Andreev-reflection spectroscopy of SrPt3P single crystals

The interplay between inversion symmetry breaking and superconducting pairing mechanisms is a long-standing puzzle of solid-state physics. Ternary platinum phosphide SrPt3P is an excellent case study in this regard, since it shares the distorted anti-perovskite crystal structure of the non-centrosymmetric superconductor CePt3Si but preserves the inversion symmetry center, enabling a direct comparison between the two compounds. Despite intense scrutiny, the superconducting pairing strength, gap symmetry, and magnitude of SrPt3P remain an open question. Here, we study the gap structure of high-quality SrPt3P single crystals with Tc = 8.6 K by using point-contact Andreev-reflection spectroscopy. The spectra systematically showed symmetric conductance maxima and a zero-bias minimum, as is typical of fully gapped superconductors, and dips at finite voltage Vdip due to the local transition of the contact region to the resistive state. In contacts with high resistance, where the dips fall sufficiently far from the gap features, the conductance curves can be well fitted with a single isotropic s-wave gap. The temperature dependence Δ(T) of the gap amplitude extracted from the fit is in good agreement with the one predicted by the BCS theory, with a zero-temperature mean value Δ(0) = 1.88 ± 0.08 meV and a mean gap ratio 2Δ/kBTc = 5.2 ± 0.2.