Discovery of Nodal‐Line Superconductivity in Chiral Crystals

Abstract

Abstract Chiral crystals, whose key feature is the structural handedness, host exotic quantum phenomena driven by the interplay of band topology, spin‐orbit coupling (SOC), and electronic correlations. Due to the limited availability of suitable chiral‐crystal materials, their unconventional superconductivity (SC) remains largely unexplored. Here, the discovery of unconventional SC in the La(Rh,Ir)Si family of materials is reported by combining muon‐spin spectroscopy, band‐structure calculations, and perturbation theory. This family, characterized by a double‐helix chiral structure, hosts exotic multifold fermions that are absent in other topological chiral crystals. While LaRhSi behaves as a fully‐gapped superconductor, the substitution of 4 d ‐Rh by 5 d ‐Ir significantly enhances the SOC and leads to the emergence of topological nodal‐line SC in LaIrSi. The developed model shows that the nodal‐line SC arises from an isotropic SOC with a specific strength. Such an exotic mechanism expands the conventional understanding of material candidates for unconventional SC, which typically rely on a significantly anisotropic SOC to promote the triplet pairing. The current work establishes a new type of phase diagram, which provides a comprehensive roadmap for identifying and engineering unconventional SC in chiral crystals. Furthermore, it calls for renewed investigations of unconventional SC in other widely studied superconductors with a chiral structure.