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The symmetry of the superconducting order parameter in single-crystalline PuCoGa5 (Tc = 18.5 K) is investigated via zero-and transverse-field muon spin relaxation (µSR) measurements, probing the possible existence of orbital and/or spin moments (time reversal-symmetry violation TRV) associated with the superconducting phase and the in-plane magnetic-field penetration depth λ(T ) in the mixed state, respectively. We find no evidence for TRV, and show that the superfluid density, or alternatively, ∆λ(T ) = λ(T ) − λ(0), are ∝ T for T /Tc ≤ 0.5. Taken together these measurements are consistent with an even-parity (pseudo-spin singlet), d-wave pairing state.

We present the first µSR measurements ever performed on elemental Pu, and set the most stringent upper limits to date on the magnitude of the ordered moment µ ord in α-Pu and δ-stabilized Pu (alloyed with 4.3 at. % Ga). Assuming a nominal hyperfine coupling field of 1 kOe/µB we find µ ord ≤ 10 −3 µB at T ∼ = 4 K.

Longitudinal-field muon spin relaxation experiments have been carried out in the paramagnetic state of singlecrystal Gd2Ti2O7 just above the phase transition at Tm = 1.0 K. At high applied fields the exponential relaxation time T1 is proportional to field, whereas T1 saturates below a crossover field Bc that is ∼2.5 T at 1.5 K and decreases as Tm is approached. At low fields the relaxation rate increases markedly as the freezing temperature is approached, as expected for critical slowing down of the spin fluctuations, but the increase is suppressed by applied field. This behavior is consistent with the very long autocorrelation function cutoff time implied by the low value of Bc.