We just got a paper accepted in Phys. Rev. Lett. where we report evidence of time-reversal symmetry breaking in LaNiGa2, which is chemically-speaking a close cousin of LaNiC2, though without the lack of inversion symmetry. We also offer a group-theoretical analysis of the pairing symmetry which, like in the case of LaNiC2, points to non-unitary triplet pairing. The paper also contains a bit of Ginzburg-Landau on which I comment some more below.
This work suggests that LaNiC2 and LaNiGa2 are two members of a new family of very exotic superconductors. Until now, all known triplet superconductors were either unitary (i.e., with equal participation in pairing by spin-up and spin-down electrons) or non-unitary, but ferromagnetic. In the case of a ferromagnet non-unitary pairing (where spin-up and spin-down electrons participate in the pairing differently) can be simply explained by a pre-existing spin polarisation of the Fermi liquid before the onset of the superconducting instability. LaNiC2 and LaNiGa2 are paramagnets, which brings up the question how the non-unitary triplet pairing comes about. In the paper we offer a possible explanation: it turns out there is a generic term (required by symmetry) in the Ginzburg-Landau theory of any paramagnetic triplet superconductor that couples the non-unitary component of the order parameter to the magnetisation. In effect, if the pairing is non-unitary, it leads to a sub-dominant magnetisation, which in turn enhances the non-unitary triplet pairing. Thus non-unitary triplets can effectively lift themselves by their bootstraps through a mechanism that is the superocnducting analogue of the Stoner instability leading to ferromagnetism in some metals.
The paper is in press, but we’ve just put it on the arXiv: A. D. Hillier, J. Quintanilla, B. Mazidian, J. F. Annett, R. Cywinski, arxiv.org/abs/1206.5905.
Note added.- published reference: Hillier, A. D., Quintanilla, J., Mazidian, B., Annett, J. F. & Cywinski, R. Nonunitary Triplet Pairing in the Centrosymmetric Superconductor LaNiGa2. Physical Review Letters 109, (2012). http://dx.doi.org/10.1103/PhysRevLett.109.097001