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. 
R. P. Singh, A. D. Hillier, B. Mazidian, J. Quintanilla, J. F. Annett,   D. McK. Paul, G. Balakrishnan, and M. R. Lees
cond-mat.supr-con cond-mat.mtrl-sci cond-mat.str-el