation/relaxation as a function of pressure of the Ca₃Ir₄Sn₁₃ and Sr₃Ir₄Sn₁₃ cubic compounds, which are members of the (Ca_1−xSr_x)₃Ir₄Sn₁₃ system displaying superconductivity and a structural phase transition associated with the formation of a charge density wave (CDW). We find a strong enhancement of the superfluid density and a dramatic increase of the pairing strength above a pressure of ≈1.6 GPa, giving direct evidence of the presence of a quantum critical point separating a superconducting phase coexisting with CDW from a pure superconducting phase. The superconducting order parameter in both phases has the same s-wave symmetry. In spite of the conventional phonon-mediated BCS character of the weakly correlated (Ca_1−xSr_x)₃Ir₄Sn₁₃ system, the dependence of the effective superfluid density on the critical temperature puts this compound in the 'Uemura' plot close to unconventional superconductors. This system exemplifies that conventional BCS superconductors in the presence of competing orders or multiband structure can also display characteristics of unconventional superconductors.