We have investigated the superconducting state of the noncentrosymmetric compound Re6Zr using magnetization, heat capacity, and muon-spin relaxation or rotation (μSR) measurements. Re6Zr has a superconducting transition temperature, Tc=6.75±0.05  K. Transverse-field μSR experiments, used to probe the superfluid density, suggest an s-wave character for the superconducting gap. However, zero and longitudinal-field μSR 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.

Muon spin relaxation experiments on the noncentrosymmetric intermetallic superconductor LaNiC2 are reported. We find that the onset of superconductivity coincides with the appearance of spontaneous magnetic fields, implying that in the superconducting state time-reversal symmetry is broken. An analysis of the possible pairing symmetries suggests only four triplet states compatible with this observation, all of them nonunitary. They include the intriguing possibility of triplet pairing with the full point group symmetry of the crystal, which is possible only in a noncentrosymmetric superconductor.

We have discovered that the Ni and C atomic positions were mistyped in the data files used to generate the image of the unit cell in Fig. 3 of our Letter [1]. The same data files were used to calculate the nuclear dipole field whose contours are shown in the same figure. We provide here a corrected version of Fig. 3, obtained using the correct Ni and C positions corresponding to the fit to neutron diffraction data shown in Fig. 1 of our Letter. The crystal symmetry does not change and the effect on the dipole fields is negligible. Thus, all our other results and conclusions are unaffected.[1] A. D. Hillier, J. Quintanilla, and R. Cywinski, Phys. Rev. Lett. 102, 117007 (2009) . FIG. 3 (color online). The crystal structure of LaNiC 2 . The large red spheres are La, medium-sized blue spheres are Ni, smaller black spheres are C, and the smallest yellow sphere represents the muon. The contour plot of the nuclear dipole fields for LaNiC 2 is shown throughout the unit cell at x ¼ 1 2 and shows the muon site is at ( 1 2 , 1 2 , 0).