Spin-triplet superconductivity in Weyl nodal-line semimetals

TL;DR

This paper reports the discovery of spin-triplet superconductivity in Weyl nodal-line semimetals, specifically in the LaNiSi, LaPtSi, and LaPtGe compounds, which exhibit a fully-gapped state and break time-reversal symmetry, indicating a topological phase transition.

Contribution

It introduces the first experimental evidence of spin-triplet superconductivity in Weyl nodal-line semimetals and models its properties assuming a purely spin-triplet pairing mechanism.

Findings

Materials exhibit fully-gapped superconducting state

Superconductivity breaks time-reversal symmetry

Model accurately describes superconducting properties

Abstract

Topological semimetals are three dimensional materials with symmetry-protected massless bulk excitations. As a special case, Weyl nodal-line semimetals are realized in materials either having no inversion or broken time-reversal symmetry and feature bulk nodal lines. The 111-family of materials, LaNiSi, LaPtSi and LaPtGe (all lacking inversion symmetry), belong to this class. Here, by combining muon-spin rotation and relaxation with thermodynamic measurements, we find that these materials exhibit a fully-gapped superconducting ground state, while spontaneously breaking time-reversal symmetry at the superconducting transition. Since time-reversal symmetry is essential for protecting the normal-state topology, its breaking upon entering the superconducting state should remarkably result in a topological phase transition. By developing a minimal model for the normal-state band structure and assuming a purely spin-triplet pairing, we show that the superconducting properties across the family can be described accurately. Our results demonstrate that the 111-family reported here provides an ideal test-bed for investigating the rich interplay between the exotic properties of Weyl nodal-line fermions and unconventional superconductivity.