studies of the superconducting order parameter in

We present transverse-field (TF) measurements of the in-plane magnetic-field penetration depth λ ( T ) in single-crystalline PuCoGa 5 ( T c = 18.5 K ) for 0.06 T applied field ( ≈ 2 – 5 × H c 1 ) . We find that the temperature dependence of the superfluid density, and therefore Δ λ ( T ) = λ ( T ) - λ ( 0 ) , is ∝ T for T / T c ⩽ 0.5 . We estimate that the measured λ ( 0 ) = 241 ( 3 ) nm . ZF measurements find no evidence for time-reversal symmetry violation. The ZF and TF measurements are consistent with an even-parity (pseudo-spin singlet), d-wave pairing state.