Electron irradiation reveals robust fully gapped superconductivity in LaNiGa2

The effects of 2.5-MeV electron irradiation were studied in the superconducting phase of single crystals of LaNiGa2, using measurements of electrical transport and radio-frequency magnetic susceptibility. The London penetration depth is found to vary exponentially with temperature, suggesting a fully gapped Fermi surface. The inferred superfluid density is close to that of a single-gap weak-coupling isotropic s-wave superconductor. Superconductivity is extremely robust against nonmagnetic point-like disorder induced by electron irradiation. Our results place strong constraints on the previously proposed triplet pairing state by requiring fine-tuned impurity scattering amplitudes and are most naturally explained by a sign-preserving, weak-coupling, and approximately momentum-independent singlet superconducting state in LaNiGa2, which does not break timereversal symmetry. We discuss how our findings could be reconciled with previous measurements that indicated magnetic signatures in the superconducting phase.