# Discovery of nodal-line superconductivity in chiral crystals

## TL;DR

This paper reports the discovery of topological nodal-line superconductivity in chiral LaIrSi crystals, revealing a new mechanism driven by isotropic spin-orbit coupling that broadens understanding of unconventional superconductivity in chiral materials.

## Contribution

It introduces the first observation of nodal-line superconductivity in a chiral crystal and demonstrates a novel isotropic SOC mechanism distinct from previous models.

## Key findings

- LaIrSi exhibits topological nodal-line superconductivity.
- Substituting Rh with Ir enhances SOC and induces unconventional SC.
- A new phase diagram for chiral superconductor engineering is proposed.

## 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, we report the discovery of unconventional SC in the La(Rh,Ir)Si family of materials 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 our conventional understanding of material candidates for unconventional SC, which typically rely on a significantly anisotropic SOC to promote the triplet pairing. Our 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.