Physical properties of Li₂Pd₃B and Li ₂Pt₃B superconductors

Superconductivity in two Li-containing compounds of Li₂Pd ₃B and Li₂Pt₃B was recently discovered. These materials, showing the superconducting transition at 7.2 K and 2.6 K, respectively, have the same cubic structure (P4₃32) composed of distorted octahedrons without mirror or inversion symmetry along any directions. This is a very interesting feature of those materials in relation to the symmetry of superconductivity. Resistivity measurements in magnetic fields gave their upper critical fields, H_c2(0) = 45 kOe and 19 kOe, respectively. Their specific heat was measured using a heat-pulse relaxation method. The Sommerfeld coefficient (γ) and Debye temperature (θ_D) terms of Li₂Pd₃B were given as γ=9.5 mJmol^-1K^-2 and θ_D=228 K. The value of C/γr at T_c was calculated to be 1.7. In the same manner, those parameters were described for Li₂Pt₃B as γ=9.6 mJmol^-1K^-2, θ_D=240 K, and C/γT_c =0.75, respectively. Since C/γT_C in the weak-coupling limit by the BCS theory is 1.43, the value of 1.7 for Li ₂Pd₃B is slightly higher. The electronic specific heat of Li₂Pd₃B at a zero magnetic field follows the typical exponetial behavior discribed in the BCS theory, while that of Li ₂Pt₃B shows quadratic behavior. This result suggests the line nodes exist in the superconducting gap of Li₂Pt₃B driven by the spin-orbit interaction.