Note added 14 March 2014: paper now published, reference Phys. Rev. Lett. 112, 107002 (2014).

New paper out with Warwick, ISIS and Bristol on the non-centrosymmetric superconductor Re6Zr (accepted for publication in Phys. Rev. Lett. – follow the link below for the arXiv preprint). Our symmetry analysis suggests this could be the first non-centrosymmetric superconductor (NCS) where we get simultaneously large singlet-triplet mixing and broken time-reversal symmetry (TRS).

Just to recap, the other NCS where broken TRS has been confirmed experimentally is LaNiC2. In that case  the crystal point group only has one-dimensional irreducible representations, and therefore the only way one can get broken TRS in the bulk for that material is if mixing is small on the relevant parts of the Fermi surface. In contrast, for Re6Zr there are higher-dimensional irreps in the point group and therefore potentially we could have broken TRS even in the case when the singlet and triplet pairing channels are strongly admixed. This could have many novel consequences.

Here’s the preprint:

Observation of Time-Reversal Symmetry Breaking in the Non-Centrosymmetric Superconductor Re6Zr

We have investigated the superconducting state of the non-centrosymmetric compound Re6Zr using magnetization, heat capacity, and muon-spin relaxation/rotation (muSR) measurements. Re6Zr has a superconducting transition temperature, Tc = 6.75 K. Transverse-field muSR experiments, used to probe the superfluid density, suggest an s-wave character for the superconducting gap. However, zero and longitudinal-field muSR data reveal the presence of spontaneous static magnetic fields below Tc indicating that time-reversal symmetry is broken in the superconducting state and an unconventional pairing mechanism. An analysis of the pairing symmetries identifies the ground states compatible with time-reversal symmetry breaking.