Unconventional superconductivity in ScIr$_2$ chiral crystal with a kagome lattice

Materials with a kagome lattice host exotic quantum phenomena driven by the interplay between band topology, spin-orbit coupling, magnetism, and electronic correlations. While magnetism of kagome materials has been widely investigated, their unconventional superconductivity (SC) remains largely unexplored due to the limited availability of suitable materials. Here, we report evidence of unconventional SC in the ScIr$_{2-x}$Si$_{x}$ family by combining muon-spin spectroscopy measurements with band-structure calculations. The parent ScIr$_2$ undergoes a structural phase transition from a high-$T$ cubic- to a low-$T$ rhombohedral phase, while the Ir kagome layer remains, albeit slightly distorted. Although the structural transition is suppressed by Si substitution, the superconducting pairing of ScIr$_{2-x}$Si$_{x}$ remains well described by a two-gap model. Since at least one of the gaps has nodes, this indicates an unconventional SC. Its unconventional nature can be explained by the distinct flat bands occurring near the Fermi level, leading to strong electronic correlations in the ScIr$_{2-x}$Si$_{x}$ family. Moreover, the low-$T$ phase of ScIr$_2$ exhibits an Ir chiral chain; therefore, it can be classified as a topological chiral crystal. Overall, the unusual properties of the ScIr$_{2-x}$Si$_{x}$ family make it an interesting, albeit rare, system for studying the interplay between unconventional SC, flat bands, and chirality.