Superconducting properties of noncentrosymmetric Li2(Pt1-xPdx)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(Pt1-xPdx)3B [x = 0, 0.5, 1] was measured. The Pd-concentration (x)-dependent electronic specific heat (Ces) of Li2(Pt1-xPdx)3B at low temperatures <<Tc showed distinctly different features on varying x. The temperature dependence of Ces for one end member, Li2Pd3B, follows an exponential behavior as in conventional superconductors, while those of Li2Pd1.5Pt1.5B 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.