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

The temperature- and field-dependent specific heat C (T,H) measurement on the noncentrosymmetric superconductors Li2 Pd3 B, Li2 Pt3 B, and Li2 (Pd0.5 Pt0.5) 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 CeS (T< Tc =6.95 K,0 T) =3 exp [-16.4/T] J/mol K while for the Pt-based compound CeS (T< Tc =2.56 K,0 T) =α T2 mJ/mol K, where α=7.3 mJ/mol K3. 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 CN (T,7T> Hc2) =9.3T+1.1 T3 mJ/mol K while that of the Pt-based isomorph shows CN (T,7T> Hc2) =9.3T+0.92 T3 mJ/mol K. For the intermediate alloy Li2 (Pt0.5 Pd0.5) 3 B, C (T,H) mimics that of the Pt-based compound; specifically, CeS (T< Tc =3.9 K,0 T) =α T2 mJ/mol K where α=4.6 (2) mJ/mol K3. The ratio of the coefficients of the quadratic terms in Li2 (Pt0.5 Pd0.5) 3 B and Li2 Pt3 B, α Pt0.5 Pd0.5 / αPt, is equal to Z Pt0.5 Pd0.5 2 / ZPt2, 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 Tc, Hc1, Hc2, and (∂ Hc2/∂T) Tc is almost linear.