Specific-heat studies of the spin-orbit interaction in noncentrosymmetric Li2(Pd1-xPtx)3B (x=0,0.5,1) superconductors

The temperature- and field-dependent specific heat C(T,H) measurement on the noncentrosymmetric superconductors Li2Pd3B, Li2Pt3B, and Li-2(Pd0.5Pt0.5)(3)B were carried out over a wide range of T and H. C(T,H) of the limit compounds is found to be distinctly different: within the superconducting phase, the electronic specific heat of the Pd-based compound follows C-e(S)(T < T-c=6.95 K,0 T)=3 exp[-16.4/T] J/mol K while for the Pt-based compound C-e(S)(T < T-c=2.56 K,0 T)=alpha T-2 mJ/mol K, where alpha=7.3 mJ/mol K-3. An increase in H tends to reduce weakly the exponential argument and the coefficient of the quadratic term. The above-mentioned findings are in agreement with the description that, due to the difference in the strength of the spin-orbit coupling, the pairing symmetry in the Pd- (Pt-) based compound is predominately a spin-singlet (spin-triplet) state. Within the very low-temperature range, the field-induced normal-state of the Pd-based limit manifests C-N(T,7T>H-c2)=9.3T+1.1T(3) mJ/mol K while that of the Pt-based isomorph shows C-N(T,7T>H-c2)=9.3T+0.92T(3) mJ/mol K. For the intermediate alloy Li-2(Pt0.5Pd0.5)(3)B, C(T,H) mimics that of the Pt-based compound; specifically, C-e(S)(T < T-c=3.9 K,0 T)=alpha T-2 mJ/mol K where alpha=4.6(2) mJ/mol K-3. The ratio of the coefficients of the quadratic terms in Li-2(Pt0.5Pd0.5)(3)B and Li2Pt3B, alpha Pd-Pt0.5(0.5)/alpha(Pt), is equal to Z(Pt0.5)Pd(0.5)(2)/Z(Pt)(2), confirming that the line nodes are driven by the spin-orbit interaction. The finding that these nodes can survive even in a 50% Pd-diluted material gives extra support to the reported claim that the x-dependent evolution of T-c, H-c1, H-c2, and (partial derivative H-c2/partial derivative T)(Tc) is almost linear.