Strong coupling superconductivity at 8.4 K in an antiperovskite phosphide SrPt ₃P

We report the discovery of a family of ternary platinum phosphides APt ₃P (A=Ca, Sr, and La), which crystallize in an antiperovskite-based structure closely related to that of the heavy fermion superconductor CePt ₃Si. All three phosphides showed superconductivity at low temperatures and the highest critical temperature T _c=8.4K was observed for SrPt ₃P. The analysis of specific heat C(T) for SrPt ₃P shows clear evidence for very strong coupling s-wave superconductivity with a large ratio between superconducting gap Δ ₀ and T _c, 2Δ ₀/k _BT _c∼5, and the presence of low-energy phonons. The presence of multiple Fermi surface pockets was inferred from the nonlinear magnetic field dependence of Hall resistivity, which we argue might play a role in realizing the strong coupling of charge carriers with the low-lying phonons.