Superconducting properties of noncentrosymmetric Li₂(Pt_1-xPd_x)₃B superconductors

Superconductivity has been found in two Li-containing compounds, Li₂Pt₃B and Li₂Pd₃B, which show superconducting transition at temperatures 2.5 K and 7.2 K, respectively, and have the same cubic structure (P4₃32) without mirror or inversion symmetry. Because the compounds are noncentrosymmetric, it is interesting to study the symmetries of the superconducting gap function and Cooper pairs. The specific heat of Li₂(Pt_1-xPd_x)₃B [x = 0, 0.5, 1] was measured. The Pd-concentration (x)-dependent electronic specific heat (C_es) of Li₂(Pt_1-xPd_x)₃B at low temperatures <<T_c showed distinctly different features on varying x. The temperature dependence of C_es for one end member, Li₂Pd₃B, follows an exponential behavior as in conventional superconductors, while those of Li₂Pd_1.5Pt_1.5B and Li₂Pt₃B follow a quadratic one, suggesting the existence of line nodes in the superconducting gap. This contrasting result conceivably originates from the difference in the strengths of the spin-orbit coupling of the Pt- and Pd-based compounds.