Superconductivity of centrosymmetric and non-centrosymmetric phases in antiperovskite (Ca,Sr)Pd${}_3$P

Abstract

In the recently discovered antiperovskite phosphide (Ca,Sr)Pd${}_3$P, centrosymmetric (CS) and non-centrosymmetric (NCS) superconducting phases appear depending on the Sr concentration, and their transition temperatures ($T_{\mathrm{c}}$) 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) (Ca${}_{0.6}$Sr${}_{0.4}$)Pd${}_3$P ($T_{\mathrm{c}}$ = 3.5 K) and NCS tetragonal (NCSt) (Ca${}_{0.25}$Sr${}_{0.75}$)Pd${}_3$P ($T_{\mathrm{c}}$ = 0.32 K) samples with a focus on explaining their large $T_{\mathrm{c}}$ difference. Specific heat measurements indicated that CSo (Ca${}_{0.6}$Sr${}_{0.4}$)Pd${}_3$P was an s-wave superconductor in a moderate-coupling regime with a 2$\Delta$${}_0$/k${}_B$$T_{\mathrm{c}}$ value of 4.0. Low-lying phonons leading to the strong coupling in the structurally analogous SrPt${}_3$P were unlikely to be present in CSo (Ca${}_{0.6}$Sr${}_{0.4}$)Pd${}_3$P. Given that CSo (Ca${}_{0.6}$Sr${}_{0.4}$)Pd${}_3$P and NCSt (Ca${}_{0.25}$Sr${}_{0.75}$)Pd${}_3$P exhibited similar Debye temperatures ($\Theta$${}_D$) of approximately 200 K, the large $T_{\mathrm{c}}$ difference could not be attributed to $\Theta$${}_D$.$T_{\mathrm{c}}$ 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 $T_{\mathrm{c}}$ in NCSt (Ca${}_{0.25}$Sr${}_{0.75}$)Pd${}_3$P can be primarily attributed to the decrease in $N$(0) associated with the structural phase transition without considering the lack of inversion symmetry.

arXiv comment14 pages, 8 figures,

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