Superconductivity of centrosymmetric and non-centrosymmetric phases in antiperovskite (Ca,Sr)Pd3P

Abstract

In the recently discovered antiperovskite phosphide (Ca,Sr)Pd3P, centrosymmetric (CS) and non-centrosymmetric (NCS) superconducting phases appear depending on the Sr concentration, and their transition temperatures (Tc) differ by as much as one order of magnitude. In this study, we investigated the superconducting properties and electronic band structures of CS orthorhombic (CSo) (Ca0.6Sr0.4)Pd3P (Tc = 3.5 K) and NCS tetragonal (NCSt) (Ca0.25Sr0.75)Pd3P (Tc = 0.32 K) samples with a focus on explaining their large Tc difference. Specific heat measurements indicated that CSo (Ca0.6Sr0.4)Pd3P was an s-wave superconductor in a moderate-coupling regime with a 2Δ0/kBTc value of 4.0. Low-lying phonons leading to the strong coupling in the structurally analogous SrPt3P were unlikely to be present in CSo (Ca0.6Sr0.4)Pd3P. Given that CSo (Ca0.6Sr0.4)Pd3P and NCSt (Ca0.25Sr0.75)Pd3P exhibited similar Debye temperatures (ΘD) of approximately 200 K, the large Tc difference could not be attributed to ΘD. Tc of each phase was accurately reproduced based on the Bardeen–Cooper–Schrieffer (BCS) theory using experimental data and the density of states of the Fermi level N(0) calculated from their band structures. We concluded that the considerable suppression of Tc in NCSt (Ca0.25Sr0.75)Pd3P can be primarily attributed to the decrease in N(0) associated with the structural phase transition without considering the lack of inversion symmetry.