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.
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