Extended Nuclear Quadrupole Resonance Study of the Heavy-Fermion Superconductor PuCoGa$_5$

TL;DR

This study uses nuclear quadrupole resonance to investigate the normal state properties of PuCoGa$_5$, revealing dominant in-plane spin fluctuations and providing insights into its unconventional superconducting pairing mechanism.

Contribution

It provides the first detailed NQR analysis of PuCoGa$_5$'s normal state, highlighting the role of anisotropic spin fluctuations near $T_c$.

Findings

No significant charge fluctuations detected.

Dominant anisotropic in-plane spin fluctuations observed.

Similar spin dynamics to PuCoIn$_5$.

Abstract

PuCoGa$_5$ has emerged as a prototypical heavy-fermion superconductor, with its transition temperature ($T_c\simeq18.5$ 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 5$f$ valence electrons. Here, we present a detailed $^{69,71}$Ga nuclear quadrupole resonance (NQR) study of PuCoGa$_5$, 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 K - 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 PuCoIn$_5$. These findings are discussed within the context of different theoretical proposals for the unconventional pairing mechanism in heavy-fermion superconductors.