Muon spin rotation study of MgCNi3

We have performed transverse field μSR measurements of the non-oxide perovskite compound MgCNi3 down to 20mK to determine the magnetic field penetration depth, finding λ(0)=2315±10Å. We have also performed zero-field measurements and can exclude any spontaneous internal fields greater than 0.02G. Thus, we conclude that the superconducting state does not possess broken time reversal symmetry.