Extended nuclear quadrupole resonance study of the heavy-fermion superconductor PuCoGa5

PuCoGa5 has emerged as a prototypical heavy-fermion superconductor, with its transition temperature (T-c similar or equal to 18.5 K) being the highest amongst such materials. Nonetheless, a clear description as to what drives the superconducting pairing is still lacking, rendered complicated by the notoriously intricate nature of plutonium's 5f valence electrons. Here, we present a detailed Ga-69,Ga-71 nuclear quadrupole resonance (NQR) study of PuCoGa5, concentrating on the system's normal state properties near to T-c and aiming to detect distinct signatures of possible pairing mechanisms. In particular, the quadrupole frequency and spin-lattice relaxation rate were measured for the two crystallographically inequivalent Ga sites and for both Ga isotopes, in the temperature range 1.6-300 K. No evidence of significant charge fluctuations is found from the NQR observables. On the contrary, the low-energy dynamics is dominated by anisotropic spin fluctuations with strong, nearly critical, in-plane character, which are effectively identical to the case of the sister compound PuCoIn5. These findings are discussed within the context of different theoretical proposals for the unconventional pairing mechanism in heavy-fermion superconductors.