Tuning the Parity Mixing of Singlet-Septet Pairing in a Half-Heusler Superconductor

In superconductors, electrons with spin s=1/2 form Cooper pairs whose spin structure is usually singlet (S=0) or triplet (S=1). When the electronic structure near the Fermi level is characterized by fermions with angular momentum j=3/2 due to strong spin-orbit interactions, novel pairing states such as even-parity quintet (J=2) and odd-parity septet (J=3) states are allowed. Prime candidates for such exotic states are half-Heusler superconductors, which exhibit unconventional superconducting properties, but their pairing nature remains unsettled. Here, we show that the superconductivity in the noncentrosymmetric half-Heusler LuPdBi can be consistently described by the admixture of isotropic even-parity singlet and anisotropic odd-parity septet pairing, whose ratio can be tuned by electron irradiation. From magnetotransport and penetration depth measurements, we find that carrier concentrations and impurity scattering both increase with irradiation, resulting in a nonmonotonic change of the superconducting gap structure. Our findings shed new light on our fundamental understanding of unconventional superconducting states in topological materials.