Nuclear spin-lattice relaxation rate in PuMGa₅ (M=Rh, Co): A two-component spin-fermion model

We examine the nuclear spin-lattice relaxation rates 1∕T₁ of PuRhGa₅ and PuCoGa₅, both in the superconducting and normal states, as well as their Knight shifts. Results for both compounds are consistent with a superconducting gap of d-wave symmetry in the presence of strong impurity scattering, though with quite different gap-over-T_c ratios 2Δ₀∕k_BT_c (eight for PuCoGa₅ and five for PuRhGa₅). In the normal state, PuRhGa₅ exhibits a gradual suppression of (T₁T)⁻¹ below 25 K, while measurements for PuCoGa₅ reveal a monotonic increase down to T_c. We propose that this behavior is consistently understood by the crossover of the two-dimensional quantum antiferromagnetic correlation of the local 5f-electron spins of Pu and the concomitant formation of the fermion pseudogap based on the two-component spin-fermion model.