Low-Energy Quasiparticle Excitations in Half-Heusler Superconductors with j = 3/2 Fermions

In noncentrosymmetric half-Heusler compounds where the strong spin-orbit coupling causes a band inversion, fermions with total angular momentum j = 3/2 contribute mainly to the transport phenomena, including the superconductivity. Interestingly, Cooper pairs formed by the j = 3/2 fermions can form not only usual singlet and triplet states but also even-parity quintet and odd-parity septet states. Recently, it has been reported that the temperature dependence of magnetic penetration depth shows a power-law behavior in YPtBi, which suggests an unconventional nodal gap structure. Several theories with the quintet or septet pairings have been proposed to explain this result, whereas it is still experimentally unclear whether the reported nodal structure is universal or not in the non-magnetic half-Heusler superconductors with j = 3/2 fermions. To study the gap structure in several half-Heusler superconductors, we have measured the magnetic penetration depth on YPtBi, LuPtBi, and LuPdBi with different spin-orbit coupling strength. Our data suggest the existence of gap minima rather than nodes in all the materials, which strongly indicates that the nodal structure is not protected by symmetries. Based on these results, we discuss the pairing symmetry of these half-Heusler compounds.