Two-gap superconductivity in LaNiGa$_2$ with non-unitary triplet pairing and even parity gap symmetry

TL;DR

This paper investigates the superconducting gap symmetry of LaNiGa$_2$, revealing a two-gap nodeless triplet pairing state with even parity, reconciling previous experimental contradictions and proposing a novel pairing mechanism.

Contribution

It introduces a new triplet superconducting state with electrons pairing on different orbitals, resulting in a nodeless, isotropic gap symmetry that breaks time reversal symmetry.

Findings

LaNiGa$_2$ exhibits two-gap nodeless superconductivity.

The proposed pairing state involves electrons of the same spin on different orbitals.

This model explains the coexistence of triplet pairing and fully gapped behavior.

Abstract

The nature of the pairing states of superconducting LaNiC$_2$ and LaNiGa$_2$ has to date remained a puzzling question. Broken time reversal symmetry has been observed in both compounds and a group theoretical analysis implies a non-unitary triplet pairing state. However all the allowed non-unitary triplet states have nodal gap functions but most thermodynamic and NMR measurements indicate fully gapped superconductivity in LaNiC$_2$. Here we probe the gap symmetry of LaNiGa$_2$ by measuring the London penetration depth, specific heat and upper critical field. These measurements demonstrate two-gap nodeless superconductivity in LaNiGa$_2$, suggesting that this is a common feature of both compounds. These results allow us to propose a novel triplet superconducting state, where the pairing occurs between electrons of the same spin, but on different orbitals. In this case the superconducting wavefunction has a triplet spin component but isotropic even parity gap symmetry, yet the overall wavefunction remains antisymmetric under particle exchange. This model leads to a nodeless two-gap superconducting state which breaks time reversal symmetry, and therefore accounts well for the seemingly contradictory experimental results.